Sensitivity of Peronophythora litchii to dimethomorph

Litchi downy blight caused by Peronophythora litchii is one of the most prevalent diseases of litchi in China. Dimethomorph has been used to control the disease for more than 10 years. The survey of the fungicide resistance was conducted in this paper. The median effective concentration (EC50) values of dimethomorph to 115 isolates of P. litchii collected from Guangdong, Guangxi, Fujian and Yunnan during 2009 to 2012 were tested by mycelia radial growth inhibition method. The results showed that EC50 values ranged from 0.0977 g/mL to 0.2226 g/mL. The average of EC50 value was 0.1702 (0.0026) g/mL. The EC50 value of the most insensitive isolate was 2.28 folds of the most sensitive one. The EC50 value frequency of all tested isolates distributed as a continuous unimodel curve. It indicated that there was no resistant subpopulation among these isolates