FORMATION OF OOSPORES OF PHYTOPHTHORA-INFESTANS IN CULTIVARS OF POTATO WITH DIFFERENT LEVELS OF RACE NONSPECIFIC RESISTANCE

Leaf discs in vitro, whole plants growing in a walk-in plastic tunnel, and field plots of up to 10 cultivars of potato were inoculated with an Al and an A2 isolate of Phytophthora infestans of recent UK origin. Numbers of oospores produced varied between repeated experiments invo lving leaf discs of four cultivars but ranking was unchanged 20 days a fter inoculation. Maximum mean numbers of oospores (approximately 2000 0 per cm(2) of leaf) were formed in the highly susceptible cv. Home Gu ard, with progressively fewer in Maris Piper, Cara, and the fewest in leaf discs of the highly resistant cv. Stirling. In Stirling, the numb er of oospores increased from approximately 1000-16000 per cm2 of leaf between 11 and 30 days post-inoculation. When leaf discs of 10 cultiv ars were inoculated, numbers of oospores were highest in cultivars wit h medium levels of race-nonspecific resistance such as Desiree and Rec ord. On Maris Piper and Cara, but not Home Guard and Stirling, asexual sporulation was significantly (P < 0.05) lower following dual inocula tion with Al and A2 isolates than following inoculation with either is olate alone. The highly susceptible cultivars Home Guard and Bintje we re rapidly destroyed after inoculation of field and tunnel-grown whole plants, and oospores were not observed. Oospores occurred in leaflets of various cultivars with medium levels of resistance to late-blight, and in stems of all cultivars except Stirling. Twenty-five isolates f rom single blight lesions from leaflets, and 16 from blighted tubers f rom the plastic tunnel-grown plants, were characterized for mitochondr ial and nuclear DNA polymorphisms. All but one were identical to paren tal isolates or confirmed as contaminants from an adjacent late-blight -affected tuber dump. One isolate, however, obtained from cv. Pimperne l 20 days after inoculation, may have been a recombinant between the A 1 and A2 parental isolates, or alternatively a product of selfing or s omatic recombination of the A1 parent.