VIEW ARTICLE

Disease Control and Pest Management

Responses to Metalaxyl of Sensitive and Resistant Isolates of Phytophthora infestans. M. D. Coffey, Associate professor, Department of Plant Pathology, University of California, Riverside 92521; Lee Hwa Young, former postgraduate student, Botany School and EM Unit, Trinity College, Dublin 2, Ireland. Phytopathology 74:615-620. Accepted for publication 23 December 1983. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-615.

Zoospore release by three isolates of P. infestans was completely inhibited by 200- 300 μg of metalaxyl per milliliter. Inhibition of cyst germination of the isolates ranged from 10 to 44% with 5 μg 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 two metalaxyl-resistant isolates were inhibited only 23- 25% at 1 μg/ml. In vitro sporulation of all three isolates was inhibited 69- 85% by metalaxyl at 1 μg/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 μg/ml in the case of the sensitive isolate, but was still profuse at 100 μg/ml for two resistant isolates on potato cultivar Kerr' s Pink. The sporulation was dependent on both the fungal isolate and host cultivar involved. On cultivar Arran Victory, metalaxyl-resistant isolate P1296 was completely inhibited by metalaxyl at 10 μg/ml, whereas isolate P1297 still sporulated at 600 μg/ml. In contrast, on cultivar Kerr' s Pink, which has foliar disease resistance identical to Arran Victory, P1296 sporulated sparsely on leaves treated with 500 μg of the fungicide per milliliter.