How to deal with bacterial diseases of tomatoes

Three bacterial diseases commonly affect tomato crops: Bacterial Spot (Xanthomonas campestris pv. vesicatora), Bacterial Speck (Pseudomonas syringae pv tomato), and Bacterial Canker (Clavibacter michiganensis pv michiganensis). Tomato pith necrosis is a less common bacterial disease that can be confused with bacterial canker. These diseases can affect foliage, fruit, and stems and distinguishing between them can be difficult. In general, bacteria are commonly introduced via overwintered weeds, crop debris, or by seed. Secondary spread of bacterial cells occurs by rain splash and movement of workers or equipment through the field. Management of bacterial diseases is difficult once they are established, so using good sanitation such as buying clean seed, hot water treating seeds, maintaining good weed control, and rotating crops are essential to preventing disease onset.

Bacterial spot caused by Xanthomonas campestris pv. vesicatora (Xcv) is present wherever tomato and peppers are grown. In general, Xanthomonas pathovars have narrow host ranges. Xcv consists of different strains that vary in their pathogenicity to tomato, pepper, and solanaceous weeds. The bacterium is able to survive on tomato volunteers and can overwinter in diseased plant debris or on tomato stakes. Seed is an important mechanism for survival and dissemination of Xcv. Disease development is favoured by temperatures between 80˚ and 90˚ F and by heavy rainfall. The bacterium is spread by wind-driven rain, workers, farm machinery, and by aerosols in humid air. It penetrates plant tissues through stomates, wounds created by insects, wind-driven sand, and tools. Xcv affects all aboveground plant parts. On leaves, the spots are generally brown, circular, and watersoaked. Bacterial spot lesions do not have concentric zones or a prominent halo. When conditions are optimal for disease development, spots can coalesce to form long, dark streaks. A general yellowing may appear on foliage with many lesions giving the plants a scorched appearance, and the plants may exhibit severe bending and twisting. Only green tomato fruit is susceptible to infection, and lesions are quite distinct, beginning as minute, slightly raised blisters with a halo that resemble the birds-eye spot caused by Clavibacter michiganense (bacterial canker). As lesions enlarge, they lose their halo and become brown, raised, and scab-like on ripe fruit. Lesions on ripe pepper fruit may be scab-like or sunken.

Bacterial speck (Pseudomonas syringae pv. tomato) causes a fruit spot and foliar blight on tomato only, not pepper and is generally of minor concern. Disease development is favoured by low temperatures and high moisture. The pathogen can be seedborne, and may persist in weed species. Secondary disease spread within fields occurs by wind driven rain, workers, farm machinery, and by aerosols in humid air. Lesions on leaves are round and dark brown to black with a halo that develops with time. Spots may coalesce, killing large areas of tissue. On fruit, small (1/16 inch), dark spots or specks develop with the tissue around them often more intensely green than unaffected areas.

Bacterial canker (Clavibacter michiganensis pv michiganensis) is one of the most destructive tomato diseases in Massachusetts. Initial symptoms are the result of primary, systemic, infections. The lower leaves are affected first, exhibiting leaf curling, wilting, yellowing, and shrivelling. In advanced stages, the pathogen spreads throughout the plant and causes poor growth, wilt, and plant death. Foliage throughout the canopy wilts, yellows, turns brown, and collapses. Stems can split resulting in open breaks or cankers and stems break easily. Secondary infections occur from rain splash onto foliage, stems, and fruit. Spots occur on green fruit and are very characteristic: white to yellow spots, 3-4 mm with raised brown centres known as “bird’s eye spots”.

Tomato Pith Necrosis is caused by Pseudomonas corrugata and other soil-borne species of Pseudomonas. While high tunnels and greenhouses provide ideal conditions for the growth of early season tomatoes, this environment also provides ideal conditions for this emerging disease. Pith necrosis generally occurs on early planted tomatoes growing when night temperatures are cool, the humidity is high, and the plants are growing vigorously because of excessive levels of nitrogen. The disease is also associated with prolonged periods of cloudy, cool weather. Initial symptoms often appear just as the first fruit clusters reach the mature, green stage, and consist of yellowing and wilting of young leaves. Serious infections can result in yellowing and wilting of upper portions of plants, with brown to black lesions on infected stems and petioles. When stems are cut longitudinally, the centre of the stem (pith) may Bacterial Spot (Xanthomonas campestris pv. vesicatora) (Xcv) Bacterial speck (Pseudomonas syringae pv. tomato) Bacterial canker (Clavibacter michiganensis pv michiganensis) Tomato Pith Necrosis (Pseudomonas corrugata) be extensively discoloured, hollow, and/or degraded. Stems may be swollen, numerous adventitious roots can form, and infected stems may shrink, crack, or collapse. The epidemiology of this disease is not well understood; it is possible that the bacteria are seed-borne and most certainly survive in the soil in association with infected tomato debris.

Preventive measures to minimize the occurrence of pith necrosis in high tunnels include: adequate ventilation to avoid high humidity levels (especially during cloudy weather), avoiding excessive nitrogen levels to prevent vigorous plant growth, incorporation of crop debris to speed decomposition of residue and associated bacteria, and crop rotation. There is no effective treatment for this disease. Affected plants may recover if environmental conditions improve (warm, sunny weather) but if not, affected plants should be removed from the field to prevent spread of the disease. Preventing losses to bacterial diseases:

1. Start with certified, disease-free seed or treat seed with hot water, hydrochloric acid, calcium hypochlorite, or other recommended materials. See the fact sheet entitled, Preventing Bacterial Diseases of Vegetables with Hot-water Seed Treatment for further details.
2. Control bacterial populations that may be present on the leaf surface of transplants in the greenhouse. Young transplants may not display symptoms of bacterial diseases. Inspect and remove suspect transplants. Lower the water pressure in irrigation equipment to avoid damaging leaves. Avoid the practice of mowing transplants to regulate transplant height or suckering plants when symptoms are present.(See New England Vegetable Management Guide for more details.)
3. Plant into a clean field using sterilized stakes. Promptly incorporate crop debris after harvest. Rotate to a non-host crop before returning to tomato and do not allow volunteer tomato or weed hosts to survive.
4. Avoid working in fields when bacterial diseases are present and the fields are wet. Work in infected fields last.
5. In general, bacterial diseases of field crops are difficult to control with pesticides. Copper products are most effective, and the addition of mancozeb products can increase their efficacy. Streptomycin (eg., 45Agri-mycin 17) is an effective product that may be used only in the greenhouse before transplanting to the field. When a significant amount of disease is present, pesticides are usually not effective. Biological disease control products that have shown efficacy in some trials on bacterial diseases in tomato include Actigard or Regalia (both plant defence activators).

Source: UMass Extension Vegetable Program, extension.umass.edu/vegetable/, article written by B. Dicklow and S. B. Scheufele.