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Effects of Simulated Acidic Mist on Germination of Alternaria solani and Phytophthora infestans in vitro and their Infection Efficiency and Sporulation on Potato. Ariena H. C. van Bruggen, Postdoctoral associate, Boyce Thompson Institute for Plant Research, Ithaca, NY 14853, Present address: Department of Plant Pathology, University of California, Davis 95616; J. F. Osmeloski, and J. S. Jacobson. Research technician, and Plant physiologist, Boyce Thompson Institute for Plant Research, Ithaca, NY 14853. Phytopathology 77:564-570. Accepted for publication 23 September 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-564.

Effects of acidity of simulated mist on germination of Alternaria solani and Phytophthora infestans in vitro and their infection efficiency and sporulation on potato were studied in the laboratory and greenhouse. Acidic mist solutions (pH 2.8, 3.4, 4.0, and 4.6) contained sulfuric and nitric acids in a 2:1 mass ratio (2.6:1 ratio on an H+ equivalent basis) and background ions similar to those in ambient rain. In vitro germination of conidia of A. solani and sporangia of P. infestans decreased curvilinearly with increasing acidity, with maximum germination at pH 4.0 and 3.4, respectively, of the original solution, corresponding to pH 6.0 and 5.2 of the spore suspensions. Rooted potato cuttings of the cultivars Norchip, Monona, and Katahdin were exposed to acidic mist (pH 2.8, 3.4, 4.0, or 4.6) for 24 hr before and after inoculation with spore suspensions of either pathogen. After 4872 hr in a greenhouse, the plants were exposed again to simulated mist at the same acidity levels to induce sporulation. When simulated mist was applied at the same acidity before and after inoculation, infection efficiency, lesion area, and sporulation of both pathogens decreased linearly or curvilinearly with increasing acidity of simulated mist. In the case of curvilinear relationships, maxima were obtained at pH 4.0. Exposure of potato leaves to simulated mist at pH 2.8 before inoculation resulted in higher infection levels than exposure to mist at pH 4.0, both before and after inoculation. However, exposure to mist at pH 2.8 after inoculation drastically reduced infection levels compared with pH 4.0, regardless of preinoculation treatment. The postinoculation acidity effect was more important than the preinoculation acidity effect. The acidity of droplets of simulated acidic mist solution on potato leaf surfaces, after termination of exposure of the leaves to simulated acidic mist, decreased rapidly to final pH levels of 6.37.7, when initial pH levels of simulated mist and droplets were 3.4 or 4.0. At pH 2.8 the neutralizing capacity of the leaves was reduced after exposure to simulated acidic mist and the pH rose only to 4.05.0, depending on the duration of exposure.

Additional keywords: acid mist, early blight, late blight, neutralizing capacity, predisposition, Solanum tuberosum.