The Comparative Root System Architecture of Declining and Non-Declining Trees in Two Apple Orchards in New York.

Rapid apple decline is a phenomenon characterized by a weakening of young apple trees in high density orchards, often followed by their quick collapse. The nature of this phenomenon remains unclear. In this work, we investigated the root system architecture (RSA) of declining and non-declining apple trees in two orchards in New York State. High-density orchard A consisted of 4-year-old 'Honeycrisp' on 'Malling 9 Nic29', and conventional orchard B consisted of 8-year-old 'Fuji' on 'Budagovsky 9'. In both orchards, a negative correlation (-0.4--0.6) was observed between RSA traits and decline symptoms, suggesting that declining trees have weaker root systems. Scion trunk diameter at the graft union, total root length, and the length of fine and coarse roots were significantly (p < 0.05) reduced in declining trees in both orchards. Additionally, internal trunk necrosis at, above, and below the graft union was observed in declining trees in orchard A but not in orchard B. Finally, latent viruses were not associated with decline, as their occurrence was documented in declining and non-declining trees in orchard A, but not in orchard B. Together, these results showed weakened root systems of declining trees, suggesting that these trees may experience deficiencies in water and nutrient uptake, although distinct RSA and trunk health traits between the two orchards were noticeable.

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.

'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.