Studies on metalaxyl‐resistant Phytophthora infestans in potato crops in south‐west Scotland

In 1981 metalaxyl‐resistant Phytophthora infestans was found in 44% of the 63 crops examined in south‐west Scotland. In 1982, when fungicides containing acylalanines (metalaxyl or ofurace) were not used in the area, it was detected in 15%. Metalaxyl‐resistant strains were found in several crops which had been sprayed only with non‐acylalanine fungicides or had received no fungicidal treatment. In neither year was resistance associated with a significant breakdown of disease control. Metalaxyl‐resistant isolates of the blight fungus were obtained from tubers from crops in which resistance was detected on the foliage. In detached leaf experiments, resistant isolates retained their ability to grow on leaves treated with 100 μg/ml of metalaxyl after 52 weekly transfers on untreated leaves, and produced sporangia on leaves treated with up to 2000 μg/ml of metalaxyl or ofurace. In mixtures with sensitive sporangia the proportions remained stable after nine weekly transfers on untreated leaves. However, at concentrations as low as 50 μg/ml, metalaxyl and ofurace reduced the extent of colonization and the numbers of sporangia produced by resistant isolates by an average of 50%.