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Epiphytic Populations of Xanthomonas campestris pv. vesicatoria and Bacterial Spot of Tomato as Influenced by Nitrogen and Potassium Fertilization. R. G. McGuire, Former postdoctoral associate, IFAS, University of Florida, Gulf Coast Research and Education Center, 5007 60th Street East, Bradenton 34203; J. B. Jones, C. D. Stanley, and A. A. Csizinszky. Associate Professors, IFAS, University of Florida, Gulf Coast Research and Education Center, 5007 60th Street East, Bradenton 34203. Phytopathology 81:656-660. Accepted for publication 30 January 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-656.

Epiphytic populations of Xanthomonas campestris pv. vesicatoria were monitored weekly on tomato plants supplied through drip irrigation with combinations of two rates of NH4NO3 and three rates of KCl. Bacterial populations and the percentage of defoliation were then related, through a repeated measures analysis of variance, to fertilization and soil salinity and also were correlated with the mineral content of leaves. Lower epiphytic populations of the pathogen developed on plants receiving combinations of the higher rates of nitrogen and potassium; an interaction between the two fertilizers was significant (P = 0.01) in each of three seasons. Defoliation was more severe at the low rate of nitrogen application, but the rate of potassium application had little influence. Epiphytic populations of X. c. vesicatoria, but not the severity of defoliation, also were associated with increases in soil salinity that resulted from high rates of fertilization. Applications of NH4NO3 increased the concentration of nitrogen in leaves throughout the season, and applications of KCl similarly increased foliar concentrations of potassium. Through fruit set, however, increased application of NH4NO3 reduced potassium concentrations while greater applications of KCl reduced nitrogen concentrations in leaves. Although a higher rate of NH4NO3 also reduced the foliar concentrations of calcium and magnesium, magnesium but not calcium was affected by the rate of KCl. Early in the season, epiphytic populations of X. c. vesicatoria were inversely correlated with the foliar concentration of potassium; at harvest, however, populations were inversely related to magnesium content. Defoliation increased significantly with increasing concentrations of both magnesium and calcium.