Bacterial Speck (Spot) on Tomatoes
The bacterium, Xanthomonas campestris pv. vesicatoria
Tomato and pepper
This disease is most important in the eastern half of Oklahoma where rainfall and humidity levels are moderate to high. The bacteria survive on diseased plant debris and on tomato seed. Leaves, stems, and fruit may be infected at any growth stage when plants are wet and temperatures range from 75-86 degrees F. The bacteria enter plants through natural openings or wounds and form leaf spots that are dark brown in color. The leaf spots appear greasy when leaves are wet, and rarely exceed 1/3 of an inch in diameter. The leaf spots produced in bacterial spot are easily confused with those produced in bacterial speck and the initial symptoms of the fungal diseases - Septoria leaf spot and early blight. Bacterial spots are less uniformly distributed over the leaf surface and lack the tiny fruiting bodies (specks) found within Septoria leaf spots, and also lack the concentric rings associated with early blight spots. In addition to the leaf spots produced, a general yellowing of heavily spotted areas on leaves may occur, followed by leaf scorch. Blighting (rapid death) of foliage progresses upward on heavily infected plants. Fruit spots are conspicuous on green fruit and appear raised and scabby, dark brown in color, and are up to 1/3 of an inch in diameter. Spots on ripe fruit and similar except that they are sunken.
Control of bacterial spot is difficult once it becomes established in a tomato planting. Therefore, production of purchase of disease-free transplants is essential. Transplants should be started from bacteria-free seed produced in an area where the disease does not occur or from treated seed (hot water, chlorine bleach). Remove and destroy diseased crop debris or incorporate it into soil soon after harvest is complete. Rotate tomato plantings with crops other than tomato and pepper to avoid carryover of bacteria from year to year. Using drip irrigation reduces bacterial spread and the leaf wetness periods that favor infection compared to sprinkler irrigation. Avoid working in plantings when foliage is wet. Intensive spraying of bactericides (mixtures of copper + fungicide) may reduce disease development. Please contact your local county extension office for current information.