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POSTERS: Chemical and Biopesticide Control II

P-094 - QoI Fungicide Resistance in Alternaria spp. Isolates from Brassica Crops and Weeds: Aggressiveness and Distribution in Virginia

Tuesday, August 5, 2025
9:30 AM - 10:15 AM HST
Location: Exhibit Hall 3
Abstract Text: In the US mid-Atlantic region, Alternaria blight and head rot (ABHR) threatens brassica crops with significant yield loss due to the emergence of Alternaria spp. genotypes resistant to quinone outside inhibitor (QoI) fungicides, the main class used by farmers. Throughout Virginia, samples displaying disease symptoms were collected from brassica crops and weeds, and non-brassica weeds to determine pathogens’ resistance to QoI fungicide and aggressiveness. A subset of Alternaria spp. isolates (n=40) was screened for azoxystrobin sensitivity using a high-throughput microplate assay that measured spore germination inhibition via optical density. A four-parameter log-logistic model was fitted to calculate the effective concentration (EC50) at which 50% of spores germinate, using the ‘drc’ package in R. Sequencing of the cytB gene, specifically targeting the G143A and F129L mutations, was carried out via PCR using specific primers to identify mutations associated with resistant isolates of Alternaria spp. QoI sensitivity, pathogenicity, and aggressiveness of isolates varied by location and host from which they were collected. Fifty-five percent of isolates were sensitive (EC50 ranging from 0.009 to 0.365 ppm) 12.5% were moderately resistant (EC50 ranging from 17.87 to 69.55 ppm), and 32.5% were highly resistant (EC50 >100 ppm). Moreover, 45% of isolates recovered from broccoli, 86% from cabbage, and 17% from the weed Crepis capillaris were classified as resistant. Resistant isolates were found in all locations. Alternative fungicide management strategies are urgently needed.