Fieberiella florii (Homoptera: Auchenorrhyncha) as a Vector of "Candidatus Phytoplasma mali"

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.

Orientus ishidae (Hemiptera: Cicadellidae): Biology, Direct Damage and Preliminary Studies on Apple Proliferation Infection in Apple Orchard

The mosaic leafhopper, Orientus ishidae (Matsumura), is an Asian species widespread in Europe that can cause leaf damage in wild trees and transmit disease phytoplasmas to grapevines. Following an O. ishidae outbreak reported in 2019 in an apple orchard in northern Italy, the biology and damage caused by this species to apples were investigated during 2020 and 2021. Our studies included observations on the O. ishidae life cycle, leaf symptoms associated to its trophic activity, and its capability to acquire "Candidatus Phytoplasma mali," a causal agent of Apple Proliferation (AP). The results indicate that O. ishidae can complete the life cycle on apple trees. Nymphs emerged between May and June, and adults were present from early July to late October, with the peak of flight between July and early August. Semi-field observations allowed for an accurate description of leaf symptoms that appeared as a distinct yellowing after a one-day exposure. In field experiments, 23% of the leaves were found damaged. In addition, 16-18% of the collected leafhoppers were found carrying AP phytoplasma. We conclude that O. ishidae has the potential to be a new apple tree pest. However, further studies are required to better understand the economic impact of the infestations.

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.

Surveying Potential Vectors of Apple Proliferation Phytoplasma: Faunistic Analysis and Infection Status of Selected Auchenorrhyncha Species

Apple proliferation (AP) is one of the economically most important diseases in European apple cultivation. The disease is caused by the cell-wall-less bacterium ' Candidatus Phytoplasma mali', which is transmitted by Cacopsylla picta (Foerster) and Cacopsylla melanoneura (Foerster) (Hemiptera: Psylloidea). In South Tyrol (Italy), severe outbreaks were documented since the 1990s. Infestation rates of AP do not always correlate with the population densities of the confirmed vectors, implying the presence of other, so far unknown, hemipterian vectors. By elucidating the species community of Auchenorrhyncha (Insecta: Hemiptera) at a regional scale, more than 31,000 specimens were captured in South Tyrolean apple orchards. The occurrence of 95 species was confirmed, whereas fourteen species are new records for this territory. Based on the faunistical data, more than 3600 individuals out of 25 species were analyzed using quantitative PCR to assess the presence of AP phytoplasma. The pathogen was sporadically detected in some individuals of different species, for example in Stictocephala bisonia Kopp and Yonk (Hemiptera: Membracidae). However, the concentration of phytoplasma was much lower than in infected C. picta and C. melanoneura captured in the same region, confirming the role of the latter mentioned psyllids as the main insect vectors of AP- phytoplasma in South Tyrol.

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.

Detection of Pathogens Associated with Psyllids and Leafhoppers in Capsicum annuum L. in the Mexican States of Durango, Zacatecas, and Michoacán

In fall 2014, 5 to 75% percent of chili and bell pepper (Capsicum annuum L.) in commercial fields located in the Mexican states of Durango, Zacatecas, and Michoacán had symptoms of deformed, small, mosaic, curled, and chlorotic leaves; shortened internodes; plant dwarfing; or phyllody and rosetting leaf tips. At the same time, leafhoppers and psyllids were observed in the fields, and more than 50 beet leafhoppers (Circulifer tenellus) and nearly 300 potato psyllids (Bactericera cockerelli) were collected from the pepper plants and adjacent weeds. Based on the insect pressure and observed symptoms, nearly 400 pepper samples were collected across this region of Mexico and tested for the presence of leafhopper- and psyllid-associated pathogens. In all, 76% of the pepper samples were found to be infected with 'Candidatus Liberibacter solanacearum', beet leafhopper-transmitted virescence agent (BLTVA) phytoplasma, a strain of a curtovirus, or a combination of any two or three of these pathogens. Additionally, 77% of the collected leafhoppers and 40% of the psyllids were infected with one or more of these pathogens, in addition to Spiroplasma citri. Specifically, the leafhoppers were infected with BLTVA phytoplasma, S. citri, or a strain of curtovirus. Of particular interest, potato psyllids were not only infected with 'Ca. L. solanacearum' but also with phytoplasmas that belong to the groups 16SrVI subgroup A and 16SrI subgroup A. The presence of mixed infections in pepper plants and the insect vectors highlights the need for growers to effectively control both leafhoppers and potato psyllids from solanaceous crops in this region of Mexico in order to prevent the spread of these bacterial and viral pathogens.

Candidate insect vectors of apple proliferation in Northwest Spain

The apple proliferation (AP) disease is spread mostly by two psyllids. Each species plays a predominant role as AP vector that changes regionally. Thus, there is an urgent need to identify the AP vectors in each region where the disease is present. This research aimed at identifying the psyllid community in apple orchards from Asturias (NW Spain) and studying their possible role in AP transmission. Yellow sticky traps were used to monitor psyllid community in five cider-apple orchards during 2 years. 3678 individuals belonging to 22 species were identified. We confirmed the presence of the two known vectors, Cacopsylla picta and Cacopsylla melanoneura, although they occurred in relatively low numbers (2.1 and 0.7 % of total catches, respectively). Most collected psyllids are not supposed to use apple as host, and their occurrence is likely favoured by landscape structure and an insect-friendly management. Phytoplasma detection was performed by squash-capture real-time PCR. The pathogen was detected in six species (Cacopsylla crataegi, Cacopsylla mali, Ctenarytaina spatulata, Ctenarytaina eucalypti and the two known AP vectors). Based on abundance and AP-detection rate C. picta is likely the main species spreading AP in our region. However, the low density of the known vectors does not match the widespread and high tree damage level observed in Asturias. The discovery of other four psyllid species carrying the phytoplasma reveals that our knowledge on the potential vectors is limited and that more research is clearly needed to unravel the role of the psyllid fauna in disease transmission in our orchards.

Molecular Identification of Two Vector Species, Cacopsylla melanoneura and Cacopsylla picta (Hemiptera: Psyllidae), of Apple Proliferation Disease and Further Common Psyllids of Northern Italy

The psyllid species Cacopsylla melanoneura (Förster) and Cacopsylla picta (Förster) are vectors of 'Candidatus Phytoplasma mali', the causal agent of apple proliferation, one of the economically most important apple diseases in Europe. Both vectors are present in apple orchards of South Tyrol and Trentino provinces in Northern Italy. As no direct treatment of the disease is possible, monitoring of the psyllids provides information about the vector presence in the orchards and enables targeted control. Thus, fast and reliable identification of the various psyllids occurring in the apple orchards is required. Morphological differentiation is problematic due to extensive resemblance of some psyllid species especially among females and is error-prone for nymphs. Here we present a rapid and cost-effective polymerase chain reaction-restriction fragment length polymorphism method based on the cytochrome c oxidase subunit I region for the molecular identification of the vector species as well as eight further Cacopsylla species present in the orchards. This method was verified through 98.9% consensus with morphologically identified males, through sequencing and subsequent phylogenetic analysis. In case of doubtful morphological identification of females, the method was able to provide a refined species assignment and could also remarkably facilitate the identification of nymphs.

Characteristics of the spread of apple proliferation by its vector Cacopsylla picta

The distribution and natural phytoplasma infection of Cacopsylla picta were investigated during a long-term field survey between 2002 and 2009 in commercial and abandoned apple proliferation-infected orchards throughout Germany, northern Switzerland, and eastern France. Comparable population dynamics were described for the different sites whereas considerable variations in the absolute population densities were observed among the years. Individual polymerase chain reaction (PCR) testing revealed, for each year, a rather stable natural infection rate with ?Candidatus Phytoplasma mali? of ?10% for overwintered adults of C. picta. Both genders were equally highly infected although more females were caught. The overall male/female ratio was 1:1.5. No direct correlation was found between the infection status of the orchard and the infection rate of overwintered C. picta. No influence of agricultural practices was seen. However, a relationship between the incidence of the disease and the vector population density became evident on a regional scale. Successful transmission of ?Ca. P. mali? occurred each year with overwintered individuals as well as with new adults. The transmission efficiency varied among the years within 8 to 45% for overwintered adults and 2 to 20% for individuals of the new generation. The load of single C. picta with ?Ca. P. mali? was determined by quantitative real-time PCR. High phytoplasma titers were measured in overwintered adults already at their first appearance in the orchards after remigration from their overwintering hosts. Thus, the data indicate the transmission of the disease on a regional scale by remigrant adults of C. picta and at a local scale within the same season by emigrant adults which developed on infected plants.

DNA-based discrimination and frequency of phytoplasma infection in the two hawthorn-feeding species, Cacopsylla melanoneura and Cacopsylla affinis, in northwestern Italy

A molecular tool, focused on the mitochondrial Control Region (CR), was developed to discriminate the two hawthorn-feeding psyllid species, Cacopsylla melanoneura (Förster) and C. affinis (Löw), and to estimate their frequencies in mixed populations. The test was carried out in paired and single-tube amplifications and validated analysing 52 male specimens previously determined by morphological analysis. The frequencies of the two species in mixed populations in the Aosta Valley (northwestern Italy) were analysed. The presence and type of 16SrX-group phytoplasmas was detected by nested PCR and RFLP tests in both species. C. melanoneura was the predominant species (86.5%; 80.4-91.2 CI); of these, 0.9% of the samples were positive for 'Ca. Phytoplasma mali' and 1.8% for 'Ca. Phytoplasma pyri'. One of 21 specimens of C. affinis was positive for 'Ca. Phytoplasma pyri'. The test also allowed us to identify two genetic variants of C. melanoneura, depending on the presence or absence of a 56 bp indel; these were named WI (with indel) and WOI (without indel), respectively. Further analyses were carried out on C. melanoneura specimens collected in apple orchards at six different locations in northern Italy where different levels of transmission efficiency have been described. Our preliminary observations suggest that some differences might exist between the two genetic variants in their ability to transmit phytoplasmas and to colonise different host plants.

Cacopsylla melanoneura has no relevance as vector of apple proliferation in Germany

Long-term field surveys on the distribution and natural infection rates of Cacopsylla melanoneura were carried out in commercial and abandoned apple-proliferation-infected orchards throughout Germany, northern Switzerland, and eastern France. Although the infection rates of some orchards reached up to 80%, only 0.09% of all C. melanoneura collected on apple were infected by the pathogen 'Candidatus Phytoplasma mali'. Despite higher population densities, no infected individual was found on wild hawthorn. Individuals of C. melanoneura were not able to transmit phytoplasmas to healthy plants, and even the acquisition of 'Ca. P. mali' from infected plants was very inefficient. Quantitative real-time polymerase chain reaction demonstrated that the very few infected individuals of C. melanoneura harbored phytoplasma concentrations 10,000 times lower than individuals of C. picta, the main vector species in Germany. Oviposition bioassays showed that hawthorn is the preferred reproduction host plant for C. melanoneura in Germany, not apple. Because hawthorn is not a suitable host plant for 'Ca. P. mali', it does not play a role in the spread of apple proliferation. In contrast to data reported from northwestern Italy, C. melanoneura developed on either apple or hawthorn has no relevance as a vector of apple proliferation in Germany. The existence of epidemiologically different populations is proposed.