‘Candidatus Phytoplasma mali’, ‘Candidatus Phytoplasma pyri’ and ‘Candidatus Phytoplasma prunorum’, the causal agents of apple proliferation, pear decline and European stone fruit yellows, respectively

Genome-Informed Real-Time PCR Assay for Detection of 'Candidatus Phytoplasma Prunorum,' Which Is Associated with European Stone Fruit Yellows

'Candidatus Phytoplasma prunorum' has been associated with severe disease in Prunus spp., which are commodities of economic importance in the USA. The introduction and establishment of 'Ca. P. prunorum' in the USA could result in huge economic losses, thus creating a need for validated diagnostic tools, which are the cornerstone of successful surveillance, quarantine, and eradication measures. Whole-genome comparisons led to the identification of a diagnostic marker gene specific to 'Ca. P. prunorum' (PE639). The PE639 assay was duplexed with an 18S rDNA plant internal control and compared to modified 23S (phytoplasmas) and imp ('Ca. P. mali') assays. The PE639 assay produced congruent results to 23S and imp assays for all metrics, demonstrating high linearity, repeatability, intermediate precision, and reproducibility. The limit of detection was comparable for all assays tested, and all demonstrated 100% analytical specificity, selectivity, and diagnostic specificity for their respective target species. Assays metrics were consistent across two platforms, the ABI QuantStudio™ 5 and Bio-Rad CFX96™ OPUS. A synthetic gBlocks™ control was designed and validated to work with all assays, as well as conventional PCR assays targeting 16S rDNA and tuf genes. These validated assays and synthetic control represent beneficial tools that support efforts to protect USA agriculture and facilitate safe trade.

Molecular Characterization of 'Candidatus Phytoplasma prunorum' in the Czech Republic and Susceptibility of Apricot Rootstocks to the Two Most Abundant Haplotypes

'Candidatus Phytoplasma prunorum' is one of the most destructive pathogens of Prunus species, where susceptible species render unproductive several years after infection. In epidemiology, the molecular characterization of phytoplasmas is based on sequence analysis of variable nonribosomal genes. In this study aceF, pnp, imp and secY genes were used for characterization of the 'Ca. P. prunorum' genotypes present in the Czech Republic. In total, 56 plant and 33 vector (Cacopsylla pruni) samples positive to 'Ca. P. prunorum' collected in seven localities were used in the study. Based on sequence analysis, four aceF, two pnp, six imp, and three secY genotypes were identified in analyzed samples. The most abundant in both plant and insect samples were the A6, P2, I4, and S2 genotypes. Most of the Czech 'Ca. P. prunorum' haplotypes clustered together in the haplotype network analysis. Next, two isolates representing the two most abundant Czech haplotypes (A6-P2-I4-S2 and A5-P2-I4-S2) were used in the susceptibility test of three apricot rootstock types (St. Julien A, M-VA-1, GF-305). Susceptibility was analyzed by phytoplasma quantification using quantitative real-time PCR and evaluation of symptom manifestation. Based on the results, the influence of the rootstock type on the phytoplasma titer and symptom manifestation was greater than of the phytoplasma isolate, while the year of analysis had no influence on the results. The results also showed that the phytoplasma titer is increasing in plant tissues during the vegetation period.

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production-Research progress and outlook

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.

Identification of spectral ranges that contribute to phytoplasma detection in apple trees - A step towards an on-site method

'Candidatus Phytoplasma mali' is the bacterial agent associated with Apple Proliferation, a disease that causes high economic losses in affected commercial apple growing regions. The identification of the disease is carried out by visual inspection performed by skilled professionals in the orchards. To confirm an infection, costly molecular laboratory methods must be applied. Furthermore, both methods are very time-consuming. Here, we analysed the potential of a non-destructive method using in-field measurements to differentiate infected from non-infected apple trees (Malus domestica) based on spectral signatures of fresh leaves. By using multivariate statistics, we were able to distinguish infected from non-infected trees and identified the wavelengths relevant for the differentiation. Factors affecting the differentiation performance were the sampling date and bacterial colonization behaviour.

Clarithromycin Suppresses Apple Proliferation Phytoplasma in Explant Cultures

Apple proliferation, caused by 'Candidatus Phytoplasma mali', is one of the most important economic threats in the field of apple production. Especially at a young age, infected trees can be affected by excessive bud proliferation and general decline. The fruit quality is also significantly reduced by this disease. To investigate treatment options, we applied a clarithromycin chemotherapy to infected in vitro cultures of 'Golden Delicious'. With increasing concentrations of clarithromycin in the media, the phytoplasma load decreased rapidly after one month of treatment, but phytotoxicity led to a pronounced mortality at 40 mg/L, which was the highest dose used in our experiment. Out of 45 initial explants, we obtained one negative mericlone and two mericlones with a concentration of phytoplasma DNA at the detection limit of PCR. The culture propagated from the mericlone that tested negative remained phytoplasma-free after 18 months of subculturing. Our results suggest the applicability of macrolide antibiotics against phytoplasma infections in vitro; however, it might be challenging to find the threshold zone where the concentration is sufficient for pathogen elimination, but not lethal for the plant material of different cultivars.

A Decade of Hidden Phytoplasmas Unveiled Through Citizen Science

Climate change is impacting agriculture in many ways, and a contribution from all is required to reduce the imminent losses related to it. Recently, it has been shown that citizen science could be a way to trace the impact of climate change. However, how can citizen science be applied in plant pathology? Here, using as an example a decade of phytoplasma-related diseases reported by growers, agronomists, and citizens in general, and confirmed by a government laboratory, we explored how to better value plant pathogen monitoring data. Through this collaboration, we found that in the last decade, 34 hosts have been affected by phytoplasmas; 9, 13, and 5 of these plants were, for the first time, reported phytoplasma hosts in eastern Canada, all of Canada, and worldwide, respectively. Another finding of great impact is the first report of a 'Candidatus Phytoplasma phoenicium'-related strain in Canada, while 'Ca. P. pruni' and 'Ca. P. pyri' were reported for the first time in eastern Canada. These findings will have a great impact on the management of phytoplasmas and their insect vectors. Using these insect-vectored bacterial pathogens, we show the need for new strategies that can allow fast and accurate communication between concerned citizens and those institutions confirming their observations.[Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Multiple factors driving the acquisition efficiency of apple proliferation phytoplasma in Cacopsylla melanoneura

Phytoplasmas are bacterial pathogens located in the plant's phloem that are responsible for several plant diseases and are mainly transmitted by phloem-sucking insects. Apple proliferation (AP) is an economically important disease associated with the presence of 'Candidatus Phytoplasma mali' which is transmitted by two psyllid species. While Cacopsylla picta is a vector in different regions, the vector efficiency of C. melanoneura varies between different populations. This species is considered the main AP vector in Northwestern Italy but plays a minor role in Northeastern Italy and other European regions. To investigate whether the psyllid and/or the phytoplasma subtype drive the phytoplasma acquisition in C. melanoneura, a phytoplasma acquisition experiment was set up using single mating couples of overwintered individuals from different psyllid populations and phytoplasma subtypes. All analyzed insect populations acquired phytoplasma, but with different efficiencies and concentrations. The main factors driving the acquisition were the phytoplasma subtype and its concentration in the leaves of the infected trees together with the psyllid lineage. The phytoplasma concentration in the psyllids was again influenced by the phytoplasma subtype, the psyllid lineage and the region of origin, whereas the phytoplasma concentration in the leaves and the psyllid haplotype defined with the cytochrome oxidase I gene had only a minor impact on the phytoplasma concentration. This is the first study evaluating the roles of both the psyllid haplotype and the phytoplasma subtype on the acquisition process and highlights the importance of C. melanoneura as an additional AP vector.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10340-023-01699-1.

Seasonal wild dance of dual endosymbionts in the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea)

Most sap-feeding insects maintain obligate relationships with endosymbiotic bacteria that provide their hosts with essential nutrients. However, knowledge about the dynamics of endosymbiont titers across seasons in natural host populations is scarce. Here, we used quantitative PCR to investigate the seasonal dynamics of the dual endosymbionts "Candidatus Carsonella ruddii" and "Ca. Psyllophila symbiotica" in a natural population of the pear psyllid Cacopsylla pyricola (Hemiptera: Psylloidea: Psyllidae). Psyllid individuals were collected across an entire year, covering both summer and overwintering generations. Immatures harboured the highest titers of both endosymbionts, while the lowest endosymbiont density was observed in males. The density of Carsonella remained high and relatively stable across the vegetative period of the pear trees, but significantly dropped during the non-vegetative period, overlapping with C. pyricola's reproductive diapause. In contrast, the titer of Psyllophila was consistently higher than Carsonella's and exhibited fluctuations throughout the sampling year, which might be related to host age. Despite a tightly integrated metabolic complementarity between Carsonella and Psyllophila, our findings highlight differences in their density dynamics throughout the year, that might be linked to their metabolic roles at different life stages of the host.

Jujube witches' broom phytoplasmas inhibit ZjBRC1-mediated abscisic acid metabolism to induce shoot proliferation

Jujube witches' broom (JWB) phytoplasmas parasitize the sieve tubes of diseased phloem and cause an excessive proliferation of axillary shoots from dormant lateral buds to favour their transmission. In previous research, two JWB effectors, SJP1 and SJP2, were identified to induce lateral bud outgrowth by disrupting ZjBRC1-mediated auxin flux. However, the pathogenesis of JWB disease remains largely unknown. Here, tissue-specific transcriptional reprogramming was examined to gain insight into the genetic mechanisms acting inside jujube lateral buds under JWB phytoplasma infection. JWB phytoplasmas modulated a series of plant signalling networks involved in lateral bud development and defence, including auxin, abscisic acid (ABA), ethylene, jasmonic acid, and salicylic acid. JWB-induced bud outgrowth was accompanied by downregulation of ABA synthesis within lateral buds. ABA application rescued the bushy appearances of transgenic Arabidopsis overexpressing SJP1 and SJP2 in Col-0 and ZjBRC1 in the brc1-2 mutant. Furthermore, the expression of ZjBRC1 and ABA-related genes ZjHB40 and ZjNCED3 was negatively correlated with lateral main bud outgrowth in decapitated healthy jujube. Molecular evidence showed that ZjBRC1 interacted with ZjBRC2 via its N-terminus to activate ZjHB40 and ZjNCED3 expression and ABA accumulation in transgenic jujube calli. In addition, ZjBRC1 widely regulated differentially expressed genes related to ABA homeostasis and ABA signalling, especially by binding to and suppressing ABA receptors. Therefore, these results suggest that JWB phytoplasmas hijack the ZjBRC1-mediated ABA pathways to stimulate lateral bud outgrowth and expansion, providing a strategy to engineer plants resistant to JWB phytoplasma disease and regulate woody plant architecture to promote crop yield and quality.

Herbivore-Dependent Induced Volatiles in Pear Plants Cause Differential Attractive Response by Lacewing Larvae

Biological control may benefit from the behavioral manipulation of natural enemies using volatile organic compounds (VOCs). Among these, herbivore-induced plant volatiles (HIPVs) provide potential tools for attracting or retaining predators and parasitoids of insect pests. This work aimed to characterize the VOCs emitted by pear plants in response to attack by Cacopsylla bidens (Hemiptera: Psyllidae), a major pest in pear orchards, to compare these with VOCs induced by a leaf chewing insect, Argyrotaenia sphaleropa (Lepidoptera: Tortricidae), and to evaluate the behavioral response of Chrysoperla externa (Neuroptera: Chrysopidae) to HIPVs from pear plants damaged by either herbivore. The results demonstrated that plants damaged by the pear psylla emitted VOC blends with increased amounts of aliphatic aldehydes. Leafroller damage resulted in increased amounts of benzeneacetonitrile, (E)-4,8-dimethylnona-1,3,7-triene, β-ocimene and caryophyllene. In olfactometer bioassays, larvae of C. externa were attracted to herbivore-damaged plants when contrasted with undamaged plants. When plant odors from psylla-damaged were contrasted with those of leafroller-damaged plants, C.externa preferred the former, also showing shorter response lag-times and higher response rates when psylla-damaged plants were present. Our results suggest that pear plants respond to herbivory by modifying their volatile profile, and that psylla-induced volatiles may be used as prey-specific chemical cues by chrysopid larvae. Our study is the first to report HIPVs in pear plants attacked by C. bidens, as well as the attraction of C. externa to psyllid-induced volatiles.

From sequences to species: Charting the phytoplasma classification and taxonomy in the era of taxogenomics

Phytoplasma taxonomy has been a topic of discussion for the last two and half decades. Since the Japanese scientists discovered the phytoplasma bodies in 1967, the phytoplasma taxonomy was limited to disease symptomology for a long time. The advances in DNA-based markers and sequencing improved phytoplasma classification. In 2004, the International Research Programme on Comparative Mycoplasmology (IRPCM)- Phytoplasma/Spiroplasma Working Team – Phytoplasma taxonomy group provided the description of the provisional genus ‘Candidatus Phytoplasma’ with guidelines to describe the new provisional phytoplasma species. The unintentional consequences of these guidelines led to the description of many phytoplasma species where species characterization was restricted to a partial sequence of the 16S rRNA gene alone. Additionally, the lack of a complete set of housekeeping gene sequences or genome sequences, as well as the heterogeneity among closely related phytoplasmas limited the development of a comprehensive Multi-Locus Sequence Typing (MLST) system. To address these issues, researchers tried deducing the definition of phytoplasma species using phytoplasmas genome sequences and the average nucleotide identity (ANI). In another attempts, a new phytoplasma species were described based on the Overall Genome relatedness Values (OGRI) values fetched from the genome sequences. These studies align with the attempts to standardize the classification and nomenclature of ‘Candidatus’ bacteria. With a brief historical account of phytoplasma taxonomy and recent developments, this review highlights the current issues and provides recommendations for a comprehensive system for phytoplasma taxonomy until phytoplasma retains ‘Candidatus’ status.

The 'Candidatus Phytoplasma mali' effector protein SAP11CaPm interacts with MdTCP16, a class II CYC/TB1 transcription factor that is highly expressed during phytoplasma infection

'Candidatus Phytoplasma mali', is a bacterial pathogen associated with the so-called apple proliferation disease in Malus × domestica. The pathogen manipulates its host with a set of effector proteins, among them SAP11CaPm, which shares similarity to SAP11AYWB from 'Candidatus Phytoplasma asteris'. SAP11AYWB interacts and destabilizes the class II CIN transcription factors of Arabidopsis thaliana, namely AtTCP4 and AtTCP13 as well as the class II CYC/TB1 transcription factor AtTCP18, also known as BRANCHED1 being an important factor for shoot branching. It has been shown that SAP11CaPm interacts with the Malus × domestica orthologues of AtTCP4 (MdTCP25) and AtTCP13 (MdTCP24), but an interaction with MdTCP16, the orthologue of AtTCP18, has never been proven. The aim of this study was to investigate this potential interaction and close a knowledge gap regarding the function of SAP11CaPm. A Yeast two-hybrid test and Bimolecular Fluorescence Complementation in planta revealed that SAP11CaPm interacts with MdTCP16. MdTCP16 is known to play a role in the control of the seasonal growth of perennial plants and an increase of MdTCP16 gene expression has been detected in apple leaves in autumn. In addition to this, MdTCP16 is highly expressed during phytoplasma infection. Binding of MdTCP16 by SAP11CaPm might lead to the induction of shoot proliferation and early bud break, both of which are characteristic symptoms of apple proliferation disease.

Study of genetic modifications of flower development and methylation status in phytoplasma infected Brassica (Brassica rapa L.)

BACKGROUND

The plants of B. rapa (syn. B. campestris) are the most important food crop of Pakistan for the production of cooking oil. Brassica plants infected by phytoplasma exhibit floral abnormalities including phyllody, virescence, hypertrophied sepal and aborted reproductive organs and affected flower developmental genes which reduces the yield manifold.

METHODS AND RESULTS

The expression level of flower developmental genes in healthy and phytoplasma infected brassica were compared by using semi-quantitative reverse transcription polymerase chain reaction and DNA hybridization. In infected brassica, LEAFY (LFY) gene, controlling the development and maintenance of floral organ, and directly involved in controlling the homeotic gene expression was affected, while APETALA2, regulate the production of sepals and petals, were not altered. Whereas the genes WUSCHEL, APETALA3 and AGAMOUS, were significantly down-regulated, that were responsible for the identity of shoot and central meristem, petals and stamens production, and stamens and carpels development, respectively. The GLUB gene, controlling the production of β-1,3-glucanases enzyme, was highly up-regulated. According to DNA hybridization results, AGAMOUS and APETALA3 were restricted to floral organs territories in healthy and phytoplasma infected brassica, indicating that their expression was tissue-specific. These outcomes indicated that flower abnormalities of phytoplasma infected B. rapa are linked with DNA methylation in the expression of homeotic genes regulating flower development.

CONCLUSIONS

Azacitidine act as a DNA demethylating reagent. By applying the foliar spray of azacitidine during the flower development, cells of Phytoplasma infected plants exhibits demethylation of DNA when treated with azacitidine chemical that incorporated as analogue of cytosine during the cell division stage. B. rapa showed the up-regulation of gene expression level significantly that restore the normal production of flowers, ultimately increase the oil production throughout the world.

Investigating the microbial community of Cacopsylla spp. as potential factor in vector competence of phytoplasma

Phytoplasmas are obligatory intracellular bacteria that colonize the phloem of many plant species and cause hundreds of plant diseases worldwide. In nature, phytoplasmas are primarily transmitted by hemipteran vectors. While all phloem-feeding insects could in principle transmit phytoplasmas, only a limited number of species have been confirmed as vectors. Knowledge about factors that might determine the vector capacity is currently scarce. Here, we characterized the microbiomes of vector and non-vector species of apple proliferation (AP) phytoplasma 'Candidatus Phytoplasma mali' to investigate their potential role in the vector capacity of the host. We performed high-throughput 16S rRNA metabarcoding of the two principal AP-vectors Cacopsylla picta and Cacopsylla melanoneura and eight Cacopsylla species, which are not AP-vectors but co-occur in apple orchards. The microbiomes of all species are dominated by Carsonella, the primary endosymbiont of psyllids and a second uncharacterized Enterobacteriaceae endosymbiont. Each Cacopsylla species harboured a species-specific phylotype of both symbionts. Moreover, we investigated differences between the microbiomes of AP-vector versus non-vector species and identified the predominant endosymbionts but also Wolbachia and several minor taxa as potential indicator species. Our study highlights the importance of considering the microbiome in future investigations of potential factors influencing host vector competence. We investigated the potential role of symbiotic bacteria in the acquisition and transmission of phytoplasma. By comparing the two main psyillid vector species of Apple proliferation (AP) phytoplasma and eight co-occurring species, which are not able to vector AP-phytoplasma, we found differences in the microbial communities of AP-vector and non-vector species, which appear to be driven by the predominant symbionts in both vector species and Wolbachia and several minor taxa in the non-vector species. In contrast, infection with AP-phytoplasma did not affect microbiome composition in both vector species. Our study provides new insights into the endosymbiont diversity of Cacopsylla spp. and highlights the importance of considering the microbiome when investigating potential factors influencing host vector competence.

Repellent Activity of Clove Essential Oil Volatiles and Development of Nanofiber-Based Dispensers against Pear Psyllids (Hemiptera: Psyllidae)

Pear psyllids are the main vectors of the pathogen 'Candidatus Phytoplasma pyri' causing pear decline. Based on earlier reports, we tested the behavioral activity of the major synthetic compounds of clove essential oil (eugenol, eugenyl acetate, and β-caryophyllene) against Cacopsylla pyri and C. pyricola. Of six mixtures tested in olfactometer assays, a formulation consisting of three specific compounds (M6 mixture) demonstrated a repellent effect on both psyllid species. In addition, this formulation masked the odor of the host Pyrus communis cv. Williams Christ, disturbing the host finding ability of C. pyri. Electrospun fibers were produced with biocompatible polymers poly(ε-caprolactone), cellulose acetate, and solvents formic acid and acetic acid, loaded with the repellent mixture to test their efficacy as dispensers of repellents in laboratory and field. The fibers produced were repellent to C. pyri and effectively masked the odors of pear plants in olfactometer tests. In a pear orchard, we compared the captures of pear psyllids in green-colored attractive traps treated with nanofibers loaded with M6 mixture or unloaded nanofibers (blank). The result showed no differences in the captures of C. pyri between treatments. The release rates of volatiles from the fibers were evaluated weekly over 56 days. The fibers were able to entrap the major compound of the M6 mixture, eugenol, but the release rates were significantly reduced after 21 days. Our results suggest that biodegradable dispensers could be produced with electrospinning, but further improvements are necessary to use repellents as a management tool for pear psyllids in the field.

Phytoplasma Taxonomy: Nomenclature, Classification, and Identification

Phytoplasmas are pleomorphic, wall-less intracellular bacteria that can cause devastating diseases in a wide variety of plant species. Rapid diagnosis and precise identification of phytoplasmas responsible for emerging plant diseases are crucial to preventing further spread of the diseases and reducing economic losses. Phytoplasma taxonomy (identification, nomenclature, and classification) has lagged in comparison to culturable bacteria, largely due to lack of axenic phytoplasma culture and consequent inaccessibility of phenotypic characteristics. However, the rapid expansion of molecular techniques and the advent of high throughput genome sequencing have tremendously enhanced the nucleotide sequence-based phytoplasma taxonomy. In this article, the key events and milestones that shaped the current phytoplasma taxonomy are highlighted. In addition, the distinctions and relatedness of two parallel systems of 'Candidatus phytoplasma' species/nomenclature system and group/subgroup classification system are clarified. Both systems are indispensable as they serve different purposes. Furthermore, some hot button issues in phytoplasma nomenclature are also discussed, especially those pertinent to the implementation of newly revised guidelines for 'Candidatus Phytoplasma' species description. To conclude, the challenges and future perspectives of phytoplasma taxonomy are briefly outlined.

Spatio-temporal variation in the root-associated microbiota of orchard-grown apple trees

BACKGROUND

The root-associated microbiome has been of keen research interest especially in the last decade due to the large potential for increasing overall plant performance in agricultural systems. Studies about spatio-temporal variation of the root-associated microbiome focused so far primarily on community-compositional changes of annual plants, while little is known about their perennial counterparts. The aim of this work was to get deep insight into the spatial patterns and temporal dynamics of the root associated microbiota of apple trees.

RESULTS

The bacterial community structure in rhizospheric soil and endospheric root material from orchard-grown apple trees was characterized based on 16S rRNA gene amplicon sequencing. At the small scale, the rhizosphere and endosphere bacterial communities shifted gradually with increasing root size diameter (PERMANOVA R²-values up to 0.359). At the larger scale, bulk soil heterogeneity introduced variation between tree individuals, especially in the rhizosphere microbiota, while the presence of a root pathogen was contributing to tree-to-tree variation in the endosphere microbiota. Moreover, the communities of both compartments underwent seasonal changes and displayed year-to-year variation (PERMANOVA R²-values of 0.454 and 0.371, respectively).

CONCLUSIONS

The apple tree root-associated microbiota can be spatially heterogeneous at field scale due to soil heterogeneities, which particularly influence the microbiota in the rhizosphere soil, resulting in tree-to-tree variation. The presence of pathogens can contribute to this variation, though primarily in the endosphere microbiota. Smaller-scale spatial heterogeneity is observed in the rhizosphere and endosphere microbiota related to root diameter, likely influenced by root traits and processes such as rhizodeposition. The microbiota is also subject to temporal variation, including seasonal effects and annual variation. As a consequence, responses of the tree root microbiota to further environmental cues should be considered in the context of this spatio-temporal variation.

Effects of Acibenzolar-S-methyl on the Probing Behaviour and Mortality of Cacopsylla pyri on Pear Plants

Simple Summary

Acibenzolar-S-methyl is an analogue of salicylic acid, and it is known as a plant elicitor able to induce plant endogenous defences against plant pathogens. Recently, it has been shown to also affect phloem-feeder pests, even though the mechanism is still unclear. Pear psylla (Cacopsylla pyri) is a serious threat for pear production in Europe, and its control is usually based on the use of chemical insecticides. The development of novel innovative control approaches is becoming more and more important, especially in integrated pest management. The present work investigated the possible indirect influence of acibenzolar-S-methyl, through the expression of pear Pathogenesis-Related protein (PR) coding genes, on the probing behaviour and on the survival of C. pyri nymphs and adults feeding on pear potted plants. The minor effects observed on the pest would suggest that acibenzolar-S-methyl cannot be used against psyllas, but it might be recommended on pear orchards in the primary control of other targets such as fire blight disease.

Abstract

European pear psylla, Cacopsylla pyri, is one of the worst pests of pear in Europe. We investigated whether acibenzolar-S-methyl (ASM) application on pear plants might affect the behaviour in C. pyri. The elicitor was applied on pear potted plants, and after 48 h, we confirmed the ASM-mediated induction of several Pathogenesis-Related protein (PR) coding genes. At the same time, an in-depth analysis was performed on the probing behaviour of adults and nymphs of C. pyri on ASM-treated pear plants by the EPG-DC system, as well as the assessment of young nymphs’ survival 7 days after the ASM application. The elicitor application weakly interfered with C. pyri nymphs probing behaviour and survival, while it did not affect adult stages. These data confirm previous observations obtained on C. pyricola and suggest that the elicitor does not represent a viable tool in the control of pear psylla species, especially if used alone, but it might be used in integrated management strategies focused on other plant pathogens such as Erwinia amylovora.

Plants and Phytoplasmas: When Bacteria Modify Plants

Plant pathogen presence is very dangerous for agricultural ecosystems and causes huge economic losses. Phytoplasmas are insect-transmitted wall-less bacteria living in plants, only in the phloem tissues and in the emolymph of their insect vectors. They are able to manipulate several metabolic pathways of their hosts, very often without impairing their life. The molecular diversity described (49 'Candidatus Phytoplasma' species and about 300 ribosomal subgroups) is only in some cases related to their associated symptomatology. As for the other plant pathogens, it is necessary to verify their identity and recognize the symptoms associated with their presence to appropriately manage the diseases. However, the never-ending mechanism of patho-adaptation and the copresence of other pathogens makes this management difficult. Reducing the huge impact of phytoplasma-associated diseases in all the main crops and wild species is, however, relevant, in order to reduce their effects that are jeopardizing plant biodiversity.

Revision of the 'Candidatus Phytoplasma' species description guidelines

The genus 'Candidatus Phytoplasma' was proposed to accommodate cell wall-less bacteria that are molecularly and biochemically incompletely characterized, and colonize plant phloem and insect vector tissues. This provisional classification is highly relevant due to its application in epidemiological and ecological studies, mainly aimed at keeping the severe phytoplasma plant diseases under control worldwide. Given the increasing discovery of molecular diversity within the genus 'Ca. Phytoplasma', the proposed guidelines were revised and clarified to accommodate those 'Ca. Phytoplasma' species strains sharing >98.65 % sequence identity of their full or nearly full 16S rRNA gene sequences, obtained with at least twofold coverage of the sequence, compared with those of the reference strain of such species. Strains sharing <98.65 % sequence identity with the reference strain but >98.65 % with other strain(s) within the same 'Ca. Phytoplasma' species should be considered related strains to that 'Ca. Phytoplasma' species. The guidelines herein, keep the original published reference strains. However, to improve 'Ca. Phytoplasma' species assignment, complementary strains are suggested as an alternative to the reference strains. This will be implemented when only a partial 16S rRNA gene and/or a few other genes have been sequenced, or the strain is no longer available for further molecular characterization. Lists of 'Ca. Phytoplasma' species and alternative reference strains described are reported. For new 'Ca. Phytoplasma' species that will be assigned with identity ≥98.65 % of their 16S rRNA gene sequences, a threshold of 95 % genome-wide average nucleotide identity is suggested. When the whole genome sequences are unavailable, two among conserved housekeeping genes could be used. There are 49 officially published 'Candidatus Phytoplasma' species, including 'Ca. P. cocostanzaniae' and 'Ca. P. palmae' described in this manuscript.

Encapsulation of the psyllid-pathogenic fungus Pandora sp. nov. inedit. and experimental infection of target insects

BACKGROUND

Pandora sp. nov. inedit. (Entomophthorales: Entomophthoraceae) (ARSEF 13372) is a recently isolated entomophthoralean fungus with potential for psyllid pest control. This study aimed to develop a formulation based on biocompatible hydrogel beads in order to transfer the fungus into an easily applicable form and to test the effects on psyllids.

RESULTS

After encapsulation in calcium alginate beads, Pandora sp. nov. grew from the beads and discharged conidia over 12 days under optimal humidity conditions at 18 °C. Conidial number was increased 2.95-fold by the addition of skimmed milk as nutritional formulation adjuvant to the beads. The virulence of the encapsulated fungus was assessed with the two target psyllid species; the summer apple psyllid, Cacopsylla picta and the pear psyllid, Cacopsylla pyri. Beads containing skimmed milk as nutritional adjuvant led to the highest mortalities (48.3% on C. picta and 75.0% on C. pyri). In a second bioassay, survival time of C. pyri exposed to beads containing different concentration (10%, 20% or 40%) of Pandora sp. nov. was tested. The survival time of C. pyri was significantly reduced when exposed to beads containing 10% or 20% Pandora sp. nov. The median survival time was reached after 5-6 days past inoculation and the cumulative mortality for C. pyri treated with Pandora sp. nov. beads showed up to 89% mortality.

CONCLUSION

The promising results of this study will ease the way for large-scale field application of a novel Pandora species in biological psyllid pest control.

Physiological, biochemical, and molecular aspects of grafting in fruit trees

Grafting is a widely used practice for asexual propagation of fruit trees. Many physiological, biochemical, and molecular changes occur upon grafting that can influence important horticultural traits. This technology has many advantages, including avoidance of juvenility, modifying the scion architecture, improving productivity, adapting scion cultivars to unfavourable environmental conditions, and developing traits in resistance to insect pests, bacterial and fungal diseases. A limitation of grafting is scion-rootstock incompatibility. It may be caused by many factors, including insufficient genetic proximity, physiological or biochemical factors, lignification at the graft union, poor graft architecture, insufficient cell recognition between union tissues, and metabolic differences in the scion and the rootstock. Plant hormones, like auxin, ethylene (ET), cytokinin (CK), gibberellin (GA), abscisic acid (ABA), and jasmonic acid (JA) orchestrate several crucial physiological and biochemical processes happening at the site of the graft union. Additionally, epigenetic changes at the union affect chromatin architecture by DNA methylation, histone modification, and the action of small RNA molecules. The mechanism triggering these effects likely is affected by hormonal crosstalk, protein and small molecules movement, nutrients uptake, and transport in the grafted trees. This review provides an overview of the basis of physiological, biochemical, and molecular aspects of fruit tree grafting between scion and rootstock.

Vector transmission and epidemiology of 'Candidatus Phytoplasma pyri' in Austria and identification of Cacopsylla pyrisuga as new pathogen vector

Pear decline, induced by the phytoplasma 'Candidatus Phytoplasma pyri', transmitted by pear psyllids, is one of the most devastating diseases on Pyrus communis in Europe and North America. Investigations of pear psyllids in 4 pear orchards in lower Austria showed the presence of Cacopsylla pyri, C. pyricola and C. pyrisuga at all locations. PCR analyses revealed overall phytoplasma infection rates for C. pyri of 5.4%, for C. pyricola, of 4.6%, for C. pyrisuga remigrants of 9.6% and for C. pyrisuga emigrants of 0%. The rates of PCR-positive C. pyri and C. pyricola individuals varied greatly in the course of the year, and the highest infection rates were observed in late summer, autumn and in late winter. In transmission experiments with healthy pear seedlings, winterform individuals of C. pyri and C. pyricola transmitted the pathogen to 19.2% (5 out of 26) and 4.8% (2 out of 41) of the test plants, respectively. The vectoring ability of C. pyrisuga was experimentally proven for the first time, and in transmission experiments with remigrants, 9.5% (2 out of 21) of the pear seedlings were infected. Our data indicate a significant risk of pathogen transmission in pear orchards during the greater part of the year, especially in late winter, early spring and autumn. Multilocus sequence analysis by aid of the genes aceF and imp allowed the discrimination between 15 phytoplasma types. Three so far undescribed aceF genotypes and four undescribed imp genotypes were identified.

Supplementary Information

The online version contains supplementary material available at 10.1007/s41348-021-00526-y.

Detection of apple proliferation disease in Malus × domestica by near infrared reflectance analysis of leaves

In this study near infrared spectroscopical analysis of dried and ground leaves was performed and combined with a multivariate data analysis to distinguish 'Candidatus Phytoplasma mali' infected from non-infected apple trees (Malus × domestica). The bacterium is the causative agent of Apple Proliferation, one of the most threatening diseases in commercial apple growing regions. In a two-year study, leaves were sampled from three apple orchards, at different sampling events throughout the vegetation period. The spectral data were analyzed with a principal component analysis and classification models were developed. The model performance for the differentiation of Apple Proliferation diseased from non-infected trees increased throughout the vegetation period and gained best results in autumn. Even with asymptomatic leaves from infected trees a correct classification was possible indicating that the spectral-based method provides reliable results even if samples without visible symptoms are analyzed. The wavelength regions that contributed to the differentiation of infected and non-infected trees could be mainly assigned to a reduction of carbohydrates and N-containing organic compounds. Wet chemical analyses confirmed that N-containing compounds are reduced in leaves from infected trees. The results of our study provide a valuable indication that spectral analysis is a promising technique for Apple Proliferation detection in future smart farming approaches.

Pathogenicity against hemipteran vector insects of a novel insect pathogenic fungus from Entomophthorales (Pandora sp. nov. inedit.) with potential for biological control

A new but still unpublished entomopathogenic fungus (ARSEF13372) in the genus Pandora (Entomophthorales: Entomophthoraceae) was originally isolated from Cacopsylla sp. (Hemiptera: Psyllidae). Several species of the genus Cacopsylla vector phloem-borne bacteria of the genus 'Candidatus Phytoplasma', which cause diseases in fruit crops such as apple proliferation, pear decline and European stone fruit yellows. To determine Pandora's host range and biocontrol potential we conducted laboratory infection bioassays; Hemipteran phloem-feeding insects were exposed to conidia actively discharged from in vitro produced mycelial mats of standardized area. We documented the pathogenicity of Pandora sp. nov. to species of the insect families Psyllidae and Triozidae, namely Cacopsyllapyri L., C.pyricola (Foerster), C.picta (Foerster, 1848), C.pruni (Scopoli), C.peregrina (Foerster), and Trioza apicalis Foerster. The occurrence of postmortem signs of infection on cadavers within 10 days post inoculation proved that Pandora sp. nov. was infective to the tested insect species under laboratory conditions and significantly reduced mean survival time for C.pyri (summer form and nymph), C.pyricola, C.picta, C.pruni, C.peregrina and T.apicalis. Assessing a potential interaction between phytoplasma, fungus and insect host revealed that phytoplasma infection ('Candidatus Phytoplasma mali') of the vector C.picta and/or its host plant apple Malus domestica Borkh. did not significantly impact the survival of C.picta after Pandora sp. nov. infection. The results from infection bioassays were discussed in relation to Pandora sp. nov. host range and its suitability as biocontrol agent in integrated pest management strategies of psyllid pests, including vector species, in orchards.

Influence of ontogenetic and migration stage on feeding behavior of Cacopsylla picta on 'Candidatus Phytoplasma mali' infected and non-infected apple plants

The summer apple psyllid Cacopsylla picta (Foerster) is the vector of 'Candidatus Phytoplasma mali', the causal agent of apple proliferation disease (AP). During its phloem-feeding activities it transmits this biotrophic bacterium from infected to healthy apple trees (Malus domestica Borkh.) causing high economic losses. During its life cycle, C.picta performs two host switches: In summer, the new adult generation (emigrants) hatch on apples before they emigrate to their overwintering host conifers. The following spring, the overwintered adult generation (remigrants) remigrate into apple orchards for mating and oviposition. The preimaginal stages (nymphs) develop on apple. It is known that phytopathogen-induced changes in plant physiology can affect insect-plant-interactions. In 12 h recordings of electrical penetration graphs (EPG) it was assessed whether 'Ca. P. mali' infection of the plant affected probing and feeding behavior of the vector C.picta. Its life stage and the infection status of the host plant (and the interaction between these factors) significantly affected the first occurrence, duration and frequency of probing and feeding phases. On 'Ca. P. mali' infected plants, the phloem salivation phase occurred later than on non-infected plants. Even though all life stages fed both on phloem and xylem, significant differences were found in the frequency and duration of phloem and xylem ingestion phases. Nymphs spent the shortest time non-probing, earlier started the first leaf penetration and longer ingested xylem compared with adults. Further, phloem phases differed between migratory stages; remigrants had higher numbers of phloem ingestion events and spent longer ingesting phloem than emigrants. For emigrants, however, phloem contact was very rarely observed during our recordings. The impact of our findings for understanding the multitrophic interactions between host plant, pathogen and behavior of vector insects are discussed with regard to the epidemiology of AP and pest control strategies of the vector.

Esterase, Glutathione S-Transferase and NADPH-Cytochrome P450 Reductase Activity Evaluation in Cacopsylla pyri L. (Hemiptera: Psyllidae) Individual Adults

Cacopsylla pyri (L.) (Hemiptera: Psyllidae) is a key pest of pear orchards in Spain. The large number of insecticide treatments necessary for control may be an important contributor to the emergence of resistance. Laboratory toxicity and biochemical assays are necessary to validate the existence of insecticide resistance and establish the underlying mechanisms. All the methodologies developed to evaluate enzyme activity in C. pyri to date have incorporated "pools" of adults to detect minimum activity ranges. In this study, we determined the optimal working conditions for evaluation of the activities of esterase, glutathione S-transferase and NADPH-cytochrome P450 reductase in individual insects via colorimetric methods using a microplate reader. The main factors affecting enzymatic analysis activity, such as enzyme source and substrate concentration, filter wavelength, buffer pH, reaction time and additives, were evaluated for optimization. Determining the frequency of resistant individuals within a population could be used as an indicator for the evolution of insecticide resistance over time. Two laboratory strains, one of them selected with cypermethrin, and two field populations were analyzed for this purpose. The data obtained revealed high values and great variation in the activity ranges of esterase (EST) in the insecticide-selected population as well as in the field populations validating the applied methodology.

History and Current Status of Phytoplasma Diseases in the Middle East

Phytoplasmas that are associated with fruit crops, vegetables, cereal and oilseed crops, trees, ornamental, and weeds are increasing at an alarming rate in the Middle East. Up to now, fourteen 16Sr groups of phytoplasma have been identified in association with more than 164 plant species in this region. Peanut witches' broom phytoplasma strains (16SrII) are the prevalent group, especially in the south of Iran and Gulf states, and have been found to be associated with 81 host plant species. In addition, phytoplasmas belonging to the 16SrVI, 16SrIX, and 16SrXII groups have been frequently reported from a wide range of crops. On the other hand, phytoplasmas belonging to 16SrIV, 16SrV, 16SrX, 16SrXI, 16SrXIV, and 16SrXXIX groups have limited geographical distribution and host range. Twenty-two insect vectors have been reported as putative phytoplasma vectors in the Middle East, of which Orosius albicinctus can transmit diverse phytoplasma strains. Almond witches' broom, tomato big bud, lime witches' broom, and alfalfa witches' broom are known as the most destructive diseases. The review summarizes phytoplasma diseases in the Middle East, with specific emphasis on the occurrence, host range, and transmission of the most common phytoplasma groups.

Specialized 16SrX phytoplasmas induce diverse morphological and physiological changes in their respective fruit crops

The host-pathogen combinations-Malus domestica (apple)/`Candidatus Phytoplasma mali´, Prunus persica (peach)/`Ca. P. prunorum´ and Pyrus communis (pear)/`Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within weeks to few years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

The agent associated with blue dwarf disease in wheat represents a new phytoplasma taxon, 'Candidatus Phytoplasma tritici'

Wheat blue dwarf (WBD) is one of the most economically damaging cereal crop diseases in northwestern PR China. The agent associated with the WBD disease is a phytoplasma affiliated with the aster yellows (AY) group, subgroup C (16SrI-C). Since phytoplasma strains within the AY group are ecologically and genetically diverse, it has been conceived that the AY phytoplasma group may consist of more than one species. This communication presents evidence to demonstrate that, while each of the two 16 rRNA genes of the WBD phytoplasma shares >97.5 % sequence similarity with that of the 'Candidatus Phytoplasma asteris' reference strain, the WBD phytoplasma clearly represents an ecologically separated lineage: the WBD phytoplasma not only has its unique transmitting vector (Psammotettix striatus) but also elicits a distinctive symptom in its predominant plant host (wheat). In addition, the WBD phytoplasma possesses molecular characteristics that further manifest its significant divergence from 'Ca. P. asteris'. Such molecular characteristics include lineage-specific antigenic membrane proteins and a lower than 95 % genome-wide average nucleotide identity score with 'Ca. P. asteris'. These ecological, molecular and genomic evidences justify the recognition of the WBD phytoplasma as a novel taxon, 'Candidatus Phytoplasma tritici'.

'Candidatus Phytoplasma stylosanthis', a novel taxon with a diverse host range in Australia, characterised using multilocus sequence analysis of 16S rRNA, secA, tuf, and rp genes

In Australia, Stylosanthes little leaf (StLL) phytoplasma has been detected in Stylosanthes scabra Vogel, Arachis pintoi Krapov, Saccharum officinarum L., Carica papaya L., Medicago sativa L., and Solanum tuberosum L. The 16S rRNA gene sequence of StLL phytoplasma strains from S. scabra, C. papaya, S. officinarum and S. tuberosum were compared and share 99.93-100 % nucleotide sequence identity. Phylogenetic comparisons between the 16S rRNA genes of StLL phytoplasma and other 'Candidatus Phytoplasma' species indicate that StLL represents a distinct phytoplasma lineage. It shares its most recent known ancestry with 'Ca. Phytoplasma luffae' (16SrVIII-A), with which it has 97.17-97.25 % nucleotide identity. In silico RFLP analysis of the 16S rRNA amplicon using iPhyClassifier indicate that StLL phytoplasmas have a unique pattern (similarity coefficient below 0.85) that is most similar to that of 'Ca. Phytoplasma luffae'. The unique in silico RFLP patterns were confirmed in vitro. Nucleotide sequences of genes that are more variable than the 16S rRNA gene, namely tuf (tu-elongation factor), secA (partial translocation gene), and the partial ribosomal protein (rp) gene operon (rps19-rpl22-rps3), produced phylogenetic trees with similar branching patterns to the 16S rRNA gene tree. Sequence comparisons between the StLL 16S rRNA spacer region confirmed previous reports of rrn interoperon sequence heterogeneity for StLL, where the spacer region of rrnB encodes a complete tRNA-Isoleucine gene and the rrnA spacer region does not. Together these results suggest that the Australian phytoplasma, StLL, is unique according to the International Organization for Mycoplasmology (IRPCM) recommendations. The novel taxon 'Ca. Phytoplasma stylosanthis' is proposed, with the most recent strain from a potato crop in Victoria, Australia, serving as the reference strain (deposited in the Victorian Plant Pathology Herbarium as VPRI 43683).

List of non-EU phytoplasmas of tuber-forming Solanum spp

Following a request from the European Commission, the EFSA Panel on Plant Health prepared a list of non-EU phytoplasmas of tuber-forming Solanum spp. A systematic literature review and search of databases identified 12 phytoplasmas infecting S. tuberosum. These phytoplasmas were assigned to three categories. The first group (a) consists of seven non-EU phytoplasmas, known to occur only outside the EU ('Candidatus Phytoplasma americanum', 'Ca. P. australiense', 'Ca. P. fragariae'-related strain (YN-169, YN-10G) and 'Ca. P. hispanicum') or having only limited presence in the EU ('Ca. P. aurantifolia'-related strains, 'Ca. P. pruni'-related strains and 'Ca. P. trifolii'). The second group (b) consists of three phytoplasmas originally described or reported from the EU. The third group (c) consists of two phytoplasmas with substantial presence in the EU, whose presence in S. tuberosum is not fully supported by the available literature. Phytoplasmas of categories (b) and (c) were excluded at this stage from further categorisation efforts. Three phytoplasmas from category (a) ('Ca. P. australiense', 'Ca. P. hispanicum' and 'Ca. P. trifolii') were excluded from further categorisation, as a pest categorisation has already been performed by EFSA. Comments provided by the EU Member States were integrated in the opinion. The main uncertainties of this listing concern: the taxonomy, the geographic distribution and prevalence and host range. The following phytoplasmas considered as non-EU and whose presence in S. tuberosum is fully supported by literature (category (a)) are categorised by the Panel in a separate opinion: 'Ca. P. americanum', 'Ca. P. fragariae'-related strain (YN-169, YN-10G), 'Ca. P. aurantifolia'-related strains and 'Ca. P. pruni'-related strains.

Identification of Plant DNA in Adults of the Phytoplasma Vector Cacopsylla picta Helps Understanding Its Feeding Behavior

Simple Summary

Cacopsylla picta is an insect vector of apple proliferation phytoplasma, the causative bacterial agent of apple proliferation disease. In this study, we provide an answer to the open question of whether adult Cacopsylla picta feed from other plants than their known host, the apple plant. We collected Cacopsylla picta specimens from apple trees and analyzed the composition of plant DNA ingested by these insects. By applying a state-of-the art sequencing approach, we show, for the first time, that Cacopsylla picta feeds from a wide range of woody and herbaceous plant species. Our results are important for a better understanding of the biology and feeding behavior of Cacopsylla picta. Since this insect is an efficient vector of apple proliferation phytoplasma, our results are also important to define potential reservoir plants that might be involved in the transmissive cycle of this pathogen. This study thus provides important data of practical relevance.

Abstract

Apple proliferation is an economically important disease and a threat for commercial apple cultivation. The causative pathogen, the bacterium ‘Candidatus Phytoplasma mali’, is mainly transmitted by Cacopsylla picta, a phloem-feeding insect that develops on the apple tree (Malus spp.). To investigate the feeding behavior of adults of the phytoplasma vector Cacopsylla picta in more detail, we used deep sequencing technology to identify plant-specific DNA ingested by the insect. Adult psyllids were collected in different apple orchards in the Trentino-South Tyrol region of northern Italy. DNA from the whole body of the insect was extracted and analyzed for the presence of plant DNA by performing PCR with two plant-specific primers that target the chloroplast regions trnH-psbA and rbcLa. DNA from 23 plant genera (trnH) and four plant families (rbcLa) of woody and herbaceous plant taxa was detected. Up to six and three plant genera and families, respectively, could be determined in single specimens. The results of this study contribute to a better understanding of the feeding behavior of adult Cacopsylla picta.

The phytopathogen ‘Candidatus Phytoplasma mali’ alters apple tree phloem composition and affects oviposition behavior of its vector Cacopsylla picta

Apple proliferation disease is caused by the phloem-dwelling bacterium ‘Candidatus Phytoplasma mali’, inducing morphological changes in its host plant apple, such as witches’ broom formation. Furthermore, it triggers physiological alterations like emission of volatile organic compounds or phytohormone levels in the plant. In our study, we assessed phytoplasma-induced changes in the phloem by sampling phloem sap from infected and non-infected apple plants. In infected plants, the soluble sugar content increased and the composition of phloem metabolites differed significantly between non-infected and infected plants. Sugar and sugar alcohol levels increased in diseased plants, while organic and amino acid content remained constant. As ‘Ca. P. mali’ is vectored by the phloem-feeding insect Cacopsylla picta (Foerster, 1848), we assessed whether the insect–plant interaction was affected by ‘Ca. P. mali’ infection of the common host plant Malus domestica Borkh. Binary-choice oviposition bioassays between infected and non-infected apple leaves revealed C. picta’s preference for non-infected leaves. It is assumed and discussed that the changes in vector behavior are attributable to plant-mediated effects of the phytoplasma infection.

Organic Control of Pear Psylla in Pear with Trunk Injection

Simple Summary

Organic pear production is challenged, in part, by short lived effects of biopesticides when applied as foliar sprays. Trunk injection may enhance their performance by delivering the biopesticides directly to the vascular system of the tree, right where pear psylla feed. The objective of this study is to compare trunk injections to foliar sprayed applications of two insecticides, azadirachtin and abamectin, on their ability to control pear psylla in pear trees. The azadirachtin and abamectin trunk injected treatments performed equally or better than two foliar applications in the control of the pear psylla. The trunk injected trees from the first season provided a moderate level of control into the second season, one year after the injections. This study suggests that trunk injection is a superior delivery system for biopesticides used in organic pear production.

Abstract

Organic production of pears is challenging in part because OMRI (Organic Materials Review Institute) approved biopesticides are short lived when applied as foliar sprays. Trunk injection is an alternative method of insecticide delivery that may enhance the performance of biopesticides for control of pear psylla. The objective of this study is to compare the efficacy of azadirachtin and abamectin in the control of pear psylla using two different application methods, airblast sprayer and trunk injection. Trunk injections of azadirachtin and abamectin were compared to airblast applications of equal labeled rates on 33-year-old Bartlett Pear trees (Pyrus communis L., var “Bartlett”). The azadirachtin and abamectin trunk injected treatments performed equally or better than the two airblast applications in the control of the pear psylla. The trunk injected trees from the first season provided a moderate level of control into the second season, one year after the injections. This study suggests that trunk injection is a superior delivery system for biopesticides used in organic pear production.

Effect of Daytime and Tree Canopy Height on Sampling of Cacopsylla melanoneura, a 'Candidatus Phytoplasma mali' Vector

The psyllids Cacopsylla melanoneura and Cacopsylla picta reproduce on apple (Malus × domestica) and transmit the bacterium ‘Candidatus Phytoplasma mali’, the causative agent of apple proliferation. Adult psyllids were collected by the beating-tray method from lower and upper parts of the apple tree canopy in the morning and in the afternoon. There was a trend of catching more emigrant adults of C.melanoneura in the morning and in the lower part of the canopy. For C.melanoneura remigrants, no differences were observed. The findings regarding the distribution of adults were reflected by the number of nymphs collected by wash-down sampling. The density of C.picta was too low for a statistical analysis. The vector monitoring and how it is commonly performed, is suitable for estimating densities of C.melanoneura. Nevertheless, above a certain temperature threshold, prediction of C.melanoneura density might be skewed. No evidence was found that other relatively abundant psyllid species in the orchard, viz. Baeopelma colorata, Cacopsylla breviantennata, Cacopsylla brunneipennis, Cacopsylla pruni and Trioza urticae, were involved in ‘Candidatus Phytoplasma mali’ transmission. The results of our study contribute to an advanced understanding of insect vector behavior and thus have a practical impact for an improved field monitoring.

Lists of names of prokaryotic Candidatus taxa

We here present annotated lists of names of Candidatus taxa of prokaryotes with ranks between subspecies and class, proposed between the mid-1990s, when the provisional status of Candidatus taxa was first established, and the end of 2018. Where necessary, corrected names are proposed that comply with the current provisions of the International Code of Nomenclature of Prokaryotes and its Orthography appendix. These lists, as well as updated lists of newly published names of Candidatus taxa with additions and corrections to the current lists to be published periodically in the International Journal of Systematic and Evolutionary Microbiology, may serve as the basis for the valid publication of the Candidatus names if and when the current proposals to expand the type material for naming of prokaryotes to also include gene sequences of yet-uncultivated taxa is accepted by the International Committee on Systematics of Prokaryotes.

Multi-scale spatial genetic structure of the vector-borne pathogen 'Candidatus Phytoplasma prunorum' in orchards and in wild habitats

Inferring the dispersal processes of vector-borne plant pathogens is a great challenge because the plausible epidemiological scenarios often involve complex spread patterns at multiple scales. The spatial genetic structure of 'Candidatus Phytoplasma prunorum', responsible for European stone fruit yellows disease, was investigated by the application of a combination of statistical approaches to genotype data of the pathogen sampled from cultivated and wild compartments in three French Prunus-growing regions. This work revealed that the prevalence of the different genotypes is highly uneven both between regions and compartments. In addition, we identified a significant clustering of similar genotypes within a radius of 50 km or less, but not between nearby wild and cultivated Prunus. We also provide evidence that infected plants are transferred between production areas, and that both species of the Cacopsylla pruni complex can spread the pathogen. Altogether, this work supports a main epidemiological scenario where 'Ca. P. prunorum' is endemic in - and generally acquired from - wild Prunus by its immature psyllid vectors. The latter then migrate to shelter plants that epidemiologically connect sites less than 50 km apart by later providing infectious mature psyllids to their "migration basins". Such multi-scale studies could be useful for other pathosystems.

Pest categorisation of the non-EU phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L

Following a request from the European Commission, the EFSA Panel on Plant Health performed a pest categorisation of nine phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L. (hereafter "host plants") known to occur only outside the EU or having a limited presence in the EU. This opinion covers the (i) reference strains of 'Candidatus Phytoplasma australiense', 'Ca. P. fraxini', 'Ca. P. hispanicum', 'Ca. P. trifolii', 'Ca. P. ziziphi', (ii) related strains infecting the host plants of 'Ca. P. aurantifolia', 'Ca. P. pruni', and 'Ca. P. pyri', and (iii) an unclassified phytoplasma causing Buckland valley grapevine yellows. Phytoplasmas can be detected by available methods and are efficiently transmitted by vegetative propagation, with plants for planting acting as a major entry pathway and a long-distance spread mechanism. Phytoplasmas are also transmitted in a persistent and propagative manner by some insect families of the Fulgoromorpha, Cicadomorpha and Sternorrhyncha (order Hemiptera). No transovarial, pollen or seed transmission has been reported. The natural host range of the categorised phytoplasmas varies from one to more than 90 plant species, thus increasing the possible entry pathways. The host plants are widely cultivated in the EU. All the categorised phytoplasmas can enter and spread through the trade of host plants for planting, and by vectors. Establishment of these phytoplasmas is not expected to be limited by EU environmental conditions. The introduction of these phytoplasmas in the EU would have an economic impact. There are measures to reduce the risk of entry, establishment, spread and impact. Uncertainties result from limited information on distribution, biology and epidemiology. All the phytoplasmas categorised here meet the criteria evaluated by EFSA to qualify as potential Union quarantine pests, and they do not qualify as potential regulated non-quarantine pests, because they are non-EU phytoplasmas.

List of non-EU phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L

Following a request from the European Commission, the EFSA Panel on Plant Health prepared a list of non-EU phytoplasmas of Cydonia Mill., Fragaria L., Malus Mill., Prunus L., Pyrus L., Ribes L., Rubus L. and Vitis L. A systematic literature review and search of databases identified 27 phytoplasmas infecting one or more of the host genera under consideration. These phytoplasmas were assigned to three categories. The first group (a) consists of 10 non-EU phytoplasmas, known to occur only outside the EU ('Candidatus Phytoplasma australiense', 'Ca. P. hispanicum', 'Ca. P. pruni'-related strain (NAGYIII), 'Ca. P. pyri'-related strain (PYLR) and Buckland valley grapevine yellows phytoplasma) or having only limited presence in the EU ('Ca. P. aurantifolia'-related strains, 'Ca. P. fraxini', 'Ca. P. phoenicium', 'Ca. P. trifolii' and 'Ca. P. ziziphi'). The second group (b) consists of three non-EU phytoplasmas, whose presence in the target plant species is not fully supported by the available literature. The third group (c) consists of 14 phytoplasmas with substantial presence in the EU (i.e. they are originally described or reported from the EU or known to occur or be widespread in some EU Member States or frequently reported in the EU). Phytoplasmas of categories (b) and (c) were excluded at this stage from further categorisation efforts. One phytoplasma from category (a) ('Ca. P. phoenicium') was excluded from further categorisation, as a pest risk assessment has been performed by EPPO. Comments provided by the EU Member States were integrated in the opinion. The main uncertainties of this listing concern: the geographic distribution and prevalence, the taxonomy, biology and host range. The phytoplasmas considered as non-EU and whose presence in target plant species is fully supported by literature (category (a)) are categorised by the Panel in a separate opinion.

A LAMP Protocol for the Detection of 'Candidatus Phytoplasma pyri', the Causal Agent of Pear Decline

Phytoplasmas are cell-wall-less bacteria that cause diseases in approximately 1,000 plant species. 'Candidatus Phytoplasma pyri', the causal agent of pear decline, induces various symptoms on its hosts, leading to weakening and dieback of the plants, reduced fruit size and yield, and, consequently, considerable financial losses in all pear-growing areas. Fighting this disease requires a reliable and inexpensive method for pathogen detection in propagation material as well as plant stocks in orchards and breeding facilities. Here, we present a field-suitable detection protocol for 'Ca. P. pyri' based on loop-mediated isothermal amplification (LAMP) targeting the phytoplasmal 16S ribosomal DNA sequence. The combination of a simplified sample preparation method based on sodium hydroxide and colorimetric visualization of LAMP results enables a laboratory-independent pathogen detection. The detection limit is comparable with analysis by polymerase chain reaction; however, the pear decline LAMP detection method is superior in terms of ease of use, cost, and time effectiveness for obtaining results.

High-Throughput Sequencing Analysis of the Bacterial Community in Stone Fruit Phloem Tissues Infected by "Candidatus Phytoplasma prunorum"

"Candidatus Phytoplasma prunorum" (CPp) is a highly destructive phytopathogenic agent in many stone fruit-growing regions in Europe and the surrounding countries. In this work, we focused on documenting entire bacterial community in the phloem tissues of 60 stone fruit trees. Nested PCR and two real-time PCR assays were used to select CPp-positive (group A) and CPp-negative samples (group B). Afterwards, high-throughput amplicon sequencing was performed to assess bacterial community compositions in phloem tissues. The bacterial composition in phloem tissue consisted of 118 distinct genera, represented mainly by Pseudomonas, Acinetobacter, Methylobacterium, Sphingomonas, and Rhizobium. Statistics showed that CPp influenced the bacterial composition of infected plants (group A) and that the bacterial community depended on the geographical origin of the sample. This is the first work focusing on an analysis of the influence of CPp on the bacteria coexisting in the phloem tissues of stone fruit trees.

Molecular diversity of "Candidatus Phytoplasma mali" strains associated with apple proliferation disease in Bulgarian germplasm collection

A quarantine organism, "Candidatus Phytoplasma mali," is the causal agent of apple proliferation, one of the most important apple diseases in Europe. The genetic diversity of this pathogen in Central and Southern Europe has already been reported; however, almost no data exists from Eastern Europe. In this study, "Ca. P. mali" strains, which were identified in 14 apple trees from the Bulgarian germplasm collection, were characterized by restriction fragment length polymorphism (RFLP) and sequence analysis of four genomic loci. In total, nine distinct genetic lineages were recognized based on the combination of the following detected RFLP profiles: two profiles for the 16S-23S rDNA region (16SrX-A2, -A3), four profiles for the secY gene (one previously known: secY(X)-A, and three new: secY-C, secY-D, secY-E), three profiles for the rpl22-rps3 genes (rpX-A, rpX-B, rpX-F), and one profile for the nitroreductase- and rhodanese-like gene (AT-1). Phylogenetic analysis of the Bulgarian and other European "Ca. P. mali" strains based on 16S-23S rRNA gene sequences confirmed RFLP grouping, regardless of the phytoplasma origin. In a phylogenetic tree based on the secY data, only German strains formed separate clade from the other strains. The tree based on rp genes did not correspond to RFLP profiles. Unexpectedly, when using nitroreductase and rhodanese-like gene sequences, the Bulgarian strains clustered separately from the other European strains. Apart from the identification of different "Ca. P. mali" strains, the paper also recommends the unification of the rpX-subgroup nomenclature to avoid future confusions. Both aims of this paper provide valuable tools to understand the epidemiology of this quarantine pathogen.

Unraveling the Host Plant Alternation of Cacopsylla pruni - Adults but Not Nymphs Can Survive on Conifers Due to Phloem/Xylem Composition

Plant sap feeding insects like psyllids are known to be vectors of phloem dwelling bacteria ('Candidatus Phytoplasma' and 'Ca. Liberibacter'), plant pathogens which cause severe diseases and economically important crop damage. Some univoltine psyllid species have a particular life cycle, within one generation they alternate two times between different host plant species. The plum psyllid Cacopsylla pruni, the vector of European Stone Fruit Yellows (ESFY), one of the most serious pests in European fruit production, migrates to stone fruit orchards (Prunus spp.) for mating and oviposition in early spring. The young adults of the new generation leave the Prunus trees in summer and emigrate to their overwintering hosts like spruce and other conifers. Very little is known about the factors responsible for the regulation of migration, reasons for host alternation, and the behavior of psyllids during their phase of life on conifers. Because insect feeding behavior and host acceptance is driven by different biotic factors, such as olfactory and gustatory cues as well as mechanical barriers, we carried out electrical penetration graph (EPG) recordings and survival bioassays with C. pruni on different conifer species as potential overwintering hosts and analyzed the chemical composition of the respective plant saps. We are the first to show that migrating psyllids do feed on overwintering hosts and that nymphs are able to ingest phloem and xylem sap of coniferous trees, but cannot develop on conifer diet. Analyses of plant saps reveal qualitative differences in the chemical composition between coniferous trees and Prunus as well as within conifer species. These differences are discussed with regard to nutritional needs of psyllid nymphs for proper development, overwintering needs of adults and restriction of 'Ca. P. prunorum' to Prunus phloem.

The influence of bacteria on multitrophic interactions among plants, psyllids, and pathogen

The recent emergence of several plant diseases caused by psyllid-borne bacterial pathogens worldwide (Candidatus Liberibacter spp.) has created renewed interest on the interaction between psyllids and bacteria. In spite of these efforts to understand psyllid association with bacteria, many aspects of their interactions remain poorly understood. As more organisms are studied, subtleties on the molecular interactions as well as on the effects of the bacteria on the psyllid host are being uncovered. Additionally, psyllid-borne bacterial phytopathogens can also affect the host plant, which in turn can impact psyllid physiology and behavior. Here, we review the current literature on different aspects of the influence of bacteria on multitrophic interactions among plants, psyllids, and pathogens. We then highlight gaps that need to be addressed to advance this field, which can have significant implications for controlling these newly emergent and other plant diseases.

'Candidatus Phytoplasma wodyetiae', a new taxon associated with yellow decline disease of foxtail palm (Wodyetia bifurcata) in Malaysia

Landscape-grown foxtail palm (Wodyetia bifurcata A. K. Irvine) trees displaying symptoms of severe foliar chlorosis, stunting, general decline and mortality reminiscent of coconut yellow decline disease were observed in Bangi, Malaysia, during 2012. DNA samples from foliage tissues of 15 symptomatic palms were analysed by employing a nested PCR assay primed by phytoplasma universal ribosomal RNA operon primer pairs, P1/P7 followed by R16F2n/R2. The assay yielded amplicons of a single band of 1.25 kb from DNA samples of 11 symptomatic palms. Results from cloning and sequence analysis of the PCR-amplified 16S rRNA gene segments revealed that, in three palms, three mutually distinct phytoplasmas comprising strains related to 'Candidatus Phytoplasma asteris' and 'Candidatus Phytoplasma cynodontis', as well as a novel phytoplasma, were present as triple infections. The 16S rRNA gene sequence derived from the novel phytoplasma shared less than 96 % nucleotide sequence identity with that of each previously describedspecies of the provisional genus 'Ca. Phytoplasma', justifying its recognition as the reference strain of a new taxon, 'Candidatus Phytoplasma wodyetiae'. Virtual RFLP profiles of the R16F2n/R2 portion of the 16S rRNA gene and the pattern similarity coefficient value (0.74) supported the delineation of 'Ca. Phytoplasma wodyetiae' as the sole representative subgroup A member of a new phytoplasma ribosomal group, 16SrXXXVI.

Detection, Identification, and Characterization of Phytoplasmas Infecting Apple and Pear Trees in Belarus

In Samochvalovichi, Belarus, apple and pear tree root samples were examined for the presence of phytoplasmas using a universal 16S rDNA-based PCR assay. Out of 27 tested apple trees, 23 were found to be infected by 'Candidatus Phytoplasma mali' and 46 out of 58 pear trees were positive for the presence of 'Ca. P. pyri.' Species were identified by sequence analysis of the 16S rDNA amplicons. The molecular diversity of the phytoplasma isolates was examined by analysis of an hflB gene using single-strand conformation polymorphism (SSCP) and sequence analysis. Therefore hflB gene amplicons from 'Ca. P. mali' and 'Ca. P. pyri' accessions were cloned after amplification. Screening of 640 cloned hflB fragments by SSCP analysis revealed the presence of eight different profiles for 'Ca. P. mali' and 12 different profiles for 'Ca. P. pyri.' The variants were sequenced and compared in multiple alignments. The nucleic acid homology among the hflB gene fragments ranged between 95.4 and 100.0% and 81.3 to 100.0% for 'Ca. P. mali' and 'Ca. P. pyri,' respectively, indicating a high genetic variability within the species. This is the first report on the occurrence of 'Ca. P. mali' and 'Ca. P. pyri' in Belarusian apple and pear trees and their molecular diversity.

Vector-Borne Bacterial Plant Pathogens: Interactions with Hemipteran Insects and Plants

Hemipteran insects are devastating pests of crops due to their wide host range, rapid reproduction, and ability to transmit numerous plant-infecting pathogens as vectors. While the field of plant-virus-vector interactions has flourished in recent years, plant-bacteria-vector interactions remain poorly understood. Leafhoppers and psyllids are by far the most important vectors of bacterial pathogens, yet there are still significant gaps in our understanding of their feeding behavior, salivary secretions, and plant responses as compared to important viral vectors, such as whiteflies and aphids. Even with an incomplete understanding of plant-bacteria-vector interactions, some common themes have emerged: (1) all known vector-borne bacteria share the ability to propagate in the plant and insect host; (2) particular hemipteran families appear to be incapable of transmitting vector-borne bacteria; (3) all known vector-borne bacteria have highly reduced genomes and coding capacity, resulting in host-dependence; and (4) vector-borne bacteria encode proteins that are essential for colonization of specific hosts, though only a few types of proteins have been investigated. Here, we review the current knowledge on important vector-borne bacterial pathogens, including Xylella fastidiosa, Spiroplasma spp., Liberibacter spp., and 'Candidatus Phytoplasma spp.'. We then highlight recent approaches used in the study of vector-borne bacteria. Finally, we discuss the application of this knowledge for control and future directions that will need to be addressed in the field of vector-plant-bacteria interactions.