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Disease Note.

Resistance to Metalaxyl in the Pseudoperonospora cubensis Population Causing Downy Mildew of Cucumber in South Florida. M. A. Moss, University of Florida, IFAS, Tropical Research and Education Center, Homestead 33031. Plant Dis. 71:1045. Accepted for publication 15 June 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-1045A.

Reduced control of Pseudoperonospora cubensis (Berk. & Curt.) Rostow. on cucumber (Cucumis sativus L.) plants treated with metalaxyl suggested that metalaxyl-resistant strains (1,2) ofthe fungus may be present in south Florida. Replicated experiments were conducted to determine the efficacy of metalaxyl and of metalaxyl + mancozeb (Ridomil MZ58) under field conditions. Eight fungicide applications were made at 3- to 5-day intervals to cucumbers (cv. Pacer) beginning at emergence. Leaf area damage of plants sprayed with metalaxyl (0.22 kg a.i./ha) reached 53% by the last application, compared with 60% on unsprayed controls. Disease levels on plants sprayed with Ridomil MZ58 (0.22 kg a.i./ha of metalaxyl + 1.0 kg a.i./ ha ofmancozeb) or with mancozeb (1.0 kg a.i./ ha) alone were only 4%. Downy mildew was not observed on plants treated with mancozeb alone at 2.7 kg a.i. / ha. Lack of disease control on plants sprayed with metalaxyl suggests resistance in the pathogen popUlation in south Florida. The control provided by Ridomil MZ58 was due primarily to the mancozeb component.

References: (I) S. G. Georgopoulos and A. C. Grigoriu. Plant Dis. 65:729,1981. (2) M. Reuveni et al. Plant Dis. 64: 1108, 1980.