PHYTOPHTHORA-INFESTANS IN POLAND FROM 1987-1989 - NUCLEAR-DNA CONTENT, MATING-TYPE DISTRIBUTION AND RESPONSE TO METALAXYL

Isolates of Phytophthora infestans were collected from all potato growing regions of Poland during the blight seasons of 1987-1989. All 1987 isolates were of A1 mating type and were sensitive to metalaxyl. In 1988 and 1989, 46.5 % and 55.3 % of the isolates were sensitive to metalaxyl, respectively. The percentage of highly resistant (R) isolates increased from 25.6 % in 1988 to 39.5 % in 1989; however the percentage of intermediately resistant (1) isolates decreased during that period from 27.9 % to 5.3 %. A significant association was observed between the A1 compatibility type and metalaxyl resistance. The A2 mating type first appeared in 1988, and its frequency increased from 4.7 % of the population in 1988 to 47.6 % in 1989. Coincident with this change in mating type frequency, changes in ploidy levels of isolates were observed. Whereas 3 % of the 1988 isolates were diploid, 90 % of the 1989 A2 isolates and 28.6 % of the 1989 A1 isolates were diploid. The approximate 1 : 1 ratio of the two mating types encountered in 1989, and the predominance of diploidy, indicates that the Polish population of P. infestans has the potential to become sexual.