Bacterial speck appears as dark brown to black lesions of various sizes and shapes on leaves, fruit, and stems. Tissue adjacent to the lesions is initially yellow. Leaf lesions are frequently concentrated near margins, causing extensive marginal necrosis (tissue death). Lesions on immature fruit are slightly raised and small, varying in size from tiny flecks to 0.125 inch (3 mm) in diameter and cause raised black spots on mature fruit. Fruit lesions are superficial, seldom penetrating more than a few cells deep.
The bacteria survive in soil, in debris from diseased plants, and on seeds. Infection is favored by cool, moist weather. The pathogen is spread by splashing rain or sprinkler irrigation. Disease progress is stopped during hot weather. In severe cases, infected plants are stunted, which may result in a delay in fruit maturity and yield reduction.
Cultural controls and copper spray generally provide adequate control of bacterial speck in early planting.
Delay planting in spring to avoid exposing tomatoes to cool, wet conditions that favor disease development. When the disease appears, change from overhead to furrow irrigation. Do not plant tomatoes in a field previously planted to tomatoes that developed the disease; instead rotate with a nonhost crop such as small grains or corn.
Copper-containing bactericides provide partial disease control. Timing is critical. Apply before rainfall and repeat at 10- to 14-day intervals when cool and moist conditions prevail. Copper is strictly a protectant and must be applied before an infection period occurs. One or two treatments are usually enough to protect tomatoes during the most susceptible stages of growth. Spraying can stop when temperatures move into the 90 F range.
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.
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 tomatoes. Avoiding sprinkler irrigation and cull piles near greenhouse or field operations, and rotating with a nonhost crop also helps control the disease.
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.