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Predisposing Effects of Soil Moisture Extremes on the Susceptibility of Rhododendron to Phytophthora Root and Crown Rot. N. S. Blaker, Graduate student, Department of Plant Pathology, University of California, Davis 95616; J. D. MacDonald, assistant professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 71:831-834. Accepted for publication 1 January 1981. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-831.

One-year-old rhododendrons were subjected to various soil-water regimes and inoculated with motile zoospores of Phytophthora cinnamomi. In the absence of stresses due to drought or flooding, plants of cultivar Purple Splendour developed severe root and crown rot following inoculation with 104 or 105 zoospores per plant, whereas the cultivar Caroline remained free from symptoms and was relatively resistant. However, if Caroline plants were drought stressed to leaf water potentials of ≤ 16 bars, or their roots were flooded for 48 hr before inoculation with P. cinnamomi, they developed severe symptoms of root and crown rot. Soil moisture extremes that commonly occur both in nursery and landscape plantings can predispose normally resistant rhododendrons to root and crown rot caused by P. cinnamomi.