Tomato Transplants & Bacterial Disease

The last few years have seen an uptick in the occurrence of three bacterial diseases in Minnesota tomatoes: bacterial spot, bacterial speck, and bacterial canker. Control of all of these diseases begins during transplant production. These diseases are known to be seedborne, and the trays, tables, and tools used in the greenhouse provide places for the disease to survive from year to year. Like all bacteria, water is key in moving the bacteria from plant to plant, and wounds provide ways for pathogens to enter the transplants.

Greenhouse Sanitation

Good greenhouse sanitation is the basis of bacterial disease control, especially if you struggled with one of these diseases last year. Try not to reuse transplant trays, and give tables and tools a good cleaning and sanitizing before the season starts. Wood tables aren’t sanitize-able, but a good cleaning followed by adequate drying time will help reduce places the bacteria can be harbored.

Starting off Right

Some seed suppliers do some level of treating or testing on seeds, while others do not. It is worth a conversation with your seed supplier to learn about what they do.

Seed treatment on your own can be an option, though it may void some guarantees from the seller. Doing seed treatment on your own is an exact science, and must be done in very particular ways to kill pathogens while not reducing germination. More information on this practice is available from Ohio State University and Michigan State University.

Scouting Transplant and Roguing

Scout transplants carefully. Symptoms on transplants can look different from the bacterial disease symptoms you may be familiar with in the field. Look for:

• Dark black or brown spots with yellow halo

• Tan spots

• Blotchy areas on steams and/or leaves

Bacterial Spot lesions on the underside of the leaves of a tomato transplant. Photo: Cheryl Truman, University of Guelph.

What to do When Diseased Plants are Found

The most important aspect of bacterial disease control is roguing diseased transplants. Research has shown these diseases can move quickly throughout a greenhouse, and that tomato transplants can have bacterial diseases before they show symptoms. This means that roguing should include both plants with disease symptoms and their neighbors.

Research in Florida on bacterial spot has shown that overhead watering allows bacterial spot to move throughout transplant production trays. In the study, one tray of tomato transplants was inoculated with bacterial spot, and placed in a transplant production greenhouse. Within 5 days, the bacteria has moved anywhere from 4 to 11 inches beyond the initial point of infection. After 12 days, the bacteria could be found five to ten feet away. The plants the bacteria were detected on didn’t necessarily show any symptoms of infection yet, with a 5-7 day lag between infection and symptoms appearing. Research in Michigan and the Netherlands has shown bacterial canker can move through transplant trays as well.

Greenhouse conditions (warm, humid, moist) are universally conducive to the movement of bacterial diseases. Symptoms on one plant this week could turn into a trays of infected tomato plants by the time transplanting rolls around (and remember, plants aren’t going to be “cured” in the field).

Bacterial canker causing the edges of leaves to die and wilt. Photo: Zachariah Hansen, University of Tennessee.

Diseased plants should not be composted, instead throw the tray away to get a potential source of future disease off the farm.

Treatment Options

If you plan to use sprays, sprays used preventatively in the greenhouse will do more for your plants than bacterial-specific sprays once they are in the field. Copper works on some strains of some bacterial tomato diseases. Products with the active ingredient streptomycin can also be used while transplants are in the greenhouse. For full information on chemical controls, see the Midwest Vegetable Production Guide.