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Protection of Persea species Against Phytophthora cinnamomi and P. citricola by Prior Inoculation with a Citrus Isolate of P. parasitica. T. E. Dolan, Postdoctoral fellow, Department of Plant Pathology, University of California, Riverside 92521; Y. Cohen(2), and M. D. Coffey(3). (2)(3)Visiting professor, and professor, respectively, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 76:194-198. Accepted for publication 10 September 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-194.

Stems of seedlings of Persea indica were inoculated with zoospores of Phytophthora parasitica, a pathogen of citrus but not of Persea, and challenged 0, 2, and 48 hr later with zoospores of the pathogenic species, P. citricola or P. cinnamomi. Severe lesions developed due to challenge by either pathogen within 3 days on stems receiving simultaneous inoculations of nonpathogen and challenge zoospores. However, lesion development was significantly reduced at a site of nonpathogen inoculation if stems were challenge-inoculated with P. citricola or P. cinnamomi following a delay of 2 or 48 hr. Seedlings of P. indica previously root-inoculated with the nonpathogenic P. parasitica were systemically protected from the development of stem lesions when stem-challenged 4 days later with zoospores of either P. citricola or P. cinnamomi. Plants of P. indica and P. americana grown in soil infested with P. parasitica were protected against root rot caused by either P. cinnamomi or P. citricola when transplanted after 6 wk into infested soil. Recovery of P. cinnamomi and P. citricola from feeder roots of protected plants was reduced by 60-85% relative to unprotected plants. The nonpathogen P. parasitica was recovered from less than 5% of the feeder roots of protected plants. Evidence suggests that this induced protection may be operating systemically.