P-169 - Confirmation of Alternaria carotiincultae as a pathogen of carrot and barley in Michigan

Black rot, caused by Alternaria radicina and A. carotiincultae, is a destructive carrot disease worldwide. In 2005, A. radicina was isolated from stored carrots in Michigan. From 2021 to 2023, foliar blight and root rot were observed in carrot fields. We sought to identify the causal agent(s), confirm pathogenicity of isolates, and assess the susceptibility of barley seedlings, which are used as a cover crop in carrot production. Alternaria isolates (95) were recovered from symptomatic carrot tissue from eight fields and from culls. Isolates had smooth colony margins, produced yellow pigment in culture, and some produced dendritic crystals in culture. The characteristics were intermediate to those reported for the two Alternaria spp. Phylogenetic analysis of 3 genes (β-tub, EF-1α, and Alt a1) was performed to confirm the species. All isolates were identified as A. carotiincultae. Representative isolates (10) were inoculated onto carrot roots; they produced black rot symptoms. The fungus was reisolated from lesions and compared to original cultures. A representative isolate was inoculated onto barley seedling foliage and roots using a conidial spray or colonized millet, respectively. Regardless of inoculation method, a reduction in root mass compared to an uninoculated control was observed. Other symptoms included foliar chlorosis, wilt, and root rot. This is the first report of A. carotiincultae on carrot in Michigan. The barley cover crop, used by carrot growers, may play a role in the disease cycle of A. carotiincultae.