VIEW ARTICLE

Physiology and Biochemistry

Components in Alfalfa Root Extract and Root Exudate that Increase Oospore Germination of Phytophthora megasperma f. sp. medicaginis. Zeinab A. El-Hamalawi, Department of Plant Pathology, University of California, Riverside 92521, Senior author is with Department of Plant Pathology, Faculty of Agriculture, Cairo University, Giza, Egypt; D. C. Erwin, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 76:508-513. Accepted for publication 21 November 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-508.

The germination of oospores of Phytophthora megasperma f. sp. medicaginis was three- to four-fold higher in root extracts (86%) or root exudates (61%) of both the susceptible alfalfa cultivar, Moapa 69, and resistant germ plasm A77-10B, than in water (23%). Oospore germination occurred at pH 3.5-10.5 with optimum at pH 6.0. Oospores germinated by germ tube only in water, but in root extract or exudate up to 50% germinated by germ sporangia. Increased concentrations of root extract or exudate (up to 92 mg of solids per 100 ml) increased germination by germ sporangia, but at higher concentrations (138-275 mg/100 ml) the percentage of oospores germinating by germ sporangia decreased. The factors in alfalfa root extract or exudate that increased oospore germination were stable to heat and proteolysis and were not restricted to a certain molecular size. Lyophilization of alfalfa root extract and exudate prior to rehydration increased the percentage that germinated by germ sporangia but did not increase total oospore germination. Charcoal treatment of root extracts and root exudates, which reduced carbohydrate and amino acid content, also reduced the percentage of total oospore germination, but increased the percentage of oospores that germinated by germ sporangia. Oospore germination was also increased by root exudate and extract of pea and tomato plants (nonhosts). The cationic fraction of alfalfa root extract and exudate, which contained amino acids, stimulated oospore germination (58%) and also increased the percentage that germinated by germ sporangia (about 30%). Amino acids and sugars in root extract were analyzed and tested for effect on germination. Aspartic acid, singly and in combination with certain other amino acids, increased oospore germination when added to dilute root exudate but were not active in water. Ribose from the neutral fraction of alfalfa root extract or exudate increased germination slightly.