Evaluation of fungicide resistance in Alternaria alternata isolates of potato in Isfahan region

Early blight of potato caused by Alternaria species is one of the most destructive and prevalent diseases of potato in the world. In recent years, the disease has become important in Iranian potato-producing regions, especially in Isfahan province, and farmers often use fungicides to control the disease. Considering the problem of resistance to some common fungi, this study aimed to evaluate the susceptibility/resistance of nine isolates of A. alternata to conventional fungicides (mancozeb, chlorothalonil, iminoctadine tris, Concento) by using vegetative growth inhibition method under in vitro and greenhouse conditions. The results revealed that most isolates collected from potato fields that were regularly treated with fungicides such as mancozeb exhibited less sensitivity to mancozeb. In contrast, against chlorothalonil, the least difference was observed in the susceptibility of different isolates. The greenhouse experiment showed that the highest control of the disease could be achieved by chlorothalonil and mancozeb treatment. These two fungicides at their recommended concentrations reduced the severity of the disease to less than 15 percent for 20 days after inoculation. Based on the results, concerning the resistance levels observed in the laboratory conditions, mancozeb application might be recommended in alternation with other fungicides such as chlorothalonil and iminoctadine tris.