Sensitivity of Soybean (Glycine max) Pathogens Diaporthe aspalathi, D. caulivora, and D. longicolla to Difenoconazole and Fluopyram Fungicides

Diseases of soybean (Glycine max L.) caused by species of Diaporthe have resulted in estimated yield losses totaling $248.2 million in the United States over the past 10 years. To effectively manage species of Diaporthe, it is important to use an integrated approach. In this study, we evaluated the in vitro sensitivity of isolates of the soybean pathogens D. aspalathi, D. caulivora, and D. longicolla from 18 U.S. states to difenoconazole (a demethylation inhibitor fungicide) and fluopyram (a succinate dehydrogenase inhibitor fungicide). The fungicides were incorporated into 2% water agar (WA) in Petri dishes at various concentrations. A mycelial plug of each isolate (n = 59 for difenoconazole and n=55 for fluopyram) obtained froma 7-dayold culture was placed at the center of the WA and incubated in the dark. After 5 days for D. caulivora and D. longicolla and 8 days for D. aspalathi, the colony diameter was measured, and the corresponding percent inhibition and effective concentration at which 50%mycelial growthwas inhibited (EC50)were determined. Significant differences in EC50 values (P < 0.0001) were observed among the isolates of D. aspalathi (0.227 μg/ml), D. caulivora (0.130 μg/ml), and D. longicolla (1.860 μg/ml) for difenoconazole. Similarly, for fluopyram, the EC50 values varied significantly (P < 0.001) among the D. aspalathi (2.233 μg/ml), D. caulivora (1.610 μg/ml), and D. longicolla (0.347 μg/ml) isolates. This study established sensitivity profiles for difenoconazole and fluopyram fungicides for D. aspalathi, D. caulivora, and D. longicolla and provides valuable information that may help in the development of a Diaporthe disease management program.