In-vitro sensitivity to fungicides of Corynespora cassiicola isolates from different hosts and geographic regions from Brazil

No fungicides are specifically registered for the control of Corynespora cassiicola on tomato or cucumber in Brazil. Current control is achieved through the use of fungicides registered for early blight on tomato or for target spot on soybean. This constitutes an illegal and inappropriate use of fungicides that may compromise the effectiveness of some products. Our objective was to determine the sensitivity of C. cassiicola isolates from different hosts and geographic regions of Brazil to several common fungicides. In-vitro tests on fungicide-amended media were performed using C. cassiicola isolates to determine the effective dose necessary to decrease mycelial growth by 50% (ED50), relative to growth on a non-amended medium. The fungicides chlorothalonil and carbendazim exhibited an average ED50 of >50 mg L-1. For tebuconazole, the ED50 varied from 0.50 to 18.79 mg L-1. The ED50 values for pyrimethanil, fludioxonil and cyprodinil were substantially lower, with average values of ≤10 mg L-1. Discriminatory rates of 1 mg L-1 for fludioxonil, 3 mg L-1 for cyprodinil, 5 mg L-1 for boscalid and fluopyram, and 10 mg L-1 for tebuconazole, azoxystrobin and pyrimethanil were used to differentiate the sensitivity of additional isolates to each fungicide based on percentage mycelial growth inhibition (MGI). Based on MGI, great variation for each fungicide was observed among the 55 C. cassiicola isolates tested. Fludioxonil showed the highest efficacy against C. cassiicola isolates compared with the other fungicides tested, with all isolates classified as sensitive (MGI >50%). Chlorothalonil showed the lowest efficacy, with 91% of tested isolates exhibiting an MGI <50%. Based on MGI, isolate EH-1068 was considered insensitive to seven of the nine fungicides tested, while isolate EH-1720 was considered sensitive to eight fungicides. The results showed that fludioxonil would be a good option for the control of target spot.