Accuracy of Methods to Monitor Sensitivity of Phytophthora infestans to Phenylamide Fungicides. D. Sozzi, Agricultural Division, Ciba-Geigy Ltd., 4002 Basel, Switzerland. T. Staub, Agricultural Division, Ciba-Geigy Ltd., 4002 Basel, Switzerland. Plant Dis. 71:422-425. Accepted for publication 24 October 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0422.

Sensitivity of Phytophthora infestans to metalaxyl was monitored on potato leaf disks, detached leaves, or whole plants. Only minor differences between the various experimental systems were found. Values obtained on amended agar showed similar resistance factors. The less differentiating slopes of the dose response curves from agar tests, however, leave more space for interpretation of sensitivity of test isolates. The drawback of all methods is that they are not sufficiently quantitative. Mixed populations of sensitive and resistant sporangia may give either a completely sensitive or resistant reaction, depending on the ratio of the two types of sporangia in the mixture. Levels of resistance lower than 0.1%, which occur at an early phase in the development of resistance, remain undetected with these methods. Lower levels of resistance down to 0.001% were detected only when large numbers of sporangia were bulked and subjected to preselection of metalaxyl-treated potato plants. Sensitivity testing was carried out subsequently with sporangia recovered from lesions developing on the treated plants. The methods described, therefore, are well suited for monitoring programs aimed at detecting fungicide resistance at higher levels late in the overall selection process. Other methods have to be developed to detect early selection with resistance frequencies lower than 10^–3.