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Effect of Salinity Stress on the Development of Phytophthora Root Rot of Chrysanthemum. J. D. MacDonald, Assistant professor, Department of Plant Pathology, University of California, Davis 95616; Phytopathology 72:214-219. Accepted for publication 22 May 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-214.

Rooted cuttings of Chrysanthemum morifolium ‘Paragon’ were grown hydroponically in ceramic crocks containing half-strength Hoagland’s solution. After 10 days, plants were subjected to pulsed exposures of salinity stress by amending the Hoagland’s solution with NaCl to a final concentration of 0.1 M or 0.2 M NaCl. Control plants had no salt added. After 24 hr in the NaCl-amended solutions, both stressed plants and controls were removed from the crocks and inoculated with 106 motile zoospores of Phytophthora cryptogea by immersing their roots for 1 hr in a measured volume of inoculum prior to returning them to crocks of fresh half-strength Hoagland’s, or adding inoculum directly into the crocks of fresh solution. Uninoculated plants subjected to the same stress treatments served as controls to monitor independent effects of the salinity stress. While there was some discoloration of roots following salt stress, and slight stunting of plants exposed to 0.2 M NaCl, all uninoculated plants appeared to recover quickly from the brief stress treatment. When stressed and unstressed plants were inoculated with zoospores of P. cryptogea, there was a positive relationship between the degree of stress and the severity of the resulting root rot symptoms. Only 20% of the roots of unstressed plants developed lesions 72 hr after inoculation, whereas 70 and 88% of roots previously exposed to 0.1 M and 0.2 M NaCl, respectively, developed lesions. Lesions that developed on roots exposed to salinity stress spread rapidly and turned large portions of the root system necrotic within 5–7 days. Microscopic examination of roots collected 1 hr after exposure to the zoospore inoculum showed large, statistically significant increases in the number of zoospore cysts attached to roots with each increment increase in salinity stress. The root hair zone was the region of greatest zoospore attachment on stressed roots, and a large proportion of the cysts that attached to stressed roots within 30 min of inoculation were observed by scanning electron microscopy to have germinated and penetrated the host tissue. The severe root rot infection that occurred as a result of NaCl stress could have broad implications in areas where water quality and soil salinity are important problems.