P-538 - Alternaria alternata: the major fungal species causing foliar disease in cucurbits in South Dakota

Cucurbits comprise the largest acreage in vegetable production in South Dakota. The prevalence of foliar fungal diseases affecting cucurbit crops in this state is unknown. The objective of this study was to identify the most common species of foliar fungal pathogens affecting cucurbits in South Dakota through state-wide farm surveys. In 2023, 13 farms were surveyed, and 422 leaves were recovered from pumpkin, cucumber, watermelon, squash, and bottle gourd. Alternaria spp. (79.2%) Fusarium spp. (10.9%), Stagonosporopsis spp. (4.5%), and other genera (3.2%) of fungi were identified using a combination of microcopy and sequencing of the ITS region. A subset of Alternaria spp. isolates (n=103) recovered from cucumber and pumpkin were selected for multi-locus sequencing to determine species. TEF, GAPDH and RPB2 loci were sequenced, and a combined maximum likelihood and Bayesian phylogeny was constructed. Unique haplotypes (n=25) of Alternaria spp. were identified. Haplotypes (9), comprising 64 isolates, clustered with Alternaria alternata and A. tenuissima (synonymous with A. alternata). Minimum spanning networks were constructed in PopART to visualize haplotype diversity in relation to potential population stratifying factors. We identified no influence of region (eastern, central or western South Dakota), production environment (open field or high tunnel) or host (pumpkin or cucumber). These findings will help better inform future management strategy decisions for South Dakota farmers.