Occurrence of the A2 mating type and oospores of Phytophthora Infestans in potato crops in Israel

Seventeen metalaxyl-sensitive and 21 metalaxyl-resistant isolates of Phytophthora infestans collected from blighted potato fields during the years 1983-1988 were tested for mating type on rye seed agar medium. All isolates except one (MS3, collected in 1986 at Sufa, in the western Negev) were found to belong to the A2 mating type. A2 isolates produced oospores within 2 weeks when cultured together with isolate 163 (A₁ from the U.S.A.) or with the A₁ isolate MS3 from Israel. When cultured singly, A₂ isolates produced some oospores within 4-8 weeks. Blighted potato tubers harvested from potato crops artificially inoculated with a mixture of A1+A2 sporangia were found to contain some oospores. No oospores were detected in blighted tubers harvested from A₂ + A₂ inoculated crops. It was concluded that the A2 mating type of P. infestans has occurred in Israel since 1983 or even earlier. The rare occurrence of the A1 mating type was unexpected and indicated that sexual reproduction of the fungus in the country might be limited.