Occurrence of Copper-Resistant Xanthomonas perforans and X. gardneri in Illinois Tomato Fields

This study was conducted to determine whether currently prevailing isolates of Xanthomonas perforans and X. gardneri, the causal agents of tomato bacterial spot in Illinois, were resistant to copper. First, 133 isolates from each pathogen were tested for their multiplication on mannitol glutamate yeast agar (MGYA) amended with 0.8 mM of laboratory-grade copper sulfate. Results showed that 59% of X. perforans and 38% of X. gardneri isolates formed colonies on the copper-amended culture. Then, six isolates of X. perforans and five isolates of X. gardneri were tested for their multiplication on MGYA amended with 0.2, 0.8, 1.2, 1.6, and 2.0 mM of laboratory-grade of either copper hydroxide or copper sulfate. All 11 isolates formed colonies on cultures with 1.2 mM or lower rates of either copper compound. Molecular analysis revealed the presence of the copper-resistance genes copA and copM in the isolates. In vitro assays showed that both X. perforans and X. gardneri formed colonies when grown on Luria–Bertani agar containing low concentrations of copper hydroxide (Kocide- 3000 46.1DF) and copper sulfate pentahydrate (Instill). Double Nickel LC, a biopesticide containing Bacillus amyloliquefaciens strain D747, was the most effective in preventing multiplication of copper-resistant isolates of both pathogens on the culture media.