Tomato scab

Tomato scab is a bacterial disease that primarily affects the stems, leaves, and fruits of tomato plants. In the early stages, water-soaked spots appear on the underside of the leaves, which gradually develop into nearly circular or irregularly shaped brown lesions. These lesions are rough and uneven, often surrounded by a yellowish chlorotic halo. As the disease progresses, the affected areas become dry, brittle, and may lead to leaf drop. On the stems, the initial symptoms appear as small, water-soaked, chlorotic spots that expand into elongated oval shapes. These lesions often have a dark center with slight depressions. On the fruits, the first signs are water-soaked, chlorotic spots that gradually grow in size. Initially, they look oily and light in color, then turn yellow-brown or dark-brown as they develop into cork-like, rough, sunken spots. These spots are typically 0.2–0.5 cm in diameter and may merge into larger, irregular patches. When the fruit stem or base becomes infected, the fruit can easily fall off. The pathogen responsible for tomato scab is *Xanthomonas campestris pv. vesicatoria*, a short, rod-shaped bacterium with rounded ends. This pathogen overwinters in infected plant debris or on contaminated seeds. In the following growing season, it spreads through wind, rain, insects, and mechanical wounds, entering the plant through stomata or injured tissues. High humidity, warm temperatures, heavy rainfall, and excessive irrigation create favorable conditions for disease development. Frequent pruning, insect damage, or storm events that cause more wounds also increase the risk of infection. To manage tomato scab, several control measures can be implemented. First, it is essential to use disease-free seeds from certified sources. Seeds can be treated with a 1% sodium hypochlorite solution for 20–30 minutes before sowing, followed by thorough rinsing with clean water. This helps eliminate bacteria on the seed surface and prevents early infections. When the disease is detected, effective chemical treatments include applying antibiotics such as streptomycin at a dilution of 4000–5000 times, or using 200-fold diluted bacterial spirit. Another option is to spray 50% copper potassium citrate wettable powder at a concentration of 400–500 times. These sprays should be applied every 7–10 days, with a total of three applications recommended for best results. In addition to chemical controls, cultural practices such as proper crop rotation, good drainage, and avoiding overhead irrigation can reduce disease pressure. Monitoring fields regularly and removing infected plants promptly can also help prevent the spread of the disease. By combining these strategies, growers can effectively manage tomato scab and protect their crops.

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