Cytology studies on effects of metalaxyl on a wild-type isolate and a metalaxyl-resistant mutant of Phytophthora sojae in vitro

The effects of metalaxyl, an acylalanine phenylamide fungicide exhibiting systemic properties, on morphology and ultrastructure of the wild-type isolate and the metalaxyl-resistant mutant of Phytophthora sojae in vitro were examined by means of electron microscopy techniques. Ultrastructral observations on the wild-type isolate revealed marked alterations at a low concentration of metalaxyl (1μg/mL), such as uneven growth of hyphae, excessive branching of hyphal tips, cessation of hyphal growth and bulb-like swelling of hyphal tips; mitochondria lost their distinct structures, number and size of vesicles increased, cellwalls became thickened and hyphal cytoplasm was seriously degenerated in presence of a high concentration of metalaxyl (10μg/mL). While, hyphae of the metalaxyl-resistant mutant displayed little morphological changes after treatment with the high concentration of metalaxyl (10μg/mL). Observations indicated hyphal tips just appeared little excessive branching, hyphal cell contained a high number of lipid bodies, but fungal cell walls did not thicken. Hyphae maintained their growth and get across the filter treated with 10μg/mL metalaxyl.