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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.