Sensitivity of Septoria lycopersici Speg. isolates to fungicides in Brazil

Disease management using synthetic fungicides is the main strategy used by tomato growers to prevent tomato yield reductions caused by Septoria leaf spot (SLS). Despite the importance of this plant disease, there is little information about the sensitivity of Septoria lycopersici isolates to fungicides. The sensitivity of 94 isolates of S. lycopersici to azoxystrobin, chlorothalonil, tebuconazole, and thiophanate-methyl was assessed in experiments using fungicide-amended 96-well microtiter plates. The highest EC 50 values above 100 mg/L were detected for azoxystrobin, thiophanate-methyl, and tebuconazole. Based on these results, five isolates classified as resistant and five as sensitive to each fungicide were selected for the experiments. Tomato plants were inoculated with sensitive (S) and resistant (R) isolates and sprayed by the fungicides in three greenhouse assays. The differences in the values of the area under the progress curve of Septoria leaf spot (AUDPC), disease severity progress, and progress rates were not so pronounced between tomato plants sprayed and non-sprayed with fungicides. The AUDPC values for plants inoculated with S and R isolates and treated with chlorothalonil, tebuconazole, and thiophanate-methyl ranged from 10 to 320, 0 to 80, and 50 to 150, respectively. For azoxystrobin, the AUDPC values were around 200 in plants inoculated with S and R isolates. There is evidence of reduced sensitivity of individuals of S. lycopersici to azoxystrobin, tebuconazole, and thiophanate-methyl in the population in Brazil. Therefore, tomato growers should rotate and combine fungicides with different mechanisms of action and low-risk of resistance to control SLS.