Formation of sex organs in the A1 mating type of Phytophthora infestans induced chemically by A2 isolates of other species of Phytophthora

Abstract

Oogonia with attached antheridia, many of which contained mature thick-walled oospores, were formed in the A1 mating type of Phytophthora infestans after it had been paired with the A2 mating type of P. parasitica, P. palmivora, P. capsici, or P. infestans, on opposite sides of a polycarbonate membrane. The A2 culture discs, in both top and bottom positions in relation to the polycarbonate membrane, induced sex-organ formation in the A1 isolate of P. infestans. Sex organs were formed in P. infestans at 12, 15, 18, 21, and 24 °C, but they were most abundant at 15 and 18 °C and least at 24 °C. More sex organs in the P. infestans A1 isolate were induced by three A2 isolates of P. parasitica than by an A2 isolate of P. infestans. Evidence was presented that a diffusible oospore-inducing substance was produced by the A2 isolate of P. parasitica in the agar medium even when the A1 isolate of P. infestans was absent on the other side of the membrane. Because both P. infestans and P. parasitica are pathogenic to tomato and potato, such interspecific induction of the formation of selfed oospores in the A1 mating type of P. infestans could play an important role in survival and variation of P. infestans throughout the world.