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Ecology and Epidemiology

Salinity Effects on the Susceptibility of Chrysanthemum Roots to Phytophthora cryptogea. J. D. MacDonald, Associate professor, Department of Plant Pathology, University of California, Davis 95616; Phytopathology 74:621-624. Accepted for publication 31 January 1984. Copyright 1984 The American Phytopathological Society. DOI: 10.1094/Phyto-74-621.

Root systems of hydroponically grown chrysanthemums were exposed to salinity stress after inoculation with zoospores of Phytophthora cryptogea. Although inoculated with similar numbers of zoospores, roots subsequently exposed to 0.2 M NaCl and 0.01 M CaCl2 in half-strength Hoagland' s solution developed severe root rot symptoms, whereas roots that were not exposed to salinity stress developed only mild symptoms of disease. Roots exposed to nutrient solution containing 0.1 M NaCl and 0.005 M CaCl2 developed symptoms of intermediate severity. When unstressed roots and roots exposed to the highest level of salinity stress were excised, placed in humidity chambers, and inoculated in a uniform manner, large differences were observed in the ability of P. cryptogea to colonize the tissues. After 48 hr of incubation, P. cryptogea was not cultured from tissue segments of unstressed roots taken 12 mm or farther from the point of inoculation, and the frequency of recovery in tissues closer to the point of inoculation was relatively low. By contrast, P. cryptogea was cultured with high frequency from tissue segments of stressed roots up to 12 mm from the point of inoculation, and with lower frequency up to 20- 24 mm. When plants were exposed to salinity stress 4 or 12 hr after inoculation, severe disease resulted, but not if stress was delayed until 24 or 48 hr after inoculation. Thus, the host appeared to be susceptible to pathogen establishment only if stress occurred within 24 hr of inoculation. These data suggest that salinity stress alters the normal, active defense responses of the plant. The potential importance of salinity stress in the occurrence or severity of Phytophthora root rot is discussed.

Additional keywords: Chrysanthemum morifolium, environmental stress, predisposition.