Bacterial spot on tomatoes is caused by four closely related strains of bacteria: Xanthomonas vesicatoria, Xanthomonas euvesicatoria, Xanthomonas gardneri, and Xanthomonas perforans. Individual strains of these bacteria infect tomato plants, pepper plants, or both. This disease is the most dangerous in hot and humid weather conditions. Under these conditions, it can cause very large losses in yields, the affected fruits are very often unusable for fresh consumption or processing. Bacterial spot attacks all above-ground parts of the plant – both leaves and petioles, as well as stems, flowers, and fruits.
The first noticeable symptom of the disease is located on the leaves, initially watery, dark green, and later dark brown, small (<1/8 inch/1-2mm), irregular spots surrounded by a yellowish border. Sometimes the centers of the leaf spots fall out and small holes appear in the leaves. As the disease progresses, lesions in the form of small, brown, round spots also appear on the stems and flower calyx. Affected fruit develops spots about 1/4 inch/2-4mm in size. The spots are slightly raised and crusted, dark brown in color, and often surrounded by a waxy white border.
As mentioned above, the disease develops fastest in warm (75°F – 86°F/24°C – 30°C) and humid environments where there is frequent rainfall or soaked leaves during watering. The disease is most often transmitted by infected seeds or seedlings. On the youngest plants, the disease may not be visible and may only show up over time. Bacteria can be transferred from plant to plant through splashing raindrops, water intended for watering, or by tools and hands of people who care for crops. For these reasons, it’s best to use drip irrigation and to only handle plants when they are dry. Bacteria can survive on plant debris and soil for one to two years, or survive on volunteer tomato or pepper plants.
In order to prevent the occurrence of bacterial spot on tomato crops, select high-quality, certified, disease-free seeds of cultivars that are resistant to bacterial spot. If you are saving your own seeds, hot water treatment and a diluted bleach solution should be used to kill bacteria. When growing your own seedlings, avoid excessive watering and touching of the plants. Greenhouses, tools, and all kinds of equipment that come into contact with the plants or are touched by people who care for the plants must be disinfected frequently.
After the growing season is over, all plant residues should be removed – it is best to burn or bury them deeply, away from your garden area. If the disease has occurred in a given area, take a break for at least one year, and don’t grow tomatoes or peppers in the same place. During this time, clear the area of all tomato and pepper volunteers on which the bacteria could survive. Spray the area with a copper-containing bactericide.