Copper Tolerance and Zinc Sensitivity of Mexican Strains ofXanthomonas campestrispv.vesicatoria,Causal Agent of Bacterial Spot of Pepper

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Abstract
Copper-sensitive strains of X. campestris pv. vesicatoria were isolated from infected pepper plants from two locations in Arizona [USA] where there is limited use of copper bactericides. Three copper-tolerant strains of the bacterium also were isolated from diseased plants from the west coast and central Mexico, where copper bactericides have been used for more than 30 yr. The Arizona strains were sensitive to various copper formulations (copper hydroxide, copper sulfate, copper ammonium carbonate, and basic copper sulfate) with and without the addition of mancozeb as determined by the presence of inhibition zones in disk assays. The Mexican strains were tolerant to all copper formulations but were slightly sensitive to copper-mancozeb combinations. Both strains were sensitive to zinc compounds (zinc sulfate and zineb) in similar assays. In dilution tests using copper sulfate and zinc sulfate with bacterial concentrations of 5 .times. 107 cfu ml, the Arizona strains were sensitive to copper at concentrations higher than 28 mg a.i. L. whereas some colonies of the Mexican strains still grew at copper concentrations of 1,800 mg a.i. L. Strains that were sensitive or tolerant to copper were sensitive to zinc concentrations higher than 56 mg a.i. L. However, a strain initially insensitive to copper and sensitive to zinc developed tolerance to zinc after repeated exposures. In the greenhouse, several copper formulations prevented infection of Cal Wonder 300 pepper plants (Capsicum annuum) by the Arizona strains but not by the Mexican strains of X. campestris pv. vesicatoria. Zineb sprays prevented infection by both strains.