First Report of Calosphaeria pulchella causing Canker and Twig Dieback of Peach (Prunus persica) in California, U.S.A

California leads the United States in peach (Prunus persica L.) production, with approximately 505,000 tons produced in 2021 and valued at $378.3 million (California Agriculture Statistics Review, 2021-2022). During the spring and summer of 2023, twig and branch dieback were observed in three peach orchards (cvs. Late Ross and Starn) in San Joaquin County, California. Wood cankers and discoloration also occurred in branches, generally initiating at pruning wounds. Approximately 8 symptomatic twigs or branches per orchard were collected to proceed with the isolation of necrotic tissues on acidified potato dextrose agar (APDA). Isolations consistently yielded colonies of the fungal pathogen Calosphaeria pulchella (Pers. : Fr.) J. Schröt. (Réblová et al. 2004; Trouillas et al. 2012). Pure cultures were obtained by transferring single hyphal tips onto new APDA Petri plates. Colonies on APDA grew dark pink to red or purple in their center, with a white margin. Conidiogenesis was phialidic, producing round conidial masses at the tip of phialides. Conidia were produced abundantly on APDA, and were hyaline, allantoid to oblong-ellipsoidal, 4 to 5.5 (7) × 1.2 to 2.3 μm (n = 60). Two representative isolates (SJC-62 and SJC-64) were selected for genomic DNA extraction and sequencing of the internal transcribed spacer region (ITS) using ITS5/ITS4 universal primers and the beta-tubulin (TUB2) gene region using primers Bt2a and Bt2b. Consensus sequences of the two genes for the two isolates (ITS: PP063990, PP063991; TUB2: PP068303, PP068304) were compared to reference sequences (Réblová et al. 2015; Trouillas et al. 2012) using BLAST analysis. The ITS sequences of SJC-62 and SJC-64 were 99.8 and 99.5% identical to that of C. pulchella ex-type strain CBS 115999 (NR145357) and reference strain SS07 (HM237297); the TUB2 sequences were at least 98.5% identical to that of C. pulchella CBS 115999 (KT716476). Pathogenicity tests were conducted in 2- to 3-year-old healthy branches on 7-year-old peach trees, cvs. Loadel, Late Ross and Starn using the two fungal isolates and a control treatment (1 branch per treatment and 3 branches per tree) on each of 8-tree replicates. Branches were inoculated in June 2023 following wounding with a 5 mm cork borer to remove the bark and placing an agar plug from the margin of 10-day-old colonies on APDA directly into the fresh wound. Sterile agar plugs were used as controls. Inoculation sites were covered with petroleum jelly and wrapped with Parafilm to retain moisture. The experiment was completed twice. After four months, cankers and vascular discolorations developed around the inoculation sites. Length of vascular discoloration in inoculated branches averaged 72, 75, and 79 mm, for the Loadel, Starn, and Late Ross cvs., respectively. Calosphaeria pulchella was re-isolated from inoculated branches at 80 to 100% recovery rate, thus fulfilling Koch's postulates. The average length of vascular discoloration in the control was 13.5 mm and no fungi were recovered from control branches. Calosphaeria canker caused by C. pulchella is a global disease of sweet cherry. Recently, it was reported to cause cankers in peach trees in Chile (Grinbergs et al. 2023). To our knowledge, this is the first report of C. pulchella causing cankers and twig dieback of peach trees in the United States. These findings improve our knowledge of the etiology of canker diseases affecting peach trees and is critical for the development of effective disease management strategies.

Genetic Variation of Monilinia fructicola Population in Korea

Brown rot disease, caused by Monilinia spp., poses a significant threat to pome and stone fruit crops globally, resulting in substantial economic losses during pre- and post-harvest stages. Monilinia fructigena, M. laxa, and M. fructicola are identified as the key agents responsible for brown rot disease. In this study, we employed the amplified fragment length polymorphism (AFLP) method to assess the genetic diversity of 86 strains of Monilinia spp. isolated from major stone fruit cultivation regions in South Korea. Specifically, strains were collected from Chungcheong, Gangwon, Gyeonggi, Gyeongsang, and Jeolla provinces (-do). A comparative analysis of strain characteristics, such as isolation locations, host plants, and responses to chemical fungicides, was conducted. AFLP phylogenetic classification using 20 primer pairs revealed the presence of three distinct groups, with strains from Jeolla province consistently forming a separate group at a high frequency. Furthermore, M. fructicola was divided into three groups by the AFLP pattern. Principal coordinate analysis and PERMANOVA were applied to compare strain information, such as origin, host, and fungicide sensitivity, revealing significant partition patterns for AFLP according to geographic origin and host plants. This study represents the utilization of AFLP methodology to investigate the genetic variability among M. fructicol isolates, highlighting the importance of continuous monitoring and management of variations in the brown rot pathogen.

Efficacy of Insecticides against the Invasive Apricot Aphid, Myzus mumecola

The invasive apricot aphid (Myzus mumecola Matsumura) is an important pest of apricot trees (Prunus armeniaca L.). In the presented study, laboratory bioassays using treated leaf disks of apricot were conducted to test the efficacy of twelve insecticides according to the maximum field dose. Additionally, dose-response curves were established for selected insecticides, and the effects on colony development were evaluated. Furthermore, a field trial was conducted to investigate the effectiveness of commonly used insecticides in apricot cultivation. The dose-response curves showed LC50 values ranging from 0.08 mg/L for flupyradifurone, 0.15 mg/L for acetamiprid, 0.70 mg/L for etofenprox, 1.89 mg/L for sulfoxaflor, 2.64 mg/L for pirimicarb, 3.97 mg/L for deltamethrin, up to 6.79 mg/L for tau-fluvalinate. These aforementioned insecticides resulted in mortality rates ranging from 95 to 100% at the field dose. Azadirachtin, flonicamid, and pyrethrins showed mortality rates of 27 to 45%. Spirotetramat reduced the colony development and decreased the number of infested shoots by 86%. Spinosad, which is not recommended against aphids, showed minimal impact; reducing the number of exuviae in nymphs in the colony development bioassay. It can be concluded that the majority of the tested insecticides are effective against M. mumecola.

Hydroxypropyl Methylcellulose and Gum Arabic Composite Edible Coatings Amended with Geraniol to Control Postharvest Brown Rot and Maintain Quality of Cold-Stored Plums

In this study, the effect of hydroxypropyl methylcellulose (HPMC) and gum Arabic (GA) edible coatings amended with 0.2% geraniol (GE) were evaluated for the control of brown rot, caused by Monilinia fructicola, on artificially inoculated plums (Prunus salicina Lindl., cv. Angeleno) stored for 5 weeks at 1 °C. Brown rot is the most important pre- and postharvest fungal disease of stone fruits, causing severe economic losses worldwide. Geraniol is an important constituent of many essential oils that can be obtained as a byproduct from different industrial procedures, such as those of the juice industry. Fruit postharvest quality was also evaluated after 5 and 8 weeks of storage at 1 °C, followed by 3 days at 7 °C plus 5 days at 20 °C, simulating packinghouse, transport, and retail shelf-life conditions, respectively. HPMC coatings containing 0.2% GE reduced the incidence and severity of brown rot by 37.5 and 64.8%, respectively, compared to uncoated fruit after 5 weeks of storage at 1 °C. HPMC-coated plums, with and without GE, showed the highest level of firmness, the lowest change in external peel color parameters (L*, a*, b*, C*, hue), and the lowest flesh bleeding compared to uncoated control and GA-coated samples throughout the entire storage period, which correlated with a higher gas barrier of these coatings without negatively affecting sensory quality. Furthermore, the HPMC-0.2% GE coating provided the highest gloss to coated plums, showing the potential of this coating as a safe and environmentally friendly alternative to conventional fungicides and waxes for brown rot control and quality maintenance of cold-stored plums.

Tranzschelia discolor as the Causal Agent of Rust on Nectarine, Peach, and Japanese Plum in Highland Areas of Northern Thailand

A severe outbreak of rust disease was observed on Prunus species, P. persica (peach), P. persica var. nectarina (nectarine), and P. salicina (Japanese plum) cultivated in northern Thailand in the rainy season. Previous reports have identified the causal agents as Tranzschelia discolor and T. pruni-spinosae based on only morphological characteristics. Thus, the aim of this study was to identify rust fungi of Prunus spp. based on morphology and molecular analyses. Between May and July in 2020 and 2022, 18 isolates were collected from five areas in Chiang Mai and Chiang Rai provinces. Symptoms of rust consisted of cinnamon brown pustules of uredinia that were hypophyllous and visible as pale greenish to chlorotic yellowish angular spots on the upper leaf surfaces. Urediniospore shape, size, and color were similar to T. discolor. Molecular analysis of internal transcribed spacer (ITS) and partial 28S large subunit (LSU) region rRNA genes confirmed the isolates to be T. discolor. This is the first report of P. salicina as a host of T. discolor in Thailand.

Variation in the Root System Architecture of Peach × (Peach × Almond) Backcrosses

The spatial arrangement and growth pattern of root systems, defined by the root system architecture (RSA), influences plant productivity and adaptation to soil environments, playing an important role in sustainable horticulture. Florida's peach production area covers contrasting soil types, making it necessary to identify rootstocks that exhibit soil-type-specific advantageous root traits. In this sense, the wide genetic diversity of the Prunus genus allows the breeding of rootstock genotypes with contrasting root traits. The evaluation of root traits expressed in young seedlings and plantlets facilitates the early selection of desirable phenotypes in rootstock breeding. Plantlets from three peach × (peach × almond) backcross populations were vegetatively propagated and grown in rhizoboxes. These backcross populations were identified as BC1251, BC1256, and BC1260 and studied in a completely randomized design. Scanned images of the entire root systems of the plantlets were analyzed for total root length distribution by diameter classes, root dry weight by depth horizons, root morphological components, structural root parameters, and root spreading angles. The BC1260 progeny presented a shallower root system and lower root growth. Backcross BC1251 progeny exhibited a more vigorous and deeper root system at narrower root angles, potentially allowing it to explore and exploit water and nutrients in deep sandy entisols from the Florida central ridge.

The Expanding Menagerie of Prunus-Infecting Luteoviruses

Members of the genus Luteovirus are responsible for economically destructive plant diseases worldwide. Over the past few years, three luteoviruses infecting Prunus trees have been characterized. However, the biological properties, prevalence, and genetic diversity of those viruses have not yet been studied. High-throughput sequencing of samples of various wild, cultivated, and ornamental Prunus species enabled the identification of four novel species in the genus Luteovirus for which we obtained complete or nearly complete genomes. Additionally, we identified another new putative species recovered from Sequence Read Archive data. Furthermore, we conducted a survey on peach-infecting luteoviruses in eight European countries. Analyses of 350 leaf samples collected from germplasm, production orchards, and private gardens showed that peach-associated luteovirus (PaLV), nectarine stem pitting-associated virus (NSPaV), and a novel luteovirus, peach-associated luteovirus 2 (PaLV2), are present in all countries; the most prevalent virus was NSPaV, followed by PaLV. The genetic diversity of these viruses was also analyzed. Moreover, the biological indexing on GF305 peach indicator plants demonstrated that PaLV and PaLV2, like NSPaV, are transmitted by graft at relatively low rates. No clear viral symptoms have been observed in either graft-inoculated GF305 indicators or different peach tree varieties observed in an orchard. The data generated during this study provide a broader overview of the genetic diversity, geographical distribution, and prevalence of peach-infecting luteoviruses and suggest that these viruses are likely asymptomatic in peach under most circumstances.

First Report of Brown Rot Caused by Monilinia polystroma on Sweet Cherry and Almond in Italy

Brown rot decay is an important disease of pome and stone fruits. In Italy, the main pathogens on stone fruits are Monilinia laxa, M. fructigena, and M. fructicola (Spitaler et al. 2022a). In addition, Monilinia polystroma (G. Leeuwen) L. M. Kohn (van Leeuwen et al. 2002), was recently found in Italy on peach (Martini et al. 2014), pear (Martini et al. 2015), plum (Abate et al. 2018), apple (Rosati et al. 2021), and quince (Spitaler et al. 2022b). In South Tyrol province, sweet cherry (Prunus avium L.) and almond (Prunus dulcis Mill. D. A. Webb), plants of the Rosaceae family and belonging to stone fruits, were observed to be frequently affected by brown rot. Affected cherries as well as almonds showed brown lesions, covered by yellowish or buff-colored stroma in concentric rings. Symptomatic cherries became shriveled, while symptomatic almonds remained firm. To determine the pathogen, single spore isolates were obtained from five symptomatic fruits, each from a cherry orchard of the cultivar Kordia in July 2021 and almond trees of the variety Dulcis in August 2021. Both sample sites were situated in Vadena/Pfatten. Infestation in the cherry orchard, covered by a rain-protection foil, was determined to be about 1 %. In almond, over 50 % of the fruits of various ripening stages showed brown rot symptoms. On potato dextrose agar (PDA) at 22 °C and a photoperiod of 16 h, isolates from both fruits matched the morphological characteristics of M. polystroma (Vasić et al. 2016) within 14 days. DNA was extracted from mycelium and the rRNA encoding gene region using ITS4 and ITS6 primers as well as a genomic sequence of unknown function using the primers UniMon_Forw and UniMon_Rev (Petróczy et al. 2012), were amplified and sequenced. MegaBLAST analysis revealed 100 % identity with M. polystroma sequences of the NCBI GenBank (rRNA encoding region: NR_154198; genomic region: JN128836). Sequences were deposited in GenBank under the accession numbers OP642545/OP654171 (cherry) and OP642546/OP654172 (almond). Pathogenicity was confirmed with mature cherries cultivar Duroncino or almost mature almond fruit of the variety Dulcis, respectively: 16 samples each for both fruits were surface-sterilized by dipping in 75 % ethanol for 10 s and subsequent rinsing with sterile water for 10 s. Mycelial plugs (1 mm) were dislodged from a 7-day old colony and inserted in a 1 mm hole into the fruits. Incubation was performed in plastic boxes under the conditions described above. PDA-inoculated fruit were used as controls. All cherries and all almonds were completely covered by brown rot lesions 7 days and 15 days post inoculation, respectively. Control fruits remained symptomless. Conidia were produced in branched chains on mycelium-inoculated fruit. Conidia were one-celled, limoniform, hyaline, measuring 13.1 to 22.2 × 9.7 to 14.8 µm (cherry) and 14.1 to 20.8 × 10.7 to 15.3 µm (almond). Additionally, 16 fruits each were inoculated with 20 µL conidial suspension (5 x 10^3 spores/mL) from mycelium-inoculated fruits. All cherries as well as all almonds were completely covered by brown rot lesions 7 days and 15 days post inoculation. Control fruits remained symptomless. To confirm identity, the fungus was isolated from five spore-inoculated fruits each for cherry and almond. The isolates showed identical morphological features and sequence identity as the original isolates. To our knowledge, this is also the first report of M. polystroma on almond, while the pathogen has already been reported on sweet cherry in Poland (Poniatowska et al. 2016). These additional host plants identified in this study suggest a broad impact of M. polystroma on Italian stone fruit production. Due to the economically important cultivation of stone fruit, further knowledge about the pathogens' host range will help to assign brown rot symptoms to M. polystroma and to improve targeted control strategies.

Mechanism of Stone (Hardened Endocarp) Formation in Fruits: An Attempt toward Pitless Fruits, and Its Advantages and Disadvantages

Stone (hardened endocarp) has a very important role in the continuity of plant life. Nature has gifted plants with various seed protection and dispersal strategies. Stone-fruit-bearing species have evolved a unique adaptation in which the seed is encased in an extremely hard wood-like shell called the stone. The lignification of the fruit endocarp layer produces the stone, a feature that separates drupes from other plants. Stone cells emerge from parenchyma cells after programmed cell death and the deposition of cellulose and lignin in the secondary cell wall. Generally, the deposition of lignin in primary cell walls is followed by secondary thickening of cell walls to form stone cells. This review article describes the molecular mechanisms and factors that influence the production of stone in the fruit. This is the first review article that describes the molecular mechanisms regulating stone (harden endocarp) formation in fruits. This article will help breeders understand the molecular and genetic basis for the stone formation in fruit, and this could lead to new and innovative directions to breed stoneless fruit cultivars in the future.

The Molecular Characterization of a New Prunus-Infecting Cheravirus and Complete Genome Sequence of Stocky Prune Virus

As part of a virome characterization of Prunus species, a novel cheravirus was discovered in two wild species, Prunus brigantina and P. mahaleb, and in an apricot (P. armeniaca) accession. The sequence of the two genomic RNAs was completed for two isolates. The Pro-Pol conserved region showed 86% amino acid (aa) identity with the corresponding region of trillium govanianum cheravirus (TgCV), a tentative Cheravirus member, whereas the combined coat proteins (CPs) shared only 40% aa identity with TgCV CPs, well below the species demarcation threshold for the genus. This suggests that the new virus should be considered a new species for which the name alpine wild prunus virus (AWPV) is proposed. In parallel, the complete genome sequence of stocky prune virus (StPV), a poorly known cheravirus for which only partial sequences were available, was determined. A phylogenetic analysis showed that AWPV, TgCV and StPV form a distinct cluster, away from other cheraviruses.

Resistance of Almond (Prunus dulcis) to Xylella fastidiosa: A Comparative Study on Cultivars

Almond leaf scorch is a disease caused by Xylella fastidiosa, which is increasingly widespread globally in the main almond cultivation areas. Previously confined to America, in the last decade this disease has been reported in Iran and southern Europe. In this comparative study, the resistance to X. fastidiosa subsp. multiplex of 13 almond cultivars (Mamaei, Non-Pareil, Sefied, Rabie, Ferragnes, Shahrood21, Thompson, Merced, Marcona, Frudeul, Kapariel, Princess, and Tuono) grafted onto seedlings of Talkhe was evaluated in controlled conditions. Plants were artificially inoculated and maintained in greenhouse conditions. Approximately 3 months after inoculation, three times at 3-week intervals, disease incidence, disease severity, and disease index were determined based on scaling, and bacterial populations were estimated. The effect of winter survival of bacteria in outdoor potted seedlings was also investigated in all almond cultivars. Findings showed a great degree of variability in response to X. fastidiosa among cultivars considering symptom development and severity, as well as bacterial titer. Thompson and Rabie cultivars scored the best results from both a symptomatological and infectious point of view, indicating resistance against the pathogen compared with other tolerant cultivars (e.g., Ferragnes, Tuono, and Kapariel), thanks to the development of mild symptoms. Mamaei, Non-Pareil, and Sefied scored worst, suggesting a susceptible behavior when infected by X. fastidiosa. Given that the pathogen was not detected by culturing and PCR during the following summer, bacterial population in potted seedlings was reduced significantly by overwintering in outdoor conditions regardless of cultivar susceptibility. This suggests that cold treatment can be used as a preventive treatment to manage nursery almond seedlings.

Agroclimatic Metrics for the Main Stone Fruit Producing Areas in Spain in Current and Future Climate Change Scenarios: Implications From an Adaptive Point of View

Stone fruit production has enormous economic importance in Spain. Cultivation locations for these fruit species (i.e., peach, apricot, plum, and sweet cherry) cover wide and climatically diverse geographical areas within the country. Climate change is already producing an increase in average temperatures with special intensity in certain areas like the Mediterranean ones. These changes lead to a decrease in the accumulated chill, which can have a profound impact on the phenology of Prunus species like stone fruits due to, e.g., difficulties to cover the chilling requirements to break endodormancy, the occurrence of late frost events, or abnormal early high temperatures. All these factors can severely affect fruit production and quality and therefore provoke very negative consequences from the socio-economic point of view in the incumbent regions. Thus, characterization of current cultivation areas in terms of agroclimatic variables (e.g., chill and heat accumulation and probabilities of frost and early abnormal heat events), based on data from 270 weather stations for the past 20 years, is carried out in this work to produce an informative picture of the current situation. Besides, future climatic projections from different global climate models (data retrieved from the Meteorological State Agency of Spain-AEMET) up to 2065 for two Representative Concentration Pathway scenarios (i.e., RCP4.5 and RCP8.5) are also analyzed. Using the current situation as a baseline and considering the future scenarios, information on the current and future adaptive suitability of the different species/cultivars to the different growing areas can be inferred. This information could be the basis of a decision support tool to help the different stakeholders to take optimal decisions regarding current and future stone fruit or other temperate species cultivation in Spain.

A Duplex-Droplet Digital PCR Assay for Simultaneous Quantitative Detection of Monilinia fructicola and Monilinia laxa on Stone Fruits

Brown rot, caused by different Monilinia species, is a most economically important disease of pome and stone fruits worldwide. In Europe and in Italy, the quarantine pathogen M. fructicola was recently introduced and rapidly spread and, by competing with the main indigenous species Monilinia fructigena and Monilinia laxa, caused relevant changes in Monilinia populations. As a result, in most areas, the pathogen almost replaced M. fructigena and now coexists with M. laxa. The availability of specific and easy-of-use quantification methods is essential to study the population dynamics, and in this work, a new method for the simultaneous quantification of M. fructicola and M. laxa based on droplet digital PCR (ddPCR) technique was established. Under the optimized reaction conditions, consisting of 250/500 nM of primers/probe sets concentration, 58°C as annealing temperature and 50 PCR cycles, the duplex-ddPCR assay was 200-fold more sensitive than duplex-real-time quantitative PCR (qPCR) assay, quantifying < 1 copy μL^–1 of target DNA in the PCR mixture. The results obtained with the validation assay performed on apricot and peach fruits, artificially inoculated with conidial suspensions containing different ratios of M. fructicola and M. laxa, showed a high correlation (R² = 0.98) between the relative quantity of DNA of the two species quantified by ddPCR and qPCR and a more accurate quantification by ddPCR compared to qPCR at higher concentrations of M. fructicola. The herein described method represents a useful tool for the early detection of Monilinia spp. on stone fruits and for the improving knowledge on the epidemiology of brow rot and interactions between the two prevalent Monilinia species.

Diversity and Relationships among Neglected Apricot (Prunus armeniaca L.) Landraces Using Morphological Traits and SSR Markers: Implications for Agro-Biodiversity Conservation

Apricot (Prunus armeniaca L.) is an economically important tree species globally cultivated in temperate areas. Italy has an ample number of traditional varieties, but numerous landraces are abandoned and at risk of extinction because of increasing urbanization, agricultural intensification, and varietal renewal. In this work, we investigated the morphological and genetic diversity present in an ex-situ collection of 28 neglected varieties belonging to the so-called "Vesuvian apricot". Our aim was to understand the level of diversity and the possible link between the promotion of specific fruit types (e.g., by public policies) and the intraspecific variation in apricot. The combination of five continuous and seven categorical traits allowed us to phenotypically distinguish the varieties; while fruit quality-related attributes displayed high variation, both apricot size and skin colour were more uniform. The twelve fluorescent-based Simple Sequence Repeats (SSRs) markers identified cultivar-specific molecular profiles and revealed a high molecular diversity, which poorly correlated with that described by the morphological analysis. Our results highlighted the complementary information provided by the two sets of descriptors and that DNA markers are necessary to separate morphologically related apricot landraces. The observed morphological and genetic differences suggest a loss of diversity influenced by maintenance breeding of specific pomological traits (e.g., skin colour and size). Finally, our study provided evidence to recommend complementary strategies to avoid the loss of diversity in apricot. Actions should pivot on both the promotion of easily identified premium products and more inclusive biodiversity-centred on-farm strategies.

Field validation of biocontrol strategies to control brown rot on stone fruit in several European countries

BACKGROUND

Brown rot caused by Monilinia spp. is the most significant disease of stone fruit. New approaches to fruit production have necessitated the development of control strategies that are more eco- and consumer-friendly. An efficient field strategy to control brown rot was previously designed based on the application of two biocontrol agents (BCAs), Bacillus amyloliquefaciens CPA-8 (CPA-8) or Penicillium frequentans 909 (Pf909), with calendar-based treatment. In the present study, the strategy was validated on different stone fruit hosts in four producing countries over two seasons.

RESULTS

The results obtained were reported according to three different scenarios: Scenario 1, in which there was no presence of disease in the field; Scenario 2, in which high disease pressure occurred in the field and treatments (biologicals or chemicals) were not effective; and Scenario 3, with low or medium to high disease presence. The results were successful because, in general, BCA strategies were shown to control brown rot to a similar extent as chemicals strategies. We found that most of the trials conducted in this study were classed under Scenario 3 (62.5%), with only 12.5% and 25% of the trials classed under Scenarios 1 and 2, respectively.

CONCLUSION

These novel findings allowed the formulation of CPA-8 and Pf909 as valuable tools for farmers to produce stone fruits more competitively and meet consumer demand for safer and more environmentally friendly products. © 2021 Society of Chemical Industry.

Deciphering the Monilinia fructicola Genome to Discover Effector Genes Possibly Involved in Virulence

Brown rot is the most economically important fungal disease of stone fruits and is primarily caused by Monilinia laxa and Monlinia fructicola. Both species co-occur in European orchards although M. fructicola is considered to cause the most severe yield losses in stone fruit. This study aimed to generate a high-quality genome of M. fructicola and to exploit it to identify genes that may contribute to pathogen virulence. PacBio sequencing technology was used to assemble the genome of M. fructicola. Manual structural curation of gene models, supported by RNA-Seq, and functional annotation of the proteome yielded 10,086 trustworthy gene models. The genome was examined for the presence of genes that encode secreted proteins and more specifically effector proteins. A set of 134 putative effectors was defined. Several effector genes were cloned into Agrobacterium tumefaciens for transient expression in Nicotiana benthamiana plants, and some of them triggered necrotic lesions. Studying effectors and their biological properties will help to better understand the interaction between M. fructicola and its stone fruit host plants.

Re-Evaluation of the Podosphaera tridactyla Species Complex in Australia

The Podosphaera tridactyla species complex is highly variable morphologically and causes powdery mildew on a wide range of Prunus species, including stone fruit. A taxonomic revision of the Po. tridactyla species complex in 2020 identified 12 species, seven of which were newly characterised. In order to clarify which species of this complex are present in Australia, next generation sequencing was used to isolate the fungal ITS+28S and host matK chloroplast gene regions from 56 powdery mildew specimens of stone fruit and ornamental Prunus species accessioned as Po. tridactyla or Oidium sp. in Australian reference collections. The specimens were collected in Australia, Switzerland, Italy and Korea and were collected from 1953 to 2018. Host species were confirmed using matK phylogenetic analysis, which identified that four had been misidentified as Prunus but were actually Malusprunifolia. Podosphaera species were identified using ITS+28S phylogenetic analysis, recognising three Podosphaera species on stone fruit and related ornamental Prunus hosts in Australia. These were Po.pannosa, the rose powdery mildew, and two species in the Po. tridactyla species complex: Po. ampla, which was the predominant species, and a previously unidentified species from peach, which we describe here as Po. cunningtonii.

Evaluation of the Intelligent Sprayer System in Peach Production

Air-blast sprayers are routinely used to apply pesticides in commercial peach orchards, with growers using both conventional air-blast and ultrasonic sensor-driven models. Even with advanced spray technologies, there are still concerns with the amount of chemicals used and lost to drift. Our study evaluated a laser sensor-based variable-rate sprayer system in three experimental peach orchards for pest and brown rot disease control, spray volume output, spray coverage, and spray drift. A single 378-liter air-blast sprayer was used for both the conventional air-blast and the Intelligent Sprayer (iSprayer) treatments. Treatments were started at the phenological stage of bloom and continued through final swell. The iSprayer treatment was as effective in controlling pests and brown rot disease as the conventional air-blast treatment. Compared with the conventional air-blast treatment, the iSprayer treatment reduced the spray volume (liters/hectare) in cultivar PF23 by 71% at bloom, 62% at pit hardening, and 55% at final swell. For Juneprince, the spray volume reduction was 50% at bloom, 40% at pit hardening, and 13% at final swell. Spray drift was significantly (P < 0.05) reduced only at bloom in the iSprayer treatment. Spray coverage was increased by 50.13 and 26.67% in the iSprayer treatment at bloom and pit hardening, respectively, but not at final swell. Our results show that the iSprayer maintained pest and disease control efficacy in peach orchards while reducing spray volume and drift compared with the conventional air-blast treatment.

Impact of Postharvest Handling on Preharvest Latent Infections Caused by Monilinia spp. in Nectarines

Latent infections caused by Monilinia spp. in nectarines cause great economic losses since they are not detected and rejected at harvest and can appear at any time post-harvest, even at the consumer’s home. The effect of a pre-cooling chamber, water dump operation, and cold-storage chamber on the activation and/or development of preharvest latent infections caused by Monilinia spp. on nectarines were studied under different postharvest conditions: (a) cold storage for 0, 1, or 3 d at 4 °C at either 75% relative humidity (RH) or 100% RH before water dumping, (b) water dumping for 10 minutes at 15 °C, and (c) cold storage for 0, 3, or 10 d at 4 °C at either 75% RH or 100% RH after water dumping. These storage conditions were transformed to fungal physiological time. For visualization of the latent infections caused by Monilinia spp., the nectarines were placed in sterile paper bags and frozen at −20 °C for 48 h in order to damage the epidermis. To compare different handling scenarios, the incidence of latent infection was modelled for physiological time description by a modified Gompertz model. The activation and/or development of preharvest natural latent infections caused by Monilinia spp. at postharvest was mainly related to temperature and incubation time at postharvest. Storing nectarines with any postharvest handling less than 11 days at 4 °C avoids brown rot symptoms and reduced the activation and/or development of pre-harvest latent infections caused by Monilinia spp., while more cold days caused the exponential phase of latent infection activation and/or development. The Gompertz model employed could be used for predicting the activation and/or development of latent infection caused by Monilinia spp. at postharvest conditions and looks at the postharvest life. To our knowledge, this is the first time that the effects of post-harvest handling on latent infections in fruit have been studied.

Trap-Based Economic Injury Levels and Thresholds for Euschistus servus (Hemiptera: Pentatomidae) in Florida Peach Orchards

Populations of the brown stink bug, Euschistus servus (Say), in Florida peach orchards were monitored during the 2017 and 2018 growing seasons using yellow pheromone-baited pyramid traps. Peaches were evaluated at harvest for the presence of stink bug injury. A relationship between E. servus trap capture and fruit injury was used to estimate the economic injury level (EIL) for varying scenarios of crop price, per-hectare yield, and control costs. Economic thresholds were then set based on observed E. servus population trends and expected rates of increase. Thresholds were lowest in the period immediately following shuck split due to the rapid increase in E. servus populations during this period. Euschistus servus trap capture trended downward at the time of harvest. Therefore, increased E. servus management early in the Florida peach season provides the greatest overall benefit, whereas late season populations decline independent of management actions. The proposed EIL for Florida peaches determined by this study is 5.53 E. servus per trap over a 14-d trapping period, which was surpassed in only 16 of the 60 recorded trapping periods. This suggests that the current recommended spray frequency may not be economically justified.

New High-Quality Draft Genome of the Brown Rot Fungal Pathogen Monilinia fructicola

Brown rot is a worldwide fungal disease of stone and pome fruit that is caused by several Monilinia species. Among these, Monilinia fructicola can cause severe preharvest and postharvest losses, especially for stone fruit. Here, we present a high-quality draft genome assembly of M. fructicola Mfrc123 strain obtained using both Illumina and PacBio sequencing technologies. The genome assembly comprised 20 scaffolds, including 29 telomere sequences at both ends of 10 scaffolds, and at a single end of 9 scaffolds. The total length was 44.05 Mb, with a scaffold N50 of 2,592 kb. Annotation of the M. fructicola assembly identified a total of 12,118 genes and 13,749 proteins that were functionally annotated. This newly generated reference genome is expected to significantly contribute to comparative analysis of genome biology and evolution within Monilinia species.

Draft Genomic Resources for the Brown Rot Fungal Pathogen Monilinia laxa

Monilinia laxa is the causal agent of brown rot on stone fruit, and it can cause heavy yield losses during field production and postharvest storage. This article reports the draft genome assembly of the M. laxa Mlax316 strain, obtained using a hybrid genome assembly with both Illumina short-reads and PacBio long-reads sequencing technologies. The complete draft genome consists of 49 scaffolds with total size of 42.81 Mb, and scaffold N₅₀ of 2,449.4 kb. Annotation of the M. laxa assembly identified 11,163 genes and 12,424 proteins which were functionally annotated. This new genome draft improves current genomic resources available for M. laxa and represents a useful tool for further research into its interactions with host plants and into evolution in the Monilinia genus.

Impact of Postharvest Storage on the Infection and Colonization of Penicillium digitatum and Penicillium expansum on Nectarine

Very few studies have investigated the host-pathogen interaction of Penicillium spp. on nectarine. Penicillium digitatum was identified as pathogenic and highly aggressive on nectarine. A strong association was made with host age/ripeness. This points to a new mechanism or life strategy used by P. digitatum to infect and colonize previously thought nonhosts. The aim of this study was to determine the effect of postharvest storage of nectarine on the infection and colonization of P. digitatum and Penicillium expansum at molecular and physical (firmness and pH) levels. The impact of environmental conditions (cold storage) and pathogen pressure (inoculum load) was also investigated. Although disease incidence was much lower, lesions caused by P. digitatum were similar in size to those caused by P. expansum on freshly harvested nectarine. Disease incidence and lesion diameter significantly increased (larger than P. expansum) on longer stored fruit. Cold storage had the largest effect on P. digitatum. Inoculum load had a meaningful effect on both Penicillium spp. Storage significantly affected pH modulation and gene expression. The pathogens not only decreased but also, increased and maintained (similar to initial pH of the host) pH of infected tissue. The polygalacturonase (PG) gene and creA were upregulated by P. digitatum on 7-day postharvest fruit (other genes were unaffected). It partly explains the larger lesions on older or riper fruit. A different expression profile was observed from P. expansum: strong downregulation in PG and slight upregulation in pacC. Very different life strategies were used by the two Penicillium spp. when infecting nectarine. Unlike what is known on citrus, P. digitatum showed an opportunistic lifestyle that takes advantage of specific host and environmental conditions. It is largely still unclear (gene expression) what specifically triggers the increase in disease incidence (infection) and lesion diameter (colonization) of P. digitatum on older or riper fruit. The differences between in vivo and in vitro studies make it difficult to directly correlate results. Additional research is still needed to differentiate and understand the infection and colonization of these pathogens on the same host.

Preventive Fungicidal Control of Cytospora leucostoma in Peach Orchards in Colorado

In western Colorado, Cytospora leucostoma is ubiquitous in peach orchards and has developed into a major limiting factor of peach production. The pathogen is unable to invade healthy intact phloem tissue of the tree, but instead, it requires a wound as a mode of entry. Bark injuries caused by cold and pruning in commercial orchard systems provide infection courts that, in suitable environment conditions, can lead to many successful fungal infections. Preventive fungicide control is an integral component of management in tree fruit production. Eighteen fungicides were tested at selected label dose rates for C. leucostoma control. All treatments were initially tested in vitro in fungicide-amended media dishes. Successful treatments were then tested under controlled conditions on detached peach branch segments. Effective fungicides identified in the laboratory assays (thiophanate-methyl, captan, lime sulfur, and copper hydroxide) were further tested as spray applications in the field and as wound sealant applications in combination with latex paint and kaolin clay. Of the treatments evaluated, thiophanate-methyl, captan, 50% latex paint, thiophanate-methyl amended in 50% latex paint, captan amended in 50% latex paint, and lime sulfur were most effective in reducing C. leucostoma necrotic area. Copper hydroxide was ineffective in all field trials and in some instances, yielded larger necrotic areas than the nontreated positive control shoots.

Co-Infection with Three Mycoviruses Stimulates Growth of a Monilinia fructicola Isolate on Nutrient Medium, but Does Not Induce Hypervirulence in a Natural Host

Monilinia fructicola and Monilinia laxa are the most destructive fungal species infecting stone fruit (Prunus species). High-throughput cDNA sequencing of M. laxa and M. fructicola isolates collected from stone fruit orchards revealed that 14% of isolates were infected with one or more of three mycoviruses: Sclerotinia sclerotiorum hypovirus 2 (SsHV2, genus Hypovirus), Fusarium poae virus 1 (FPV1, genus Betapartitivirus), and Botrytis virus F (BVF, genus Mycoflexivirus). Isolate M196 of M. fructicola was co-infected with all three viruses, and this isolate was studied further. Several methods were applied to cure M196 of one or more mycoviruses. Of these treatments, hyphal tip culture either alone or in combination with antibiotic treatment generated isogenic lines free of one or more mycoviruses. When isogenic fungal lines were cultured on nutrient agar medium in vitro, the triple mycovirus-infected parent isolate M196 grew 10% faster than any of the virus-cured isogenic lines. BVF had a slight inhibitory effect on growth, and FPV1 did not influence growth. Surprisingly, after inoculation to fruits of sweet cherry, there were no significance differences in disease progression between isogenic lines, suggesting that these mycoviruses did not influence the virulence of M. fructicola on a natural host.

Volatile Compounds Related to 'Stone Fruit' Aroma Attributes in Viognier and Chardonnay Wines

A 'stone fruit' aroma is important in many white wine varieties and styles, but little is known about the chemical basis of this wine aroma attribute. A set of Viognier and Chardonnay wines that featured 'stone fruit' aroma attributes were selected by a panel of wine experts. The selected wines were characterized by sensory descriptive analysis and detailed volatile chemical composition analyses. This comprehensive data also allowed Viognier wine to be profiled for the first time. By partial least-squares regression, several esters and fatty acids and benzaldehyde were indicated as contributing to the 'peach' attribute; however, a reconstitution sensory study was unsuccessful in mimicking this attribute. A mixture of γ-lactones, monoterpenes, and aldehydes were positively correlated to the 'apricot' aroma, which were generally higher in the Viognier wines. Reconstitution studies confirmed that the monoterpenes linalool, geraniol, and nerol were the most important compounds for the mixture being perceived as having an 'apricot' aroma.

Thyme and Savory Essential Oil Vapor Treatments Control Brown Rot and Improve the Storage Quality of Peaches and Nectarines, but Could Favor Gray Mold

The effect of biofumigation, through slow-release diffusors, of thyme and savory essential oils (EO), was evaluated on the control of postharvest diseases and quality of peaches and nectarines. EO fumigation was effective in controlling postharvest rots. Naturally contaminated peaches and nectarines were exposed to EO vapors for 28 days at 0 °C in sealed storage cabinets and then exposed at 20 °C for five days during shelf-life in normal atmosphere, simulating retail conditions. Under low disease pressure, most treatments significantly reduced fruit rot incidence during shelf-life, while, under high disease pressure, only vapors of thyme essential oil at the highest concentration tested (10% v/v in the diffusor) significantly reduced the rots. The application of thyme or savory EO favored a reduction of brown rot incidence, caused by Monilinia fructicola, but increased gray mold, caused by Botrytis cinerea. In vitro tests confirmed that M. fructicola was more sensitive to EO vapors than B. cinerea. Essential oil volatile components were characterized in storage cabinets during postharvest. The antifungal components of the essential oils increased during storage, but they were a low fraction of the volatile organic compounds in storage chambers. EO vapors did not influence the overall quality of the fruit, but showed a positive effect in reducing weight loss and in maintaining ascorbic acid and carotenoid content. The application of thyme and savory essential oil vapors represents a promising tool for reducing postharvest losses and preserving the quality of peaches and nectarines.

Factors Limiting Peach as a Potential Host for Drosophila suzukii (Diptera: Drosophilidae)

The spotted wing drosophila, Drosophila suzukii Matsumura, has widely established in North America and become an economic concern for a variety of fruit crops. To better understand fruit susceptibility, we evaluated peach surface characteristics on the pest's oviposition success. The number of D. suzukii eggs laid into the fruit flesh was tested on 1) peaches with or without indumenta (commonly referred to as peach fuzz), 2) peaches physically damaged by harvest operations, 3) peaches damaged by the peach twig borer Anarsia lineatella Zeller or the forktailed bush katydid, Scudderia furcata Brunner von Wattenwyl, and 4) peaches with punctures that simulated stink bug damage. Female D. suzukii did not lay eggs in intact fuzzy sections of the fruit or into small punctures (0.3 or 0.5 mm), but readily laid eggs in sections without fuzz, with insect damage, and with large punctures (1 mm). The number of eggs per treatment was positively related to the area of the damaged section; the overall fruit firmness and sugar content was not related to the number of eggs laid in treated or damaged spots. Direct observations of D. suzukii oviposition confirmed that peach fuzz appeared to be an obstacle for the fly's oviposition success, and female flies ceased ovipositional attempts on fuzzy peach sections after a short period of time. Successful oviposition times were associated with substrate firmness, with shorter oviposition time in damaged spots than in cherry fruit or shaved spots of the peach. The results indicate that intact, preharvest peach fruit are unlikely to be infested by the fly, but any surface damage could render the fruit susceptible to the fly.

The Use of Hierarchical Sampling in the Surveillance Program for Plum pox virus Incidence in the United States

Plum pox virus (PPV) was first reported in North America in late 1999. As part of the eradication effort mounted in response to this finding, a Plum Pox Virus National Surveillance Program has been implemented in the United States. The survey method adopted for this program is a form of "group testing" that utilizes relationships between disease incidence at two levels in a spatial hierarchy, known as hierarchical sampling. Leaf material is collected from groups of four trees and tested by enzyme-linked immunosorbent assay (ELISA) for PPV infection without distinguishing between the individual trees in a group. PPV incidence at the scale of the individual tree is then calculated by means of a formula. Incidence data were collected during an initial survey conducted by the Pennsylvania Department of Agriculture from nine PPV-infected orchards in the fall of 1999 and from an additional 11 PPV-infected orchards during the spring and summer of 2000. The pattern of PPV-infected trees at the within-group scale was weakly aggregated. As a result, an effective sample size of v = 3.2, rather than the nominal group size of four, should be used in the formula to calculate an estimate of incidence at the scale of the individual tree from observations of incidence collected at the group scale.

Analysis of Factors Affecting Latent Infection and Sporulation of Monilinia fructicola on Prune Fruit

Two studies were conducted to determine the effects of water content (WC) on sporulation on thinned fruit and the effects of wetness duration, inoculum density, and temperature on secondary infection of prune fruit by Monilinia fructicola, the main causal pathogen of brown rot in California. In the first study, sporulation intensity and duration of sporulation of the pathogen were tested on inoculated thinned fruit with five levels (67.2, 53.8, 40.3, 26.9, and 13.4%) of WC. Regression analyses showed that both sporulation intensity and duration of sporulation increased as WC of thinned fruit increased. The predicted difference in duration of sporulation between fruit with 13.4 and 67.2% WC was about 3 days. In the second study, three inoculum concentrations (8,000, 16,000, and 24,000 conidia per milliliter) of M. fructicola were atomized onto prune fruit on trees in an orchard. Inoculated fruit and shoots were covered with plastic bags to maintain wetness duration for 4, 8, 12, or 16 h. An overnight freezing and incubation technique was used after harvest to determine the proportion of fruit with latent infection. Regression analysis demonstrated that inoculum concentration and wetness duration were significant factors affecting secondary infection. Temperature was less important. Increased inoculum concentration and wetness duration increased the percentage of fruit with latent infections. Increased temperature decreased the percentage of fruit with latent infections.

Fumigation of Fruit with Short-Chain Organic Acids to Reduce the Potential of Postharvest Decay

Vapors of acetic (1.9 or 2.5 μl/liter), formic (1.2 μl/liter), and propionic (2.5 μl/liter) acids were tested for postharvest decay control on 8 cherry, 14 pome, and 3 citrus fruit cultivars. Surfacesterilized fruit were inoculated with known fungal pathogens by drying 20-μl drops of spore suspension on marked locations on each fruit, placing at 10°C to equilibrate for approximately 24 h, and fumigating by evaporating the above acids in 12.7-liter airtight fumigation chambers for 30 min. Immediately after fumigation, the fruit were removed, aerated, aseptically injured, and placed at 20°C until decay occurred. All three fumigants controlled Monilinia fructicola, Penicillium expansum, and Rhizopus stolonifer on cherry. Formic acid increased fruit pitting on six of eight cultivars and was the only organic acid to increase blackening of cherry stems when compared to the control. Decay of pome fruit caused by P. expansum was reduced from 98% to 16, 4, or 8% by acetic, formic, and propionic acids, respectively, without injury to the fruit. Decay of citrus fruit by P. digitatum was reduced from 86 to 11% by all three acids, although browning of the fruit peel was observed on grapefruit and oranges fumigated with formic acid.