VIEW ARTICLE

Etiology

Pathogenicity and Relative Virulence of Phytophthora spp. from Walnut and Other Plants to Rootstocks of English Walnut Trees. M. E. Matheron, Former graduate student, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616, Present address of senior author: Department of Plant Pathology, University of Arizona, Yuma Mesa Agricultural Center, 6425 West Eighth Street, Yuma 85364; S. M. Mircetich, professor, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 75:977-981. Accepted for publication 26 March 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-977.

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 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 x 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.