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Tomato Diseases | Bacterial Leaf Spot Fact Sheets
Bacterial Leaf Spot of Tomato
Bacterial spot can produce lesions on all above ground parts of the plant. Foliar symptoms of bacterial spot and speck are identical and can’t be used to reliably identify these diseases. On infected seedlings, small (1/8 inch), dark water-soaked lesions appear on the edges and tips of the leaflets. Water soaked lesions can also be observed on infected transplants, plants and fruit. Lesions on older leaves are dry, necrotic, and chlorotic. As the number of lesions increase they begin to coalesce forming dead areas and eventually cause plant defoliation. Necrotic lesions also appear on the stem and pedicels. Lesions on the pedicels can cause flower abortion, resulting in split fruit set and loss of yield.
Fruit lesions are initiated on immature (green) fruit. Lesions start off as small, greasy water-soaked spots with a light grey to tan colored center. Lesions on older, mature (red) fruit are larger (1/4 inch) than those on immature fruit. Lesion symptoms can vary depending on the Xanthomonas species causing the disease. Typical lesions are irregular in shape, light brown to black, slightly sunken and have a scabby surface texture that is not easily removed from the surface of the fruit skin.
Infections originate from infested seed or by being introduced into the crop on infected crop debris. Volunteer tomato plants and contaminated equipment or tools can also be a source of these bacteria. Bacteria enter the plant through natural openings, such as stomata or hydathodes, or through wounded tissue. In the greenhouse, high plant density and humidity and warm temperatures provide optimal growth conditions for these bacteria and overhead irrigation contributes to spread. In the field, bacteria are spread through splashing water, wind-driven rain, and overhead irrigation water. Plants are more susceptible to infection after a wounding event such as a hail storm or plant maintenance operation (i.e. pruning, cultivation, pesticide applications with high-pressure sprayers).
Favorable Environmental Conditions
Optimal conditions for bacterial leaf spot are high moisture, high relative humidity (≥ 80%) and warm temperatures (75-90 °F).
Often Confused With
- Bacterial Canker – Although bacterial spot fruit lesions may have white halos similar to bird’s eye spotting seen with bacterial canker, the white halo disappears as the fruit matures.
- Bacterial Speck – Look for small black pin-point lesions on the fruit with no white halo as indicators of bacterial speck. Bacterial speck lesions on fruit are superficial and can be easily scraped off of the fruit surface.
The pathogen can be active from the time of plant emergence through to harvest. Because foliar symptoms of bacterial spot and speck are identical, fruit symptoms should be used to distinguish between the two diseases. Unlike bacterial canker, bacterial spot produces lesions on the entire leaf surface. However, caution should be taken as co-infections of the two diseases can occur.
Greenhouse seedlings and plants in the field should be monitored weekly for early symptoms. Symptomatic plant tissue should be sampled and submitted for plant disease diagnosis.
No thresholds have been established for this disease however tolerance is low due to marketability and peeling issues.
Bacterial leaf spot is very difficult to manage once it has been introduced into the crop. Preventative measures are the most cost effective way to manage bacterial leaf spot. Fruit lesions can only be initiated on immature fruit so management strategies used prior to fruiting are most beneficial.
- Start with clean seed – Purchase certified, disease-free seed or sanitize seed with hot water, sodium hypochlorite (bleach) or hydrochloric acid. Here is a link to The Ohio State University Extension Fact Sheet for Hot Water and Chlorine Treatment of Vegetable Seeds to Eradicate Bacterial Plant Pathogens.
- Start with clean transplants – Scout plants daily and destroy plants once a plant disease diagnostic laboratory has confirmed the disease. Apply one or two preventative copper fungicide applications and one application of streptomycin to the seedlings before transplanting them into the field.
- Start with clean equipment and tools – Clean and disinfect all tools and farm equipment prior to working with the transplants or plants. Good sanitation practices are critical to prevent contamination and cross contamination of plants by the bacterial leaf spot pathogen.
- Start with a clean field – The bacterial leaf spot pathogen can survive in the field as long as there is infected crop debris present. Rotate with a non-host crop before re-planting the field with tomato. Avoid rotations with crops in the same family as tomato (pepper, eggplant and tobacco) for 3-4 years. Plant into a field free of volunteer tomato plants.
- Use best cultural practices – Use management strategies that maintain reduced-stress growing conditions. Provide plants with adequate but not excessive nitrogen, improve the organic matter content of the soil through the use of composted green or animal waste or cover crops, avoid overhead irrigation if possible and avoid performing crop maintenance operations while plants are wet.
- Use crop protectants – Field applications of copper fungicides, applied early and often, may slow bacterial leaf spot development over the growing season.