Bacterial speck and spot are common bacterial diseases of tomatoes in the southeastern United States.
Signs and Symptoms
Symptoms of bacterial speck can occur on leaves, stems, petioles, peduncles, pedicels, sepals, and fruit. Round, dark-brown to black lesions develop on leaves infected with the bacterial speck pathogen; over time, a yellow halo may develop around leaf lesions (Figure 1). These lesions extend to both leaf surfaces but are more prominent on the lower leaf surface. Lesions on stems, petioles, peduncles, pedicels, and sepals tend to be more elongated. Small, dark lesions that are commonly flat or slightly raised develop on infected fruits; as fruits enlarge, these lesions become sunken or pitted (Figure 2). A dark green halo may also develop around fruit lesions (Figure 2).
Symptoms of bacterial spot also can develop on all aboveground plant parts. Brown, circular lesions develop on leaves, stems, and fruit spurs. The tissue surrounding lesions on leaflets may become chlorotic (yellow) (Figure 3). In some bacterial spot infections, leaves may have a shot-hole appearance. As lesions coalesce, the foliage becomes blighted and may defoliate or remain attached to the plant. Lesions on fruits begin as small, slightly raised blisters that grow and become scab-like (Figures 4 and 5). Bacterial spot can also occur on peppers, and symptoms on peppers are similar to those observed in tomatoes (Figures 6 and 7).
In both bacterial speck and spot, lesions may coalesce to kill large areas of leaf tissue or cause streaking. Fruit lesions of bacterial spot tend to be rough compared to those of bacterial speck. However, the symptoms of bacterial speck and spot are very similar and may easily be confused with one another. Pathogen isolation and laboratory testing are often necessary to confirm a diagnosis. Symptoms associated with bacterial speck and spot on leaflets may also be confused with other diseases of tomatoes, such as early blight, gray leaf spot, and target spot.
Pathogen and Disease Spread
Bacterial speck is caused by the bacterium Pseudomonas syringae pv. tomato. This pathogen is seedborne and can overwinter in crop residue in temperate climates. Disease development is favored by high humidity and temperatures between 64°F and 75°F.
Bacterial spot is caused by several species of Xanthomonas: X. vesicatoria, X. euvesicatoria, X. gardneri, and X. perforans. Xanthomonas perforans has been confirmed on tomatoes in Mississippi (Abrahamian et al., 2017). These pathogens can be spread by seed and can overwinter in crop residue. Disease development is favored by high precipitation, high humidity, and temperatures between 75°F and 86°F.
The bacterial speck and bacterial spot pathogens can be spread easily during transplant production in greenhouses when plants are densely grown and watered overhead. These bacteria may survive on the surface of transplants without causing disease. These pathogens may also be spread by splashing water and wind-driven rain and on tools, machinery, and workers’ hands. The bacteria enter plants through natural openings and wounds. Volunteer tomatoes and, in the case of bacterial spot, peppers can serve as sources of inoculum.
