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Twenty‐three field isolates of Phytophthora infestans from North America, Europe and Israel were tested for sensitivity to fosetyl‐Al on rye‐seed‐agar medium and on leaf discs from 12 potato cultivars. Sensitivity varied greatly between fungal isolates; their sensitivity in vitro was not significantly correlated with sensitivity in vivo. In vivo, there was a significant effect of potato cultivars on sensitivity, which interacted with fungal isolates. The fungicidal effect of fosetyl‐Al was greater on older than on younger leaves, and greater with a 12‐h photoperiod than in continuous light. Sensitivity of isolates to fosetyl‐Al was significantly correlated with sensitivity to phosphorous acid (tested in vitro) but was not correlated with sensitivity to metalaxyl (tested in vitro and in vivo). The results explain some of the controversial reports on the efficacy of fosetyl‐Al in controlling potato late blight.

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