Responses to Metalaxyl of Sensitive and Resistant Isolates ofPhytophthora infestans

Publisher Website
Google Scholar
Abstract
Zoospore release by 3 isolates of P. infestans was completely inhibited by 200-300 .mu.g of metalaxyl/ml. Inhibition of cyst germination of the isolates ranged 10-44% with 5 .mu.g of the fungicide per milliliter. An ultrastructural analysis of the effects of metalaxyl revealed no observable change in fine structure of either cysts or zoospores. In vitro mycelial growth of a metalaxyl-sensitive isolate was inhibited 69%, while 2-metalaxyl-resistant isolates were inhibited only 23-25% at 1 .mu.g/ml. In vitro sporulation of all 3 isolates was inhibited 69-85% by metalaxyl at 1 .mu.g/ml; there was no correlation between the in vitro and in vivo metalaxyl resistance of the isolates. In vivo sporulation was reduced to zero by metalaxyl at 0.1 .mu.g/ml in the case of the sensitive isolate, but was still profuse at 100 .mu.g/ml for 2 resistant isolates on potato cv. Kerr''s Pink. The sporulation was dependent on both the fungal isolate and host cultivar involved. On ''Arran Victory'', metalaxyl-resistant isolate P1296 was completely inhibited by metalaxyl at 10 .mu.g/ml, whereas isolate P1297 still sporulated at 60 .mu.g/ml. In contrast, on ''Kerr''s Pink'', which has foliar disease resistance identical to ''Arran Victory'', P1296 sporulated sparsely on leaves treated with 500 .mu.g of the fungicide per milliliter.