Diagnostic Guide for Bacterial Spot of Tomato and Pepper

Abstract

Bacterial spot of tomato and pepper, caused by four Xanthomonas species, X. euvesicatoria, X. vesicatoria, X. perforans, and X. gardneri (renamed X. hortorum pv. gardneri), is a disease that affects pepper and tomato production worldwide. Symptomatic plants often show dark brown or black lesions on all aboveground tissue including fruit, stems, and foliage. Defoliation, fruit spots, and fruit drop are the most important symptoms that contribute to yield loss. To manage the disease, a combination of cultural management tactics is recommended. Unfortunately, effective commercially available copper-based agrochemicals are limited due to the pathogen’s ability to develop tolerance in the field. Multiple breeding efforts have focused on generating genetically resistant cultivars; however, host resistance has been observed to be lost over time due to the pathogen’s adaptation to a deployed genotype and the evolution of new pathogenic races. Isolation of the pathogen from infected tissue can be performed by surface sterilization followed by tissue maceration and streaking the supernatant on nutrient agar or yeast–dextrose–calcium carbonate agar. Identification tools for these pathogens include semiselective media (i.e., Chang Kama Tween Medium and Tween agar), serological methods, biochemical tests, and molecular techniques. For the latter method, one can use species-specific primers to run conventional PCR, qPCR, and multiplex PCR. Lastly, differential genotypes can be used for identifying races.