Pathogenicity and Relative Virulence ofPhytophthoraspp. from Walnut and Other Plants to Rootstocks of English Walnut Trees

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Abstract
Phytophthora cryptogea, P. drechsleri, P. parasitica, and two unidentified Phytophthora spp, were isolated from declining English walnut (Juglans regia) trees affected by root and crown rot in California [USA] orchards. All five Phytophthora spp. were pathogenic to Northern California black walnut (J. hindsii) seedlings grown in artificially infested potting mix in greenhouse tests. J. hindsii and Paradox (J. hindsii .times. J. regia), standard rootstocks of English walnut were highly susceptible to P. cryptogea, whereas Paradox was significantly more resistant than J. hindsii to the two unidentified Phytophthora spp. A total of thirty-one different isolates from among P. cactorum, P cinnamomi, P, citricola, and P. cryptogea isolated from walnut and numerous trees, shrubs, and other hosts were pathogenic to seedlings of J. hindsii grown in artificially infested potting mix. This is the first report directly implicating P. cryptogea, P. parasitica, and the two unidentified Phytophthora spp. (isolates W489R and 2833) in decline and death of English walnut trees in California''s commercial walnut orchards.