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Influence of Simulated Acidic Rain on Phytophthora cinnamomi and Phytophthora Root Rot of Blue Lupine. S. R. Shafer, Research plant pathologist, USDA-ARS, and assistant professor, Department of Plant Pathology and Soil Science, North Carolina State University, Raleigh 27695; R. I. Bruck(2), and A. S. Heagle(3). (2)Associate professor, Departments of Plant Pathology and Forestry, North Carolina State University, Raleigh 27695; (3)Research plant pathologist, Departments of Plant Pathology and Crop Science, North Carolina State University, Raleigh 27695. Phytopathology 75:996-1003. Accepted for publication 5 April 1985. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1985. DOI: 10.1094/Phyto-75-996.

Effects of acid deposition on Phytophthora cinnamomi were examined with simulated rain solutions (pH 5.6, 4.0, 3.2, or 2.4), Lakeland sand, and blue lupine seedlings. Infected radicles were buried in soil that was subsequently saturated for 15 min with solutions and then drained to - 3.0 to - 2.3 kPa. The number of sporangia formed on radicles decreased linearly with increasing solution acidity (47% fewer sporangia formed in soil treated with pH 2.4 solution than with pH 5.6 solution). Release of zoospores from sporangia incubated in soil extracts was unaffected by the acidity of solutions used to prepare extracts. Seedlings grown in soil, inoculated with zoospores, and exposed to simulated rainfall (2.4 cm, 1 hr) at pH 2.4 had 44% fewer infection sites on roots than did seedlings exposed to rain at pH 5.6. Effects of rain acidity on onset of disease symptoms and rate of disease increase were not consistent among seedlings maintained for 28 days in infested soil and repeatedly exposed to simulated rains. Although simulated acidic rain significantly affects epidemiologically important steps in the life cycle of P. cinnamomi, gradual deposition of H+ in rain probably has little short-term effect on Phytophthora root rot if plants remain exposed to inoculum.

Additional keywords: acid deposition, Lupinus angustifolius, soil microorganisms.