Bacterial Canker Disease in Tomato

Mike Davis, Extension Plant Pathologist, UC Davis

What causes the disease?
The disease is caused by a bacterium named
Clavibacter michiganensis subsp. Michiganensis
(formerly called Corynebacterium michiganense).

What are the symptoms?
On seedlings – usually none.

On young plants – poor growth and temporary wilting of branches. Lower leaves yellow and shrivel. However, symptoms may not show until flowering.

On mature plants – two kinds, those resulting from systemic infections, i.e., the bacteria enter the vasculature and invade much of the plant, and those resulting from secondary infections, i.e., the bacteria cause local infections of leaves, stem, and fruit.

In systemic infections, leaflets of the oldest leaves curl, yellow, wilt, and finally turn brown and collapse (known as firing). Sometimes, only 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. Round to irregular spotting of leaves also occurs. Fruit may be spotted, especially near the calyx.

On fruit – Yellow to brown spots, slightly raised, surrounded by a persistent white halo (“bird’s eye”). Spots are usually less than ¼ inch diameter. Vascular tissue under calyx scar and leading to seeds may be brown.

What is the primary source of the bacteria?
In California, probably seed and transplants. Overwintering in our soils does not seem to be important. However, bacteria can overwinter in California on tomato residue that was not incorporated into the soil.

Can assayed seed eliminate the disease?
Seed contaminated 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 chance of infections, but is no guarantee of contaminated-free seed. A seedlot contaminated with very few infested seeds can cause serious problems in a greenhouse.

How does the disease spread?
When the seed germinates, the bacteria enter the seedling through leaf hydathodes (pores) or small wounds in the cotyledon, probably through broken trichomes (hairs). The bacteria move systemically through the xylem and then invades the phloem, pith, and cortex. In a highly favorable 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 diseased plants in the greenhouse and subsequent rows of infected transplants in the field. During field planting, which invariably causes wounds, transplants may be inoculated after a contaminated plant is handled, especially if plants are wet.

Secondary spread occurs in splashing water, on contaminated equipment, during clipping, cultivation, or vine training operations, and other activities. In the field, such spread usually results in only local infections, i.e., leaf, stem, and fruit spots. In the greenhouse, these sources can lead to local and systemic infections.

How long do bacteria survive?
In the field, the bacteria will survive indefinitely in tomato tissue. Once that tissue has decomposed in the ground, however, the bacteria will die since they are not soil inhabitors. 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 is disinfest the surface of benches and equipment after cleaning to prevent spread to subsequent trays of transplants. In fields of more mature plants, disinfesting equipment is not as critical since any spread to other plants would probably result in local, and not systemic, infections. It is cautious to wash equipment that has been through a heavily infested field.

What are the hosts of the causal organism?
Almost exclusively tomato. Several nightshades, including perennial nightshade (Solanum douglasii), black nightshade (S. nigrum), and S. triflorum, are naturally infected. Pepper and eggplant can be successfully artificially inoculated, but they are probably not important in the epidemiology of the disease on tomato. It is not known how long the bacterium can persist on nightshade.

Does canker cause problems in direct-seeded tomato fields?
In California, economic losses in direct-seeded fields are probably very uncommon. During unusually wet weather or sprinkler irrigation, however, secondary spread can occur. Also, frequent vine-training, cultivation, or other operations when the foliage is wet may cause extensive leaf loss. Canker probably occurs at a low incidence in many direct-seeded fields but almost always goes unnoticed.

What are some control measures?
Planting clean transplants is the most important control measure. Seed fields should be vigilantly monitored and strict quality control measures implemented. Seeds should be assayed for detectable levels of contamination and lots discarded if the bacterium is found. It may be prudent to soak all seed in a dilute solution of hydrochloric acid as an extra precaution, although this does not guarantee complete elimination of the bacterium. Sodium hypochlorite (bleach) also fails to completely eliminate the bacterium from seed. In the greenhouse, potting mix and flats should be steamed or washed with a 1% solution of sodium hypochlorite or other surface disinfectant.

Sanitation
Greenhouses should be emptied between crops of transplants to allow time to clean and disinfest benches, irrigation hoses, etc. Overhead water pressure should be low to prevent wounding. Applications of streptomycin or copper (check registrations) may be necessary to reduce the efficient, yet unnoticeable spread between plants.

In the field, special measures should 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.

Bactericides
Copper or streptomycin applications offer limited benefits since systemic infections can not be affected and localized infections (the most probable scenario if other precautions are taken) pose a small economic threat. During wet weather, however, bactericides are probably 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.

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