Salinity Effects on the Susceptibility of Chrysanthemum Roots toPhytophthora cryptogea

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Abstract
Root systems of hydroponically grown chrysanthemums were exposed to salinity stress after inoculation with zoospores of P. 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 h 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 fromtissue 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 h after inoculation, severe disease resulted, but not if stress was delayed until 24 or 48 h after inoculation. Thus, the host apparently was susceptible to pathogen establishment only if stress occurred within 24 h of inoculation. Salinity stress probably 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.

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