How to Manage Pests
UC Pest Management Guidelines
Pathogen: Xanthomonas campestris pv. vesicatoria
(Reviewed 12/13, updated 12/13)
In this Guideline:
Bacterial spot develops on seedlings and mature plants. On seedlings, infections may cause severe defoliation. On older plants, infections occur primarily on older leaves and appear as water-soaked areas. Leaf spots turn from yellow or light green to black or dark brown. Older spots are black, slightly raised, superficial and measure up to 0.3 inch (7.5 mm) in diameter. Larger leaf blotches may also occur, especially on the margins of leaves. Symptoms on immature fruit are at first slightly sunken and surrounded by a water-soaked halo, which soon disappears. Fruit spots enlarge, turn brown, and become scabby.
Comments on the Disease
The bacterial spot bacterium persists from one season to the next in crop debris, on volunteer tomatoes, and on weed hosts such as nightshade and groundcherry. The bacterium is seedborne and can occur within the seed and on the seed surface. The pathogen is spread with the seed or on transplants. Secondary spread within a field occurs by splashing water from sprinkler irrigation or rain. Infection is favored by high relative humidity and free moisture on the plant. Symptoms develop rapidly at temperatures of 68°F (20°C) and above. Night temperatures of 61°F (16°C) or below suppress disease development regardless of day temperatures. Some pathogen strains are virulent on either tomato or pepper and some may be virulent on both.
Cultural practices and preventive sprays of copper help to manage bacterial spot.
Bacterial spot occurs commonly in tomatoes throughout California. Using pathogen-free seed and disease-free transplants, when possible, is the best way to avoid bacterial spot on tomato. Avoiding sprinkler irrigation and cull piles near greenhouse or field operations, and rotating with a nonhost crop also helps control the disease.
Organically Acceptable Methods
Cultural controls and some copper formulations are acceptable for use on organically certified produce.
Monitoring and Treatment Decisions
Copper-containing bactericides provide partial disease control. Apply at first sign of disease and repeat at 10- to 14-day intervals when warm, moist conditions prevail. Copper is strictly a protectant and must be applied before an infection period occurs.
Resistance to copper has been observed, but can be somewhat overcome by combining copper with mancozeb.
UC IPM Pest Management Guidelines: Tomato
R. M. Davis, Plant Pathology, UC Davis