Bacterial spot ("Doa bakteria" in Swahili) is a disease attacking all growth stages of tomatoes. It causes water-soaked spots on leaves, stems, fruits. Small round or irregular greasy dark green spots are first seen on the undersides of leaves. As spots grow to 2-3mm, they become purple-grey with a black centre. Spots are sometimes surrounded by a yellow ring. On the fruits, small pale green raised areas can appear surrounded by a water-soaked border. Old spots turn brownish and become raised and scabby, are sometimes surrounded by a white ring. Spots do not go deeply into fruit, but allow other fungi and bacteria to rot the fruit.
The bacteria can be carried on tomato and pepper seeds. It is capable of surviving on plant debris in the soil and on volunteer plants in abandoned fields. This bacterium does not live for very long in the soil. Wet weather, rain splash and crowded planting favour the spread of the disease. The bacteria enter the plant through natural openings and wounds on the leaves and fruit caused by mechanical damage, for example by wind-blown sand. It is easier to prevent this disease than to control it.
To prevent the problem:
- Use non-infected clean seeds from healthy tomatoes, or certified seeds, because this disease lives on seeds.
- Kill disease on <1 year old tomato seeds by soaking them in 50°C water for 25 minutes, then cooling, drying. But this slightly reduces germination.
- Keep out volunteer tomato plants or other Solanaceae of fallow fields.
- Select nursery sites away from tomato fields.
- Use furrow irrigation instead of sprinkler irrigation to reduce disease spread.
- Interrupt tomato growing for at least 2 cycles. Do not rotate with Solanaceae (Irish potatoes, red pepper…), as they can have this disease.
- Avoid working in fields under wet conditions.
If typical symptoms appear on leaves of about 5 to 10% of the plant population, action is suggested:
- Remove infected seedlings with roots and surrounding soil and burn or deeply burrow everything. Do not re-plant tomatoes into the same spot.
- Prune infected leaves and burn them.
- You may reduce further infections by spraying copper oxychloride or copper hydroxide. Both are toxic (WHO toxicity class II: moderately hazardous). Wear protective clothing. Spray earlier than at least 21 days before harvest.
When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.