Symptoms and Signs
There are usually no symptoms of bacterial canker on seedlings; however, on young plants symptoms consist of poor growth and temporary wilting of branches. Lower leaves yellow and shrivel, but symptoms may not show until flowering. On mature plants there are two kinds of symptoms. One kind is from systemic infections (i.e., the bacteria enter the vasculature and invade much of the plant) and the other resulting from secondary infections (i.e., the bacteria cause local infections of leaves, stem, and fruit).
In systemic infections of mature plants, leaflets of the oldest leaves curl, yellow, wilt, and finally turn brown and collapse (known as firing). Sometimes, one side of a leaf is affected. Plants grow poorly and wilt. Pith of stems becomes yellow and later reddish brown, especially at the nodes, and has a mealy appearance. The pith may later become somewhat hollow. In advanced infections, cankers may or may not form at the nodes. Light and later dark streaks may develop on stems. Branches break off easily. Plants may die.
In secondary infections, infection of the margins of leaves is common. Lesions are dark brown to almost black. Round to irregular spotting of leaves also occurs. Fruit may be spotted, especially near calyx.
On fruit bacterial canker symptoms appear as yellow to brown spots, slightly raised, surrounded by a persistent white halo ("'bird's eye spot"'). Spots are usually about 0.125 inch (3 mm) in diameter. Vascular tissue under the calyx scar, leading to seeds that may be brown.
Comments on the Disease
In California, the source of the pathogen is probably seed and transplants, although local contamination within greenhouses is a potential source. In California, the pathogen only overwinters in the soil when the previous crop residue is not thoroughly incorporated and does not decompose. In colder climates, the bacterium may overwinter on undecomposed plant residue.
Tomato is the most important host of the pathogen. Several nightshades, including perennial nightshade (Solanum douglasii), black nightshade (S. nigrum), and S. triflorum, are naturally infected. It is not known how long the bacterium can persist on nightshade. Pepper and eggplant can be artificially inoculated, but they are probably not important in the epidemiology of the disease on tomato.
In California, economic losses in direct-seeded fields are very uncommon. During unusually wet weather, however, secondary spread from frequent vine-training, cultivation, or other operations may cause extensive leaf loss. Canker probably occurs at a low incidence in many direct-seeded fields but almost always goes unnoticed.
Seed contamination with only a few bacterial cells, apparently below the level of detection, can result in relatively high numbers of infected transplants. For that reason, certified seed reduces the chances of infections, but is no guarantee of contaminated-free seed. A seed lot contaminated with very few infested seeds can cause serious problems in a greenhouse.
When the seed germinates, the bacteria enter the seedling through small wounds in the cotyledon, probably through broken trichomes. The bacteria move systemically through the xylem from which it invades the phloem, pith, and cortex. In a highly conducive environment, like a greenhouse, bacteria on the surface of infected plants are then splashed to surrounding plants during overhead irrigation. This kind of spread accounts for the occurrence of groups of plants or trays in the greenhouse and subsequent rows of infected transplants in the field. During planting, which invariably causes wounds, transplants may also be infected after an infected plant is handled, especially if the plants are wet.
Secondary spread occurs in splashing water, on contaminated equipment, during clipping, cultivation, vine-training operations, and other activities. In the field, such spread usually results in local infections (i.e., leaf, stem, and fruit spots). In the greenhouse, these sources can lead to local and systemic infections.
In the field, the pathogen will survive indefinitely in tomato tissue. Once that tissue has decomposed in the ground, however, the bacteria will die because they are not soil inhabitants. Thus, it is very important to turn under infected plant residue at the end of the season. Once that residue decomposes, the bacteria will die and the field does not pose a problem for subsequent plantings. It is prudent, however, to rotate to another crop for at least one season to assure that the tomato residue is completely gone.
In research trials, bacteria have survived as long as 10 months on contaminated wooden stakes. Hence, in the greenhouse it may be extremely important to disinfest the surface of benches and equipment to prevent spread to subsequent trays of transplants. In fields of more mature plants, disinfesting equipment is not as critical because any spread to other plants would probably result in local, and not systemic, infections. It is prudent, however, to wash equipment that has been through a heavily infested field. Surface disinfectants include bleach solutions (0.5 to 1% calcium or sodium hypochlorite) and Physan, among other products.
Planting clean transplants is the most important control measure.
- Vigilantly monitor seed fields and implement strict quality control measures. Assay seed for detectable levels of contamination and discard lots if the bacterium is found. Soak all seed in 130°F water for 25 minutes.
- In the greenhouse, steam potting mix and flats; flats may also be washed with a 1% solution of calcium or sodium hypochlorite. Empty greenhouses between crops of transplants to allow time to disinfest benches and irrigation hoses. Overhead water pressure should be low to prevent wounding during irrigation.
In the field, special measures may have to be taken once canker has been identified.
- Do not work fields when the foliage is wet. Frequent field operations at the wrong time can result in spread of the disease throughout the entire field. Unless the number of infected plants is small, it may do more harm than good to try to remove the symptomatic plants.
- Copper applications offer limited benefits because systemic infections cannot be affected and localized infections (the most probable scenario if other precautions are taken) pose a small economic threat.
- During wet weather, however, bactericides may be justified.
- At the season's end, incorporate all plant tissue. Tissue that remains on the surface and doesn't decompose is a real risk to subsequent tomato crops. Once the tomato residue decomposes, however, canker is no longer a threat.
- To be absolutely certain that the bacterium has been eliminated from the field, rotate out of tomatoes for at least one year.