Reduced sensitivity of tomato early blight pathogen (Alternaria solani) isolates to protectant fungicides, and implication on disease control

The sensitivity of Alternaria solani isolates to the fungicides mancozeb and chlorothalonil was evaluated, to determine if inadequate disease management by these fungicides could be attributed to reduced sensitivity of A. solani isolates to these fungicides. The sensitivity of 60 isolates of A. solani was assessed using the inhibition of radial mycelial growth (RG) method, using fungicide concentrations of 0, 1.0, 10, 100, 500 and 1000 μg a.i ml−1 medium. EC50 was calculated for each isolate and fungicide combination. The EC50 values of different A. solani isolates to mancozeb ranged from 9.05 to 712.65 μg ml−1. EC50 values of different isolates to chlorothalonil ranged from 4.25 to 849.4 μg ml−1. The percentage of isolates with reduced sensitivity was 46.7 and 53.3% for mancozeb and chlorothalonil, respectively. Results of the in vivo tests demonstrated decline in disease control by the two fungicides with the reduced-sensitivity isolates compared to the sensitive ones.