Wild tuber-bearing species ofSolanum and incidence ofPhytophthora infestans (Mont.) de Bary on the Western slopes of the volcano Nevado de Toluca. 2. Distribution ofPhytophthora infestans

Summary An area of 15 000 ha on the Western slopes of the volcano Nevado de Toluca was surveyed during 1982–1986. The occurrence of late blight (Phytophthora infestans) was studied in 1986 along five long (7–10 km) transects from an altitude of 2900 m to 3900 m, the upper limit ofSolanum demissum (dms) and in all years in some other sites. Late blight was confined to scattered pockets, 100–500 m in diameter, separated by several kilometers. In 789 colonies of wildSolanum spp., each consisting of 4 to >100 plants per colony ofdms, verrucosum (ver), iopetalum, brachycarpum, × edinense, stoloniferum or one unidentified species, symptoms were found in 10% of the colonies or less than 1% of the plants in each colony. Plants had few (1–2) lesions which did not affect vigour. No infected tubers were found. Most (64%) of the infected colonies were ofdms, the predominant species (69%). Colonies ofver were less frequent (11%) and 17% were infected. No infected plants were found in the unidentified species, thought to bedms×ver.