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Start with Clean Tomato Seed and Transplants

M. Grabowski, UMN Extension Educator
MN Department of Agriculture

In 2015, the Minnesota Department of Agriculture found a bacterial pathogen of tomato responsible for the disease bacterial canker in community gardens and small vegetable farms. Although this tomato disease is common in other states, it had been rarely reported in Minnesota. That changed with thirteen confirmed cases in nine different counties in 2015.

Bacterial canker is caused by the bacteria Clavibacter michiganensis subsp. michiganensis (CMM). This bacterial pathogen is capable of infecting tomato, pepper and the weeds cutleaf nightshade and eastern black night shade. In tomato, when disease is mild, the pathogen causes browning of leaf edges and fruit spots. When disease is severe, the stem cracks and becomes discolored. Leaves wilt and the entire plant may collapse and die.

This disease is difficult to control once established in the garden so prevention is an important management tool. Bacterial canker can be brought into the garden on infected tomato seed or transplants. The bacteria can be attached to the outside of the seed coat or carried within the seed. Infected transplants and seeds rarely show obvious symptoms of infection.

There are several steps that a gardener can take to improve the chances of starting with healthy seeds or transplants.

Purchase seeds from a reputable supplier. Most seed companies will not guarantee disease free seed but a good seed company will take steps to reduce the chances of seed borne pathogens. 

If you are saving seed or swapping seed with neighbors, save seeds only from healthy plants. In Minnesota it is difficult to grow a completely disease free tomato in the garden. There are many fungal and bacterial pathogens that infect garden tomatoes. Choose healthy fruit from healthy plants whenever possible. If you suspect seed may be contaminated, there are two seed treatment options that can help to clean seed. Both treatments can reduce germination of seed that is old or of poor quality, but have minimal effect on fresh, good quality seed.

Bleach Treatment
Make a solution with one part bleach (5.25% hypochlorite) and four parts water. Add a few drops of dish soap. Add seed to the solution and allow it to sit for one minute, stirring occasionally. Seed should be able to float freely so that all surfaces come in contact with the solution. Pour the solution through a thin mesh sieve or a cheese cloth. Rinse the seed in cool running tap water for 5 minutes. At this point seed can be directly planted or dried completely on a screen, then stored. Direct planting after treatment is preferable.

Bleach seed treatment can be used on any kind of seed including tomato. It will remove pathogens from the surface of the seed coat but not from within the seed. This means for bacterial canker, bleach treatment only partially reduces the risk of infection from contaminated seed.

Hot Water Treatment
Sous vide set up as a water bath
Soak tomato seeds in water heated to 100 F for 10 minutes. Then move seed into water heated to 122 F and soak the seeds for 25 minutes. Pour the seed through thin meshed sieve or a cheese cloth. Rinse the seed in cool running tap water for 5 minutes. It is critical that the exact time and temperature requirements be met with precision. This is often accomplished with a laboratory quality hot water bath. A sous vide, is a cooking device designed specifically to maintain exact temperatures in water and could be used in the absence of a water bath. Finally seed can be directly planted or dried completely on a screen, then stored. Direct planting after treatment is preferable.

Seed in a plastic cup, ready to treat
Hot water seed treatment is effective in eliminating the majority of bacterial plant pathogens from both the surface of the seed coat and from within the tomato seed. The time and temperature requirements for hot water treatment varies by plant and some seeds like peas, beans and squash may be seriously injured by hot water treatment.  The description above covers only tomato seeds.

Tomato Transplants

Tomato transplants should be purchased from a reputable local grower. Inspect plants carefully and reject any transplants with discoloration of leaves or stems, or any signs of wilting. Avoid any transplants that have been pruned or cut back, as bacterial pathogens can easily spread on tools. 

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