These bacteria attack the foliage, stems, and fruit of tomatoes. The first symptoms are small, yellow-green lesions on young leaves, which usually appear deformed and twisted. On older foliage, lesions are limited by veins, acquiring an angular aspect over time. They are first dark-green, greasy in appearance, and often surrounded by a yellow halo. They are usually more numerous on leaf margins or tips. If conditions are met, they enlarge rapidly to a size of 0.25 to 0.5 cm wide and become tan to brownish-red. Eventually, the spots look like shot holes because the center dries up and disintegrates. Fruit spots (up to 0.5 cm) start off as pale-green, water-soaked areas with a yellow halo and eventually roughen, becoming brown and scabbed.
Bacterial spot is caused by several species of bacteria of the genus Xanthomonas. It occurs worldwide and is one of the most devastating diseases on tomatoes grown in warm, moist environments. The pathogen can survive in or on seeds, on plant debris and on specific weeds. It has a very limited survival period of days to weeks in the soil. When conditions are favorable, it spreads through rain or overhead irrigation onto healthy plants. It enters the plant tissues through leaf pores and wounds. Optimum temperatures range from 25 to 30°C. Once the crop is infected, the disease is very difficult to control and can lead to total crop losses.
Bacterial spot is very difficult and expensive to treat. If the disease occurs early in the season, consider destroying the entire crop. Copper-containing bactericides provide a protective cover on foliage and fruit. Bacterial viruses (bacteriophages) that specifically kill the bacteria are available. Submerging seeds for one minute in 1.3% sodium hypochlorite or in hot water (50°C) for 25 minutes can reduce incidence of disease.
Always consider an integrated approach with preventive measures together with biological treatments if available. Copper-containing bactericides can be used as protectant and give partial disease control. Application at first sign of disease and then at 10 to 14 day intervals when warm, moist conditions prevail. As the development of resistance to copper is frequent, a combination of copper-based bactericide with mancozeb is also recommended.