Assessment of Fungicide Resistance via Molecular Assay in Populations of Podosphaera leucotricha, Causal Agent of Apple Powdery Mildew, in New York.

Podosphaera leucotricha, causal agent of apple powdery mildew, is a pathogen endemic worldwide where apples are produced. In the absence of durable host resistance, the disease is most effectively managed in conventional orchards with single-site fungicides. In New York State, increasingly erratic precipitation patterns and warmer temperatures due to climate change may create a regional environment more conducive to apple powdery mildew development and spread. In this scenario, outbreaks of apple powdery mildew may supplant the apple diseases of current management concern: apple scab and fire blight. Presently, there have been no reports from producers of fungicide control failures for apple powdery mildew, though increased disease incidence has been reported to and observed by the authors. As such, action was needed to assess the fungicide resistance status of populations of P. leucotricha to ensure key classes of single-site fungicides (FRAC 3, demethylation inhibitors, DMI; FRAC 11, quinone outside inhibitors, QoI; and FRAC 7, succinate dehydrogenase inhibitors, SDHI) remain effective. In a 2-year survey (2021 to 2022), we collected 160 samples of P. leucotricha from 43 orchards, representing conventional, organic, low-input, and unmanaged orchards from New York's primary production regions. Samples were screened for mutations in the target genes (CYP51, cytb, and sdhB) historically known to confer fungicide resistance in other fungal pathogens to the DMI, QoI, and SDHI fungicide classes, respectively. Across all samples, no nucleotide sequence mutations that translated into problematic amino acid substitutions were found in the target genes, suggesting that New York populations of P. leucotricha remain sensitive to the DMI, QoI, and SDHI fungicide classes, provided no other fungicide resistance mechanism is at play in the population.

Characterizations of an Emerging Disease: Apple Blotch Caused by Diplocarpon coronariae (syn. Marssonina coronaria) in the Mid-Atlantic United States.

Apple orchards with minimal or reduced fungicide inputs in the Mid-Atlantic region of the United States have experienced outbreaks of severe premature defoliation with symptoms that matched those of apple blotch disease (ABD) caused by Diplocarpon coronariae. Fungal isolates obtained from symptomatic apple leaves and fruit produced uniform slow-growing, dark-gray colonies on peptone potato dextrose agar and had conidia. Internal transcribed spacer DNA sequences matched with D. coronariae and Koch's postulates were fulfilled when typical ABD symptoms occurred when reinoculated onto apple leaves and fruit. Spore dispersal in nonfungicide-treated orchards detected with quantitative PCR was low in early spring and dropped to undetectable levels in late May and early June before rising exponentially to highs in July and August, which coincided with symptom development. Only low spore numbers were detected in fungicide-treated orchards and nearby forests. In preliminary fungicide tests, fluxapyroxad, thiophanate methyl, and difenoconazole effectively inhibited mycelial growth of isolates in vitro. When apple cultivars Fuji and Honeycrisp were inoculated with D. coronariae, Honeycrisp showed delayed onset of symptoms and lower disease severity, and the transcription profile of seven host defense-related genes showed that PR-2, PR-8, LYK4, and CERK1 were highly induced in Honeycrisp at 2 and 5 days postinoculation. This is the first report of ABD in the Mid-Atlantic United States, which includes studies of seasonal D. coronariae spore dispersal patterns, preliminary fungicide efficacy, and host defense-related gene expression to assist development of best ABD management practices.

'Honeycrisp' Bitter Pit Response to Rootstock and Region under Eastern New York Climatic Conditions.

There are still unknown factors at play in the causation of bitter pit in 'Honeycrisp' as well as in other apple varieties. To investigate some of these factors, we conducted a survey of 34 'Honeycrisp' orchard blocks distributed across two disparate production regions in eastern New York State, representing a variety of rootstocks, over three growing seasons. Weather, soil, horticultural traits, fruit quality traits, pick timing, leaf and peel minerals were evaluated for their impact on bitter pit (BP) incidence; factors were further evaluated for their interaction with region and rootstock. 'Honeycrisp' trees on B.9 rootstock were smaller but with comparable terminal shoot growth when compared to those on M.26 and M.9 rootstocks. B.9 fruits, which had similar fruit size to M.26 and M.9 and had good fruit quality at harvest and after storage, were much less likely to express bitter pit symptoms compared to M.9 and M.26 rootstocks. Not all traits evaluated individually correlated significatively with bitter pit incidence after a period in storage. Depending on rootstock and region, the correlation could be significant in one situation, with no correlation at all in another. In this study, peel Mg/Ca ratio and peel Ca correlated with BP for all three rootstocks, with the strongest correlations associated with the M.9 clones. These same traits correlated with BP for both regions. Pick timing had a significant influence on BP incidence following storage, with later picks offering better bitter pit storage performance. While excessively large fruits, those in the 48 and 56 count size categories, were found to be highly susceptible to BP regardless of rootstock, B.9 BP fruit susceptibility for smaller sizes was found to be size neutral. A PLSR prediction model for each rootstock and each region showed that different variables correlated to BP depending on the situation. Thus, the results could suggest that in addition to the variables considered in this study, there are other less studied factors that can influence the expression of BP symptoms. We strongly suggest that rootstock BP performance be considered a critical parameter when planning a commercial 'Honeycrisp' orchard and be evaluated in rootstock breeding and development programs prior to wide commercial release.

Xylosandrus germanus (Coleoptera: Curculionidae: Scolytinae) Occurrence, Fungal Associations, and Management Trials in New York Apple Orchards.

Xylosandrus germanus (Blandford) has caused increasing damage in high-density New York apple orchards since 2013, resulting in tree decline and death. We documented their occurrence and timing in > 50 orchards using ethanol-baited traps from 2014 to 2016. First captures ranged from 48 to 83 degree days (base 10 °C) from 1 January. Captures were numerically higher at the orchard-woods interface than within the orchard interior, but differences were not significant in locations with lower populations. Control using insecticide trunk sprays was tested in potted, waterlogged apple trees placed in orchards and nurseries, and inside wooded areas adjacent to orchards. A verbenone repellent was used in combination with trunk sprays to improve control. Overall, insecticide sprays were inconsistent and marginal in preventing new infestations. Chlorpyrifos significantly reduced infestations versus lambda-cyhalothrin and untreated trees at one location in the 2015 orchard trials, and versus untreated trees at one location in the 2016 nursery trials, but otherwise performed no better than other treatments. The addition of verbenone to either the check or permethrin treatments resulted in significantly fewer attack sites containing brood at one orchard site in 2016. Chlorpyrifos, lambda-cyhalothrin, and permethrin significantly reduced the number of attack sites containing adults compared with untreated trees at one nursery trial location in 2016, but were otherwise ineffective in reducing numbers of trees in other locations and infestation categories. We found several fungal and bacterial species associated with X. germanus and its infestation of apples. These microbes likely play a minimal role in apple decline.