Formation of oospores of Phytophthora infestans in cultivars of potato with different levels of race‐nonspecific resistance

Abstract

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 A1 and an A2 isolate of Phytophthora infestans of recent UK origin. Numbers of oospores produced varied between repeated experiments involving leaf discs of four cultivars but ranking was unchanged 20 days after inoculation. Maximum mean numbers of oospores (approximately 20 000 per cm2 of leaf) were formed in the highly susceptible cv. Home Guard, with progressively fewer in Maris Piper, Cara, and the fewest in leaf discs of the highly resistant cv. Stirling. In Stirling, the number of oospores increased from approximately 1000–16 000 per cm2 of leaf between 11 and 30 days post‐inoculation. When leaf discs of 10 cultivars were inoculated, numbers of oospores were highest in cultivars with medium levels of race‐nonspecific resistance such as Desiree and Record. On Maris Piper and Cara, but not Home Guard and Stirling, asexual sporulation was significantly (P < 0.05) lower following dual inoculation with A1 and A2 isolates than following inoculation with either isolate alone. The highly susceptible cultivars Home Guard and Bintje were 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 from single blight lesions from leaflets, and 16 from blighted tubers from the plastic tunnel‐grown plants, were characterized for mitochondrial and nuclear DNA polymorphisms. All but one were identical to parental isolates or confirmed as contaminants from an adjacent late‐blight‐affected tuber dump. One isolate, however, obtained from cv. Pimpernel 20 days after inoculation, may have been a recombinant between the A1 and A2 parental isolates, or alternatively a product of selfing or somatic recombination of the A1 parent.