Phytophthora Root and Crown Rot of Walnut Trees

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Abstract
P. 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 (J. regia) affected by root and crown rot in California [USA] commercial orchards. In artificially infested soil, P. cactorum, P. citricola, P. cinnamomi, P. citrophthora and 1 unidentified Phytophthora sp. (isolate 1029) induced root rot and/or crown rot; P. megasperma caused feeder root necrosis only in seedlings of Northern California black walnut (J. hindsii) and Paradox (J. hindsii .times. 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: 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 1st 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.

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