Predisposing Effects of Soil Moisture Extremes on the Susceptibility of Rhododendron to Phytophthora Root and Crown Rot

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Abstract
One-year-old rhododendrons were subjected to various soil-water regimes and inoculated with motile zoospores of P. cinnamomi. In the absence of stresses due to drought or flooding, plants of cv. Purple Splendour developed severe root and crown rot following inoculation with 104 or 105 zoospores/plant, whereas ''Caroline'' remained free from symptoms and was relatively resistant. However, if ''Caroline'' plants were drought stressed to leaf water potentials of .ltoreq. -16 bars, or their roots were flooded for 48 h 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.

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