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Poster: Biology & Disease Mgmt: Mycology


Identity, characteristics, and fungicide sensitivity of isolates in the Diaporthe species complex associated with soybean stem disease in Minnesota
D. MALVICK (1), C. Floyd (2) (1) University of Minnesota, U.S.A.; (2) University of Minnesota, U.S.A.

Late season stem diseases of soybean with symptoms of browning, canker, and top dieback are common in Minnesota. Characteristics of causal agents and factors that influence disease development are poorly understood. The objectives of this study were to: (i) assess the identity and relatedness among isolates in the Diaporthe spp. complex associated with both symptoms and asymptomatic infection, (ii) compare virulence among the isolates, and (iii) assess fungicide sensitivity. Isolates of D. longicolla (n=13) and D. phaseolorum var. caulivora (n=11) were obtained from symptomatic and asymptomatic plants across MN. The ITS and beta-tubulin genes were sequenced for all isolates and a phylogenetic tree with four branches was formed. Seedling stems of three soybean cultivars were inoculated with the isolates, and lesion length was measured to reveal significant differences in virulence among isolates. The groups in the consensus tree were associated with isolate species and virulence. No significant differences in sensitivity to the fungicides Folicure® 3.6F and Headline® were detected among isolates in vitro, and the mean EC50 for all isolates was 0.37 to 0.38 ug/ml. These studies reveal genotypic and virulence differences, but not differences in fungicide sensitivity, among isolates in the Diaporthe spp. complex from symptomatic and asymptomatic soybean stems, and suggest explanations for inconsistent disease development and avenues for disease management.