Persistence of Fungicide Resistance in Grape Powdery Mildew
The fitness of Erysiphe necator resistant to QoI/FRAC 11 fungicides was examined in terms of growth under various environmental stresses and overwintering survival and persistence. Eight Oregon E. necator isolates from the 2017 growing season were used. Four isolates were G143A mutants/QoI resistant and four were wild type/QoI sensitive, with two isolates each of MAT1-1 and MAT1-2 mating types. To analyze fitness under stress, a series of experiments were run where isolates were exposed to 10-32oC and germination, generation time, and mating were measured. The G143A mutant isolates had significantly greater germination at 14oC while all other temperatures had similar germination and growth. These results suggest that there may be a higher probability of G143A mutants infecting grapes in the early season and that QoIs should not be used during the early spring. The interaction of temperature and UVA and UVB radiative stress was examined, with results currently inconclusive as to whether there is a growth difference between wild type and G143A isolates. The same isolates were used in mating studies. Resulting chasmothecia have not fully matured and are still being observed. Previously designed real-time polymerase chain reaction (RT-PCR) primers for mating types were modified and optimized and determined to be poor at identifying mating type ratios in mixed samples. A digital droplet PCR was developed to quantify mating type ratios in mature chasmothecia, and will be used for future mating type analysis. Leaf swab samples were collected over 2018 and 2019 from 11 commercial vineyards untreated with QoIs. Samples showed increased G143A frequency from May to September. Chasmothecia sampling showed decreased G143A frequency in spring, suggesting fitness costs associated with overwintering.