Baseline and differential sensitivity to mandipropamid among isolates of Peronophythora litchii, the causal agent of downy blight on litchi

Abstract

Litchi downy blight caused by Peronophythora litchii is a devastating disease of litchi plants in China. Control of litchi downy blight requires numerous fungicide applications. A new carboxylic acid amide (CAA) fungicide, mandipropamid, was examined for its in vitro effects on multiple asexual stages of four single-sporangium P. litchii isolates and protective activity against downy blight in detached fruit assays. Though mandipropamid did not affect discharge of zoospores from sporangia, it strongly inhibited mycelial growth (mean EC50=0.0048μgml−1), sporangia production (mean EC50=0.0032μgml−1), germination of encysted zoospores (mean EC50=0.0023μgml−1), and germination of sporangia (mean EC50=0.0061μgml−1). On detached fruit, 0.39, 1.56 and 6.25μgml−1 of mandipropamid were superior in reducing downy blight compared to metalaxyl and flumorph, however, the 25μgml−1 application rate was necessary for all three CAA fungicides to completely inhibit the disease. In 2007, 100 isolates from Fujian, Guangdong, and Guangxi Provinces of China were characterized for the baseline sensitivity to mandipropamid. The isolates obtained from different provinces showed similar baseline sensitivities to mandipropamid. Baseline sensitivities formed a unimodal curve with mean EC50 values of 0.0055±0.0012μgml−1 for inhibition of mycelial growth. The described baseline sensitivities of P. litchii populations will be useful for monitoring possible shifts in sensitivity to mandipropamid.