Bacterial Speck of Tomato
Pseudomonas syringae pv. tomatobacteria
The bacterium may attack plants at all stages of development. The symptoms are mainly visible on leaves and fruits and are characterized by the appearance of tiny, round, black spots with a narrow yellow halo. The spots are usually scattered and small, but in severe cases they may coalesce or overlap, resulting in larger and irregular blemishes. They also tend to aggregate towards the tip of the leaves, which may curl up. On fruit, minute, slightly raised, black specks develop but only affect superficial tissue. When small fruit are infected, the specks may be sunken.
The symptoms are caused by a bacterium known as Pseudomonas syringae pv. tomato, that survives in soil, on infected plant debris, and on seeds. Infected seeds used for planting are the first source of inoculum, as the bacterium grows and colonizes the developing plant. It can affect both the tomato foliage and the fruit. Secondary source of infection are the bacteria growing on leaves and fruits, that are later spread between plants by rains splashes and cool damp conditions. Serious disease outbreaks are relatively infrequent, and are favored by high leaf wetness, cool temperatures and cultural practices that allow the bacteria to be disseminated between host plants. In severe cases, infected plants are stunted, which may result in a delay in fruit maturity and yield reduction.
Seed treatments include soaking seeds in 20 % bleach solution for 30 minutes to reduce the bacterial load. Because this may affect germination rates, seeds can also be treated with water at 52 °C for 20 minutes. When harvesting seeds, allow the seeds to ferment in the tomato pulp for one week in order to kill the pathogen.
Always consider an integrated approach with preventive measures together with biological treatments if available. Copper-containing bactericides can be used preventively or curatively, directly after detection of the first signs of the disease, to provide partial disease control. Repeat treatment in 7-14 day intervals when cool, rainy and moist conditions prevail. As the development of resistance to copper is frequent, bactericide combination with mancozeb is also recommended.