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Effect of Ion Concentration and Sodium:Calcium Ratio of a Nutrient Solution on Phytophthora Root Rot of Tomato and Zoospore Motility and Viability of Phytophthora parasitica. N. Bouchibi, Graduate student, Department of Plant Pathology, University of California, Davis 95616; A. H. C. van Bruggen, and J. D. MacDonald. Assistant professor, and Associate professor, respectively, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 80:1323-1329. Accepted for publication 30 May 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1323.

In two sets of greenhouse experiments, the effect of four Na:Ca equivalent ratios (0, 1, 5, and 10) on Phytophthora root rot of tomato was studied at two ionic concentrations (2.5 vs. 25 meq L?1 or 25 vs. 50 meq L?1) of a modified Hoagland?s solution. Two weeks after planting, the plants were either kept at the same ionic concentration or were shifted from low to high or high to low concentration, and half of the plants in each treatment were inoculated with zoospores of Phytophthora parasitica. The percentage of root rot was assessed visually 2 wk after inoculation. Root rot severity increased significantly with increasing Na:Ca ratios at ion concentrations of 2.5 and 25 meq L?1 before or after inoculation. Salt stress at 50 meq L?1 before inoculation increased root rot. Salt stress at 50 meq L?1 after inoculation reduced root rot caused by an isolate of P. parasitica originating from nonsaline soil, particularly at higher Na:Ca ratios of 5 and 10. Root rot caused by an isolate originating from saline soil was not reduced. Percentages of motile and germinated zoospores decreased in vitro, and those of encysted and lysed zoospores increased with increasing salt concentrations and Na:Ca ratios. These effects were more pronounced for the isolate from nonsaline soil than for the isolate from saline soil. The isolate from saline soil lost its relative salt tolerance after 2 mo in culture. Inoculation of tomato seedlings with this isolate after 2 mo in culture resulted in root rot severity similar to that caused by the isolate from nonsaline soil when salt stress was applied during and after inoculation.