Aggressive with Reduced Fungicide Sensitivity Can Be Associated with Naturally Infested Broccoli Seeds.

is a part of the complex that causes leaf blight and head rot (ABHR) in brassica crops. Infested broccoli seeds can play an important role in introducing in transplant houses and production fields. However, characterization of natural seed infestation and seed-to-seedling transmission of in broccoli is yet to be demonstrated. In this research, we characterized spp. isolates from commercial broccoli seedlots for their species identity, pathogenicity, and aggressiveness on broccoli and their sensitivity to a quinone-outside inhibitor (QoI) fungicide (azoxystrobin). Two hundred commercial seedlots from two broccoli cultivars, Cultivar 1 (EC; = 100 seedlots) and Cultivar 2 (ED; = 100 seedlots) were, evaluated for the presence of under conditions using a seedling grow-out assay. spp. was detected in 31 and 28% of the commercial seedlots of Cultivar 1 and Cultivar 2, respectively. The seed-to-seedling transmission (%) varied considerably within each positive-infested seedlot, which ranged from 1.3 to 17.3%. Subsequent molecular identification of single-spore cultures ( = 138) was made by sequencing four housekeeping genes: actin, the major allergen (Alta1), plasma membrane ATPase, and glyceraldehyde-3-phosphate dehydrogenase (GPD), and the sequences were concatenated and compared for the phylogenetic distance with diverse species. Ninety-six percent ( = 133) of the isolates formed a cluster with a known . based on a multigene phylogeny, which were later confirmed as using a species-specific PCR assay. One hundred percent of the seed isolates ( = 133) were either highly or moderately aggressive on broccoli (cultivar Emerald Crown) based on a detached leaf assay. Sensitivity of representative isolates ( = 58) to azoxystrobin was evaluated using a spore germination assay, and the EC values (effective fungicide concentration [ppm] at which germination of conidia of isolates were reduced by 50% compared to control) for each isolate was determined. isolates from naturally infested commercial broccoli seeds were sensitive to azoxystrobin with considerably low EC values in the range of <0.0001 to 0.33 ppm; however, there were a few isolates (14%) that showed 100-fold reduced sensitivity from the most sensitive isolate (EC = 0.0001 ppm). Our results confirm that commercial broccoli seedlots can be naturally contaminated with pathogenic and aggressive . We also provide evidence for the potential presence of isolates with reduced azoxystrobin-sensitivity in naturally infested commercial broccoli seedlots, which has never been reported before. Together, these findings may have implications in considerations for seed-health testing, seed treatments, and greenhouse scouting to limit introduction of infested seedlots in commercial broccoli fields.