Control of Metalaxyl-Resistant Causal Agents of Late Blight in Potato and Tomato and Downy Mildew in Cucumber by Cymoxanil

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Abstract
A relatively high dosage of cymoxanil foliar spray was required to control late blight in potato [Solanum tuberosum] and downy mildew in cucumber [Cucumis sativus] in growth chambers. ED90 values for control of metalaxyl-sensitive field isolates of Phytophthora infestans ranged between 164 and 459 .mu.g/ml, and for metalaxyl-resistant isolates, between 112 and 525 .mu.g/ml. ED90 values for control of metalaxyl-resistant isolates of Pseudoperonospora cubensis ranges between 201 and 878 .mu.g/ml. Complete control of both pathogens was achieved at concentrations between 500 and > 1,000 .mu.g/ml. Preventive and curative efficacy of the fungicide lasted for 5 and 3 days, respectively. Cymoxanil was readily taken up by leaves, roots, and stems. When applied to roots and stems, it showed acropetal systemic translocatin in cucumber and tomato [Lycopersicon esculentum] but not in potato. No systemic translocation occurred when applied to leaf laminae. Translaminar translocation occurred in potato and tomato but not in cucumbers. The fungicide was toxic to roots of tomato plants.