Management of Bacterial Diseases in Tomatoes

Bacterial speck, spot, and canker can cause serious yield and quality problems for the tomato grower. Although there is no simple control for these diseases, management practices from seeding through to field establishment can minimize the chances of an outbreak.

These three bacterial diseases can affect tomato yield and quality in a number of ways. Defoliation of the plant due to high numbers of bacterial lesions on the foliage can lead to reduced yields as well as sunscald on the tomato fruit. Loss of blossoms can occur, reducing fruit set, while lesions on the fruit itself directly reduce quality. Bacterial spot has been shown to cause significant reductions in fruit size. Localized infections of bacterial canker do not usually affect total yield, but may cause lesions on the leaves and fruit, reducing marketable yield, especially under wet conditions. The more severe systemic infections can result in lower fruit weights, stunted fruit, and earlier ripening and rotting. If the systemic canker infection occurs early in the season (from a seed or greenhouse infection), the entire plant may collapse and die.

Cultural Management

As bacterial infections cannot be cured, they must be prevented. If you do not grow your own transplants, ensure that your transplant grower follows a disease management program. Starting a bacterial disease management program at transplanting time is far too late.

Prevention starts with the seed. Verify with your seed supplier (or transplant grower) that the seed has been adequately acid or chlorine treated to eradicate bacterial pathogens. Through research, guidelines have been established for the concentrations needed and the length of time the seed needs to be in contact with the acid or chlorine solution in order for the bacterial spot, speck, and canker organisms to be eradicated. Starting with clean seed is absolutely critical to bacterial disease control in the tomato crop. One infected seed in 10,000 can be enough to infect an entire greenhouse.

In the transplant greenhouse, ensure that these management practices are followed:

Start with clean trays

Start with clean trays either new or sterilized. Trays can be sterilized by individually washing them to remove growing media or other debris, then dipping or spraying them individually with a disinfecting solution. If bleach is used as a disinfectant, ensure that no residues remain to damage the seedlings. Trays can also be solarized. Stack them and cover them with cloth or plastic and leave them in the empty greenhouse during the summer. The heat will super-dry the disease organisms, while allowing some non-disease organisms to survive to act as biological control agents.

Keep the greenhouse and equipment clean

Wash and disinfect (or solarize) the greenhouse and equipment to avoid contaminating trays, growing media, seeds, or plants. Control weeds around the greenhouse. They can harbour disease organisms.

Minimize human traffic within the greenhouse and handling of the plants.

Use disposable plastic gloves if plants have to be handled, to protect against spreading disease.

Separate seed lots, crops from one another.

Isolate seed lots, if possible, to prevent spread from a contaminated seed lot. Separate tomato and pepper transplant production to avoid cross-infection.

Maintain good ventilation.

Good ventilation is one of the most important management factors in controlling bacterial diseases in the greenhouse. Never allow dew to form on the plants. Minimize the length of time the leaves are wet.

Use good watering practices.

Use a fine, gentle mist or spray to avoid damaging the plant. Water early in the day, under conditions in which the foliage will dry in 1-2 hours. Using warm water 15-20° C (59-68° F) avoids shocking the plants.

Avoid stressing the transplants.

Avoid causing water, nutrient, temperature stress… Plants under stress will be more susceptible to disease.

Monitor the plants in the greenhouse.

At the first sign of bacterial disease symptoms, remove the affected plants and as many surrounding trays as possible. Plants in many surrounding trays could have been infected and could spread the pathogen even though they are not yet showing symptoms.

Copper in the disease management program

Managing bacterial disease in the tomato crop requires a multi-pronged approach. No single control measure will be effective. The first part of this article outlines cultural management practices which should be followed to minimize or prevent a bacterial disease outbreak. The following section will outline a spray program to complement the cultural management practices.

There are no products available to cure bacterial diseases, but copper fungicides are able to slow bacterial disease development. If used correctly - PREVENTATIVELY - they can avert an outbreak, even if the pathogen is present, by keeping bacterial populations low. A number of copper products are registered for bacterial disease control in field tomatoes. In May of 2001, Kocide 101 received a minor use registration for use in tomato (and pepper) transplant greenhouses.

The bacterial disease control program must begin in the greenhouse. If you do not grow your own transplants, talk to your transplant supplier to ensure that he or she follows an adequate disease management program.

In the greenhouse, apply three to four applications of Kocide 101 at 0.9 kg/ac, at approximately 5 day intervals, over a four week period prior to transplanting. For best results, use a low volume of water - less than you would normally use for watering. Tomato transplant growers have found that the spray is much more effective when applied in lower water volumes. Always consult the product label before applying any crop protection product and wear appropriate protective equipment.

After transplanting to the field, the Kocide program should be continued. If you know that the spray program has been followed in the greenhouse, then you should apply a copper spray with your first one or two fungicide sprays, timed according to TOM-CAST (The TOM-CAST program, for weather-timed fungicide scheduling, is delivered by the Ontario Weather Network. See http://www.weatherinnovations.com/ or phone 519-674-1532 for more information.). The greenhouse applications of Kocide should protect the young plant for the first several weeks after transplanting.

If your plant supplier did not follow the Kocide spray program in the greenhouse, then one or two additional copper sprays should probably be applied soon after transplanting, before you would normally start your fungicide spray program.

If this program, along with other disease management practices, has been followed, then you should not have to continue the copper spray program after the first couple of regular fungicide sprays. If you have bacterial populations under control early, then by the time the tomato plant has small green fruit you should be finished with your copper spray program.

Don't be confused by this - your bacterial disease control program may be finished - but your fungicide spray program must continue through the season. Consult OMAF Publication 363, Vegetable Production Recommendations, for more information.