A Decision Support System Based on Degree-Days to Initiate Fungicide Spray Programs for Peach Powdery Mildew in Catalonia, Spain

Modeling the Airborne Inoculum of Polystigma amygdalinum to Optimize Fungicide Programs Against Almond Red Leaf Blotch

Red leaf blotch (RLB) of almond, caused by the ascomycete Polystigma amygdalinum, is a severe foliar disease endemic in the Mediterranean Basin and Middle East. Airborne ascospores of P. amygdalinum were monitored from 2019 to 2021 in two almond orchards in Lleida, Spain, and a Bayesian beta regression was used to model its seasonal dynamics. The selected model incorporated accumulated degree-days (ADD), ADD considering both vapor pressure deficit and rainfall as fixed effects, and a random effect for the year and location. The performance of the model was evaluated in 2022 to optimize RLB fungicide programs by comparing the use of model predictions and action thresholds with the standard program. Two variants were additionally considered in each program to set the frequency between applications, based on (i) a fixed frequency of 21 days or (ii) specific meteorological criteria (spraying within 7 days after rainfalls greater than 10 mm, with daily mean temperatures between 10 and 20°C, and with a minimum frequency of 21 days between applications). Programs were evaluated in terms of RLB incidence and number of applications. The program based on the model with periodic fungicide applications was similarly effective as the standard program, resulting only in a 2.6% higher RLB incidence but with fewer applications (three to four, compared with seven in the standard program). When setting the frequency between applications by using the meteorological criteria, a higher reduction in the number of applications (two to three) was observed, while RLB incidence increased by roughly 16% in both programs. Therefore, the model developed in this study may represent a valuable tool toward a more sustainable fungicide schedule for the control of almond RLB in northeast Spain.

Refining Management of Apple Powdery Mildew in New York State with Weather-Based Fungicide Application Timing Programs

In the absence of durable host resistance among commercial cultivars, chemical management continues to be an essential component of disease control in apple production. Apple powdery mildew, caused by the ascomycete Podosphaera leucotricha, is largely managed with regular fungicide applications from the host phenological stages of tight cluster to terminal bud growth set, with applications typically being made in a prophylactic manner irrespective of existing disease pressure. Here we evaluated two management programs that aligned fungicide applications to specific weather thresholds conducive to powdery mildew development using a rotation of single-site fungicides and sulfur. In three separate orchards among four cultivars, we compared powdery mildew disease progression over the growing season for each of the weather factor-based programs and a typical calendar-based application program. In each year of the trial, we found that management programs with weather-based fungicide applications provided levels of disease control similar to the calendar program but required 50 to 83.3% fewer mildew-specific fungicide applications throughout the growing season. Our results provide a framework with which to evaluate future weather-based management programs for apple powdery mildew management. This knowledge could be implemented in the creation of a powdery mildew disease management decision support system to better inform and aid fungicide application programs for continued sustainable apple production in the northeast United States.

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.

Fine mapping and identification of candidate genes for the peach powdery mildew resistance gene Vr3

Powdery mildew is one of the major diseases of peach (Prunus persica), caused by the ascomycete Podosphaera pannosa. Currently, it is controlled through calendar-based fungicide treatments starting at petal fall, but an alternative is to develop resistant peach varieties. Previous studies mapped a resistance gene (Vr3) in interspecific populations between almond ('Texas') and peach ('Earlygold'). To obtain molecular markers highly linked to Vr3 and to reduce the number of candidate genes, we fine-mapped Vr3 to a genomic region of 270 kb with 27 annotated genes. To find evidence supporting one of these positional candidate genes as being responsible of Vr3, we analyzed the polymorphisms of the resequences of both parents and used near-isogenic lines (NILs) for expression analysis of the positional candidate genes in symptomatic or asymptomatic leaves. Genes differentially expressed between resistant and susceptible individuals were annotated as a Disease Resistance Protein RGA2 (Prupe2G111700) or an Eceriferum 1 protein involved in epicuticular wax biosynthesis (Prupe2G112800). Only Prupe2G111700 contained a variant predicted to have a disruptive effect on the encoded protein, and was overexpressed in both heterozygous and homozygous individuals containing the Vr3 almond allele, compared with susceptible individuals. This information was also useful to identify and validate molecular markers tightly linked and flanking Vr3. In addition, the NILs used in this work will facilitate the introgression of this gene into peach elite materials, alone or pyramided with other known resistance genes such as peach powdery mildew resistance gene Vr2.