EFFECT OF PLANT-AGE AND INOCULUM CONCENTRATION ON EXPRESSION OF MAJOR GENE RESISTANCE TO PHYTOPHTHORA-INFESTANS IN DETACHED POTATO LEAFLETS

Ten genotypes of a differential host series, each possessing different major (R) genes effective against races of Phytophthora infestans, were grown in a glasshouse to produce plants ranging between 3- and 10-wk-old at the time of inoculation. Detached leaflets were inoculated with a suspension of 2·5, 5, 10, 20 or 50 × 103 zoospores ml−1 applied as a single drop or a covering of fine droplets. Plants expected to show susceptibility uniformly gave spreading sporulating lesions, but resistance was variably expressed, with symptoms ranging from hypersensitive or symptomless reactions indicating immunity to spreading sporulating lesions covering all the leaf surface. Immune reactions were most prevalent on leaflets of flowering plants (8–10-wk-old). The incidence of immune reactions was greater with the drop than with the spray method of inoculation and decreased as spore concentration increased. © 1990, British Mycological Society. All rights reserved.