Diseases of Vegetable Crops

Prepared by Dr. Robert L. Wick, Department of Plant Pathology, Fernald Hall, University of Massachusetts Amherst, MA 01003. Tel. (413) 545 1045 Fax. (413) 545 2532

Bacterial canker of tomato is caused by Clavibacter michiganensis (formerly Corynebacterium michiganense). The disease occurs world wide wherever tomatoes are grown. In New England, bacterial canker occurs less frequently than other tomato diseases but it is potentially more destructive. The bacterium is seed-borne but may survive on plant debris in soil for at least one year. It can also survive in the greenhouse on wooden stakes and flats. Horse-nettle, nightshade and jimsonweed have been reported to harbor the bacterium. Clavibacter can survive and multiply on tomato leaf surfaces even when symptoms are not present. The bacterium can be spread from infected plants by rain, strong winds, farm workers, machinery and fungicides sprayed at high pressure.

Wilt, scorch of the foliage, canker, pith necrosis and fruit spot may occur singly or in combination depending on the circumstances. When the bacterium is carried in the seed, the vascular system becomes colonized, resulting in wilt, pith necrosis and external cankers. Wilt initially occurs on one side of a leaf or one half of a plant because only a portion of the vascular system is blocked. Cankers and pith necrosis are a later stage of disease development. Cankers are dark and water-soaked in appearance and often exude bacteria which are easily spread to adjacent plants. Pith necrosis is first evident as a darkening of the center of the stem which soon becomes chambered or hollow. When leaf scorch occurs, the petioles usually bend downward while the leaf edges curl up. The margins of the leaves become brown with a yellow border to the inside. Scorching of the foliage often develops in the absence of wilt or stem canker. Fruit spots do not occur often but are very characteristic. The spots are roughened, slightly raised in the center and surrounded by a white halo.

With staked tomatoes, activities such as pruning and tying provide an ideal means for the bacterium to spread from plant to plant and enter through pruning wounds resulting in systemic colonization. In ground tomatoes, where wounding and handling of plants is minimal, the bacterium is spread by wind, rain and pesticide applications. Spread in this manner results in entry through the leaf margin resulting in scorch. Surveys indicate that about 25% of the scorched plants may become systemically colonized. Generally, scorch results in less yield loss than does systemic colonization.

Bacterial canker can be difficult to diagnose in the field. For example, cankers can be caused by fungi such as Sclerotinia, Botrytis and Alternaria. Furthermore, Clavibacter does not always result in canker development. Pith necrosis can be caused by Pseudomonas and Erwinia. Wilt can be caused by Pseudomonas solanacearum (from southern transplants only), Sclerotinia, Fusarium, Verticillium and walnut toxicity. The fruit spot is quite distinct but can be mimicked by Xanthomonas (bacterial spot). Marginal leaf scorch is characteristic of canker but could be confused with early blight (Alternaria). Plants suspected of having bacterial canker should be brought to the Plant Disease Diagnostic Clinic in Amherst for confirmation. The specimen should include a portion of the root system with soil. If possible, fruit with symptoms should also be submitted.

Buy certified seed from a reputable source. The use of hot water-treated seed is encouraged. Ideally, seed should be custom-treated by the seed company. If this is not possible, seed can be hot-water treated at home. Some lots of seed can be vulnerable to heat treatment. Always treat a small amount of seed (50-100) of each lot before treating the remainder of the lot. After the test treatment, air dry completely and then remoisten for a germination test. Include untreated seed of the same lot for comparison. Hot-water treatment should be done as follows: Place seed in cheesecloth and tie-up in a loose pouch. Preheat seed in water at about 100 F for several minutes. Bring a large kettle of water up to 127-130 F and drop in the preheated seed. Use an accurate laboratory thermometer. Higher temperatures may result in significant reduction in germination. Soak the seed for 20 to 25 minutes. Maintain the correct temperature for the duration. Remove the seed and thoroughly dry at 70-75 F.

Transplants should be grown in a soilless medium or a pathogen-free soil. Containers, flats and tools should be new or washed with soapy water and disinfested with 10% household clorox. Discard plants that show signs of wilting or any other symptoms of disease. Do not plant tomatoes in the same field for two seasons following an occurrence of bacterial canker. During this time, solanaceous weeds should be controlled. Remove diseased tomato plants as soon as symptoms are evident. Plant late tomatoes in fields separate from early plantings of tomatoes.

The bacterium can be spread by physically handling diseased plants, particularly during wet weather. Avoid working with wet plants when bacterial canker or other bacterial diseases are present. Wash hands with soap and water after handling diseased plants or fruit. To avoid spreading the bacterium to other plantings, work in affected fields or areas after working in disease-free sites. After harvest, plow under plant debris to stimulate decay and reduce the ability of the bacterium to survive. Plow contaminated land after plowing disease-free sites. After plowing, hose-off soil from tractor wheels and equipment.

Fungicides alone will not control bacterial canker, and in fact, will probably contribute to the spread of disease especially when applied with air-blast equipment. When bacterial diseases are present, copper should be mixed with a dithiocarbamate such as Mancozeb®. Bravo CM® is formulated specifically for bacterial canker but will control fungal diseases as well. Bravo CM contains chlorothalonil, copper and maneb. Both Bravo CM and Mancozeb have a 5 day to harvest restriction. Agri-mycin 17® sprays in the greenhouse may reduce populations of bacteria on the foliage.