Copper-tolerance in Pseudomonas syringae pv. tomato and Xanthomonas spp. and the control of diseases associated with these pathogens in tomato and pepper. A systematic literature review

A large body of literature exists on the use, efficacy and pathogen resistance associated with the application of copper-based products to control bacterial diseases of Solanaceous crops. In particular, the bacterial pathogens Pseudomonas syringae pv. tomato (Okabe) Young et al. which cause bacterial speck in tomato (Solanum lycopersicum) and multiple Xanthomonas spp. which cause bacterial spot in tomato and pepper (Capsicum annum). There is also an increasing number of studies reporting on alternative products to copper, either biological or chemical, for control of the diseases caused by these pathogens. This systematic literature review provides a synthesis of the published data reporting on the prevalence of copper-tolerance, how this affects the efficacy of copper-based bactericides and key emerging alternative products to copper for disease control. A total of 133 publications were analysed within these three areas. Of all the P. syringae pv. tomato and Xanthomonas spp. isolates screened, 78.5% and 54.0% respectively, were identified as copper-tolerant. However, the methods used for in vitro screening of the isolates varied between studies, with evidence that differences in methodology may affect test results. In studies on efficacy of copper products when applied alone, rather than in a mixture with other compounds, only 38% reported highly effective control of bacterial speck and spot. The remaining studies reported variable, weak or no control. Even though the bacteria present weren't specifically screened for copper-tolerance in the majority of these studies, the efficacy data suggest tolerance was present. Around 100 different alternative products to copper for control of P. syringae pv. tomato and Xanthomonas spp. were identified in the literature. Of these, acibenzolar-s-methyl, Pseudomonas fluorescens, Bacillus subtilis, bacteriophages, Pseudomonas syringae Cit7 and the antibiotics streptomycin, oxytetracycline and kasugamycin were the most studied, efficacy data for these products from the analysed publications are reported.