Benomyl fungicide (Benlate) is used worldwide to control ascomycete pathogens, but resistance has developed in several pathogen populations (1). On the Canadian prairies, benomyl is used to reduce injury caused by Sclerotinia sclerotiorum (Lib.) de Bary on canola (Brassica napus, B. rapa) and alfalfa (Medicago sativa) seed crops. To determine if populations are resistant to benomyl, isolates of S. sclerotiorum collected from 15 fields (12 alfalfa and 3 canola, one isolate per field) in 2000 were grown on potato dextrose agar amended with benomyl at 0, 0.05, 0.5, 5, 50, and 500 mg/liter. Plugs of mycelium from the margin of an actively growing colony were placed in the center of a 10-cm-diameter petri dish containing 15 ml of test medium and incubated on a laboratory bench. Linear growth (mean of maximum width and right angle) of each colony (three replicates each) was measured after 5 to 6 days. The growth of isolates from 13 fields was inhibited by low concentrations of benomyl (EC50 < 8 mg/liter), but two isolates were very resistant (EC50 > 200 mg/liter). Resistant cultures were isolated from infected canola plants in the only two fields in the study in which reduced efficacy of benomyl was suspected. The distribution and importance of benomyl-resistant populations of S. sclerotiorum in the region remains to be determined.
Reference: (1) T. R. Pettitt et al. Mycol. Res. 97:1172, 1993.