Bacterial speck can be a major, early-season foliar disease of tomatoes. The pathogen, Pseudomonas syringae pv tomato, favors cool, rainy weather conditions. Disease can develop on young seedlings as well as mature plants.
Greater impact appears to occur at earlier crop growth stages. Plant vigor can be reduced along with yield and soluble solids.
The symptoms of the disease on foliage are dark lesions often initially surrounded by yellowing tissue. Margins of the leaves, where moisture collects, may be most affected. While symptoms on leaf margins may be confused with salt burn, speck also infects stems, branches and fruit.
Major disease outbreaks are usually not the result of a single rain event. A series of storms followed by overcast days with cool temperatures increases risk of outbreaks.
Weather conditions that maintain high leaf wetness such as morning dews, extend conditions favorable for disease development. Switching from sprinkler to furrow irrigation during favorable disease conditions is helpful.
In fields with a recent history of speck, rotation out of tomatoes can reduce the incidence of speck in succeeding years. Since the disease can survive in debris of diseased tissue, tillage to cover the residue more thoroughly is helpful.
Resistant varieties offer an effective control measure. However, even when speck resistant varieties are used, remain vigilant when disease conditions are favorable as resistant strains exist. Another reason to continue to monitor fields is other common bacterial pathogens exist, such as bacterial spot (Xanthomonas campestris).
Copper is effective in reducing infection when applied as a preventive application. Mancozeb tank mixed with copper slightly increases effectiveness, although EBDCs are not allowed by some processors.
Application timing may be the most important factor in controlling bacterial speck. While a preventive spray may not be economically warranted, an application made at first sign of the disease is more reasonable. The ability to accurately forecast weather conditions helps guide management. Repeat applications may be required if rainy weather conditions persist. Note that resistance to copper exists, and at best, copper is only partially effective in reducing the impact of susceptible strains.
The chemical control program is only a temporary mechanism to reduce the disease level until warmer, dryer weather arrests the disease.
Visit the UC IPM
According to the University of California IPM website, the telltale bacterial speck dark leaf lesions are frequently concentrated near margins, causing extensive marginal necrosis. Lesions on immature fruit are slightly raised and small, varying in size from tiny flecks to 0.125 inch in diameter and cause raised black spots on mature fruit. Fruit lesions are superficial, seldom penetrating more than a few cells deep.
There are two bacterial speck races in California: Race 0 and Race 1. Many varieties are resistant against Race 0 but none currently possess resistance against Race 1 and the occurrence of this race is increasing throughout the state.