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POSTERS: New and emerging diseases

Hemp leaf spot, caused by a Bipolaris sp., is a major disease threat for industrial hemp
Desiree Szarka - University of Kentucky. Jolanta Jaromczyk- University of Kentucky, Edward Dixon- University of Kentucky, Nicole Ward-Gauthier- University of Kentucky, Julie Beale- University of Kentucky, Christopher Schardl- University of Kentucky, Bernadette Amsden- University of Kentucky, Hua

Hemp leaf spot has become a major disease threat for field-grown hemp. Between 2014 and 2018, disease was confirmed in nine counties among at least 16 different fields in Kentucky; similar cases have been reported in at least five other states. Disease affected fiber, grain, and cannabidiol (CBD) cultivars similarly. Disease symptoms began in July or August and became progressively more severe until harvest in September. During rainy weather or in cases of early infection, growers deemed fields too severely affected for harvest. Leaf spots typically expanded to 1-2 mm, and spots frequently coalesced. Affected plants became blighted and sometimes stunted; floral materials such as calyces and sugar leaves became necrotic. Morphological characteristics of the causal fungus matched published descriptions of Drechslera gigantea. Large conidia and conidiophores, each documented at over 400 ?m long, deemed this pathogen distinct among other leaf-spotting fungi. Microconidia developed in culture, but none were identified from field samples. Sclerotia-like structures developed in culture, as well. Phylogenetic analysis placed this pathogen within the genus Bipolaris, with sequence matches to D. gigantea. Thus, we propose that the causal agent of hemp leaf spot is D. gigantea, with support for reclassification to Bipolaris. Implications of this finding are mostly unknown, as all previous reports of D. gigantea are from monocot hosts, in contrast to Cannabis sativa, a dicot. Expanding acreage of industrial hemp in the US may experience severe losses as a result of this new disease.