<em>Phytophthora infestans</em> 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 Al 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 (J) isolates decreased during that period from 27.9% to 5.3 %. A significant association was observed between the Al 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. [ABSTRACT FROM AUTHOR]