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Effect of Soil Salinity on the Formation of Sporangia and Zoospores by Three Isolates of Phytophthora. N. S. Blaker, Graduate student, Department of Plant Pathology, University of California, Davis 95616; J. D. MacDonald, associate professor, Department of Plant Pathology, University of California, Davis 95616. Phytopathology 75:270-274. Accepted for publication 15 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-270.

Formation of sporangia and zoospores by three isolates of Phytophthora was studied in naturally and artificially salinized soils and soil extracts. Sporangium formation by isolates of P. cryptogea and P. parasitica 153, both orginally obtained from plants growing in nonsaline soils, gradually decreased in soils in which the electrical conductivity of the saturated soil extract (ECe) was >5 decisiemens per meter (dS/m). In contrast, sporangium formation by an isolate of P. parasitica from a saline soil in the Coachella Valley, CA, was greatest in saline soils (ECe = 5- 44 dS/m) and was reduced in nonsaline soil or at very high salinity (ECe ≥50 dS/m). When sporangia of all three isolates were formed in soils at various levels of salinity and placed in salt solutions or saline soil extracts to stimulate indirect germination, zoospore release was greatest at ECe ≤2.5 dS/m and was restricted at ECe >10 dS/m. Sporangia of P. cryptogea formed in soil at ECe ≥10 dS/m failed to release zoospores in any solution, while sporangia of both isolates of P. parasitica formed in soil at ECe ≤37 dS/m released zoospores in solutions at ECe ≤10 dS/m. These results show that isolates of Phytophthora differ in their tolerance of high levels of salinity but that all three isolates studied are likely to produce sporangia and zoospores at salinity levels stressful to most crop plant species.

Additional keywords: citrus.