POPULATION-DYNAMICS OF XANTHOMONAS-CAMPESTRIS PV VESICATORIA ON TOMATO LEAFLETS TREATED WITH COPPER BACTERICIDES

Populations of copper-resistant (Cu(r)) strains of Xanthomonas campestris pv. vesicatoria were monitored in the field on nonsymptomatic tomato leaflets treated with copper or with a copper and mancozeb mixture over three and four seasons, respectively. Copper and a combination of copper and mancozeb reduced epiphytic populations of X. c. vesicatoria, compared to those in the untreated control. Populations of X. c. vesicatoria on leaflets receiving copper and the copper-mancozeb combination differed significantly in only one of three seasons. A positive correlation was observed between epiphytic populations and disease severity. In a greenhouse study, where a Cu(r) strain of X. c. vesicatoria was applied to tomato foliage, bacterial populations were significantly less on plants treated with copper or with a copper and mancozeb mixture than on untreated plants. However, leaflets treated with the copper and mancozeb combination had significantly lower Cu(r) populations than leaflets treated with copper alone. In another study, where a Cu(r) and copper-sensitive (Cu(s)) strain were separately inoculated on treated plants, disease severity was significantly reduced by both copper treatments compared to that of the control. However, there were no significant differences in disease severity between the two copper treatments when plants were inoculated with either a Cu(r) or Cu(s) strain. Ionic and total soluble copper in dew collected from bactericide-treated leaves were not significantly different between the two copper treatments. Although ionic and soluble copper may be factors in toxicity to Cu(r) strains, they do not appear to be the primary components involved in the toxicity of the copper and mancozeb mixture to Cu(r) strains of X. c. vesicatoria.