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Fitness of Phytophthora infestans Isolates from Metalaxyl-Sensitive and -Resistant Populations. David Kadish, Graduate student, Department of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel; Yigal Cohen, Professor, Department of Life Sciences, Bar-Ilan University, Ramat-Gan, Israel. Phytopathology 78:912-915. Accepted for publication 25 January 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-912.

Twenty field isolates of Phytophthora infestans from different locations in Israel were compared on potato plants in growth chambers, and six isolates were compared on plants grown in walk-in plastic tunnels. Although lesion areas induced in potato plants (cv. Alpha) by metalaxyl-resistant (MR) isolates were significantly (P < 0.0001) larger than those induced by metalaxyl-sensitive (MS) isolates, no significant differences were recorded among isolates in sporulation capacity or infection efficiency. Composite fitness indices in growth chambers and epidemic development in plastic tunnels were significantly higher for MR than for MS isolates, thus providing a possible explanation for the severe MR-induced late blight epidemic outbreaks in commercial potato fields in Israel.

Additional keywords: epidemiology, fungicide resistance, oomycetes.