Effects of metalaxyl on elicitor activity, stimulation of glyceollin production and growth of sensitive and tolerant isolates of Phytophthora megasperma f.sp. glycinea

Mutants of Phytophthora megasperma f.sp. glycinea races 1,4 and 6, tolerant to high concentrations of metalaxyl (50- > 500 μg ml -1 ), were isolated from colonies irradiated with ultraviolet light. When these metalaxyl-tolerant isolates were used to inoculate hypocotyls of compatible soybeans treated with metalaxyl, a majority caused symptoms typical of compatible interactions with rapidly spreading lesions and low levels of glyceollin. This contrasted with the restricted brown lesions with high glyceollin content that developed following inoculation with the parent (metalaxyl-sensitive) isolates. However, a few of the mutant isolates that tolerated 500 μg metalaxyl ml -1 in vitro were sensitive (≤ 10 μg ml -1 ) in inoculated plants and caused restricted brown lesions. Comparisons were made of the release of elicitors of glyceollin into culture fluids of a metalaxyl sensitive and a tolerant isolate following addition of metalaxyl to the culture medium. Elicitor activity was increased significantly by metalaxyl treatment in culture fluids of the sensitive isolate but not in those of the tolerant isolate. After fractionation by gel filtration most elicitor activity was detected in fractions corresponding to the second of two carbohydrate peaks. In general, these results are consistent with the possibility that in infected plants, at marginally inhibitory concentrations of metalaxyl, release of elicitors from fungal hyphae may stimulate host-defence responses in otherwise compatible interactions. © 1989.