Abstract
Two bacteriological culture media were compared for determination of copper resistance among strains of Xanthomonas euvesicatoria and Xanthomonas perforans from pepper and tomato and Xanthomonas campestris pv. vitians from lettuce. Of 94 strains tested, 73 grew on glucose-nutrient agar (GNA) amended with 200 μg ml −1 of copper sulfate. None of the 94 strains grew on modified casitone-yeast extract (CYE) agar amended with copper. In vitro population studies supported the preliminary classification of strains as copper resistant. Reduction in populations of strains found to be resistant on GNA+Cu were less than one log unit after 2-h exposure to copper hydroxide suspensions in vitro. Addition of mancozeb to the copper hydroxide resulted in population reductions of 4–8 log units. Therefore, GNA+Cu is more appropriate for screening of Florida Xanthomonas strains for resistance to copper. Copper hydroxide alone, mancozeb alone, or a combination of these two compounds reduced bacterial spot of tomato and pepper significantly under low disease pressure. Reductions in disease ratings were as much as ten-fold in some experiments. Only the combination of copper and mancozeb was effective at higher disease levels.
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