VIEW ARTICLE

Etiology

Phytophthora Root and Crown Rot of Walnut Trees. S. M. Mircetich, Research plant pathologist, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616; M. E. Matheron, agricultural research technician, Agricultural Research Service, U.S. Department of Agriculture, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 73:1481-1488. Accepted for publication 13 April 1983. 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, 1983. DOI: 10.1094/Phyto-73-1481.

Phytophthora cactorum, P. citricola, P. cinnamomi, P. citrophthora, P. megasperma, P. cryptogea, and four different, but unidentified, Phytophthora spp. were isolated repeatedly from decayed roots or trunk cankers of dead and dying English walnut trees (Juglans regia) affected by root and crown rot in California commercial orchards. In artificially infested soil, P. cactorum, P. citricola, P. cinnamomi, P. citrophthora, and one unidentified Phytophthora sp. (isolate 1029) induced root rot and/or crown rot, whereas P. megasperma caused feeder root necrosis only in seedlings of Northern California black walnut (Juglans hindsii) and Paradox (J. hindsii x J. regia) which are the standard rootstocks of J. regia in California commercial orchards. Symptoms developed on seedlings of rootstocks of J. regia in artificially infested soil were the same as those observed in the rootstock portion of naturally infected orchard trees of J. regia, P. citricola, P. cactorum, P. cinnamomi, and P. citrophthora were more virulent than P. megasperma or Phytophthora sp. (1029) to seedlings of J. hindsii used as rootstocks. In artificially infested soil, seedlings of Paradox rootstock were significantly more resistant than those of J. hindsii to P. cactorum, P. citrophthora, Phytophthora sp. (1029), and P. megasperma, whereas seedlings of J. regia used as a rootstock was as highly susceptible as was J. hindsii to P. cactorum, P. cinnamomi, P. megasperma, and P. citricola. This is the first report implicating P. cinnamomi, P. citricola, P. citrophthora, P. megasperma, and Phytophthora sp. (1029) directly in root and/or crown rot and decline and/or death of J. regia trees in California commercial orchards.