Baseline and differential sensitivity of Peronophythora litchii (lychee downy blight) to three carboxylic acid amide fungicides




Downy blight, caused by Peronophythora litchii, is an important disease of lychee (litchi) plants in China. The in vitro sensitivities of various asexual stages of P. litchii to the three carboxylic acid amide (CAA) fungicides dimethomorph, flumorph and pyrimorph were studied with four single-sporangium isolates. None of the three fungicides affected zoospore discharge from sporangia, but they strongly inhibited mycelial growth (mean EC50 values of 0·075, 0·258 and 0·115 mg L−1, respectively); sporangial production (mean EC50 values of 0·085, 0·315 and 0·150 mg L−1, respectively); germination of cystospores (mean EC50 values of 0·140, 0·150 and 0·645 mg L−1, respectively); and germination of sporangia (mean EC50 values of 0·203, 0·5 and 0·743 mg L−1, respectively). As mycelial growth was the most sensitive stage to dimethomorph and pyrimorph, it was chosen to test baseline sensitivities to the three fungicides. In 2007, from 131 isolates collected in Fujian, Guangdong and Guangxi provinces, 127, 116 and 113 isolates were used to establish baseline sensitivity for dimethomorph, flumorph and pyrimorph respectively. Isolates from different provinces exhibited similar baseline sensitivity to the same fungicide. Baseline sensitivities to dimethomorph, flumorph and pyrimorph were distributed as unimodal curves, with mean EC50 values of 0·082 (± 0·01), 0·282 (± 0·047), and 0·115 (± 0·032) mg L−1, respectively. This information will serve as a baseline for tracking future changes in sensitivities of P. litchii populations to these three CAA fungicides.