Predisposition Effect of Water Saturation of Soil on Phytophthora Root Rot of Alfalfa

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Abstract
Root rot caused by P. megasperma f. sp. medicaginis was more severe (disease index 4.8, based on a 0-5 rating scale) on alfalfa seedlings grown in saturated soil 1 wk before inoculation than on those pregrown in unsaturated soil before inoculation (disease index 2.5). Yield of foliage was reduced from 60 to 30% of that of uninoculated controls by preinoculation saturation. The percent kill of seedlings axenically inoculated with motile zoospores ranged from 50% in unsaturated soil to 70% in saturated soil. The increased severity and incidence of root rot was associated with increased chemotactic attraction of zoospores to roots grown in saturated soil; breaks on the surface of roots (visible by scanning electron microscopy) subjected to soil saturation; increased electrical conductivity of root exudate from 15 to 22 .mu.mhos/cm; root exudation of amino acids increased by 30% and sugars by 10%. Water saturation of soil predisposed alfalfa roots to Phytophthora root rot by increasing root damage and the exudation of nutrients which in turn increased the chemotactic attraction of zoospores to the roots.