WILD TUBER-BEARING SPECIES OF SOLANUM AND INCIDENCE OF PHYTOPHTHORA-INFESTANS (MONT) DEBARY ON THE WESTERN SLOPES OF THE VOLCANO NEVADO-DE-TOLUCA .3. PHYSIOLOGICAL RACES OF PHYTOPHTHORA-INFESTANS

A survey of races of Phytophthora infestans was made in 1985 and 1986 on the western slopes of the volcano Nevado de Toluca, Mexico. Isolates were collected from infected plants of wild Solanum species found in five long (7 - 10 km) and some short (300 - 500 m) transects. The study are of 15 000 ha was between 2900 and 3900 m altitude. Isolates were collected from Solanum demissum (35 isolates), verrucosum (11), iopetalum (2), brachycarpum (7), stoloniferum (2), r-genotype (9) and from differential hosts (11). No isolate was identified as race 0. Ten virulence genes were identified with a set of differential hosts and the most common were numbers 2 and 4. Isolates with eight virulence genes were the most common and those with ten were found on eight occasions in the natural habitat and on three in the experimental fields.