WILD TUBER-BEARING SPECIES OF SOLANUM AND INCIDENCE OF PHYTOPHTHORA-INFESTANS (MONT) DEBARY ON THE WESTERN SLOPES OF THE VOLCANO NEVADO-DE-TOLUCA .2. DISTRIBUTION OF PHYTOPHTHORA-INFESTANS

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 (Phylophthora infestans) was studied in 1986 along five long (7 - 10 km) transects from an altitude of 2900 m to 3900 m, the upper limit of Solanum 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 wild Solanum spp., each consisting of 4 to >100 plants per colony of dms, verrucosum (ver), iopetalum, brachycarpum, x 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 of dms, the predominant species (69%). Colonies of ver were less frequent (11%) and 17% were infected. No infected plants were found in the unidentified species, thought to be dms x ver.