Jan 27, 2023

Avoid spreading tomato disease when transplanting

When tomato growers call in the winter, it is often to review last year’s production. I’m a plant pathologist, so when last year’s production is reviewed, we discuss tomato diseases. The foliar diseases we most often discuss are bacterial diseases: bacterial speck, bacterial spot and bacterial canker.

Our discussion is often about whether the appropriate management techniques were applied in the field, such as crop rotation, sanitation and, of course, what disease control products were applied, and when. It is often the case that the grower made many of the correct choices and yet had significant disease. So what went wrong?

I would like to discuss here another aspect of managing bacterial diseases of tomato that is too often neglected: transplant production. Foliar diseases that start at the transplant stage may follow the crop all season long. Before I discuss tomato transplant disease management, I will briefly consider important aspects of biology that differ between these three bacterial diseases.

Bacterial spot and bacterial speck cause lesions on tomato foliage, including fruit. The bacteria that cause these diseases spreads by splashing rain or overhead irrigation. If either of these diseases is introduced on seed or transplants, the disease may spread in the transplant greenhouse (most tomato transplant greenhouses are watered overhead). If the transplants are then planted into the field, bacterial spot and speck can spread from leaf to leaf and from plant to plant in the field given sufficient rain or overhead irrigation.

If the transplants are planted in a greenhouse or high tunnel (without overhead irrigation), the lack of rain will not allow bacterial spot or speck to spread.

Bacterial canker is caused by a bacterium that becomes systemic in the tomato plant, possibly moving in the vascular system of the plant. If bacterial canker is introduced on tomato seed or transplants, the disease may spread in the transplant greenhouse. If diseased transplants are planted in a greenhouse or high tunnel, the disease may move to additional leaves, stems and fruit. Bacterial canker in a greenhouse or high tunnel won’t spread to additional plants if there is no overhead irrigation. However, the disease may appear to progress because the disease on individual plants increases in severity.

If transplants with bacterial canker are planted in the field, the disease may increase in severity in individual plants in addition to spread with rain or overhead irrigation in the field. If bacterial canker spreads to plants after first fruit, then the disease may not be severe until late in the season. Bacterial canker that is introduced on transplants, in contrast, is likely to become a very severe disease.

Management of bacterial diseases

Sanitation: The bacterium which causes these bacterial diseases of tomato may survive in seed, crop debris, volunteer tomatoes and equipment such as wooden stakes. All equipment should be cleaned and sanitized between generations of transplants. It is important to clean all surfaces before sanitation. Surfaces that are dirty may not be properly sanitized. Sanitation products include bleach (sodium hypochlorite), quaternary ammonium, hydrogen peroxide products or hydrogen peroxide/peroxyacetic acid products, chlorine dioxide, etc. Follow the directions on the product carefully! 

Seed/transplant considerations: The diseases discussed here may be seedborne. Seed companies spend a great deal of time trying to make sure the seeds are healthy, but there is a small chance that these bacteria may be found in or on the seed. If the seed does harbor the bacteria, it is often only a small percentage of the seed involved. However, the disease may spread quickly in a transplant house where seedlings are close together and watered overhead. With transplants from seed or purchased transplants, one should be aware of the possibility of foliar diseases.  

Seed treatment

It is possible to heat or chlorine treat seed to reduce contamination, by using these methods:

• Hot water and chlorine treatment of vegetable seeds to eradicate bacterial plant pathogens. S. A. Miller and M. L. Ivey, 2005. Ohio State University Extension Bulletin HYG-3085-05. https://u.osu.edu/vegprolab/grafting-publications/hot-water-and-chlorine-treatment-of-vegetable-seeds-to-eradicate-bacterial-plant-pathogens/
• Gatch, E. 2016. Organic seed treatments and coatings. eOrganic Community of Practice. Extension Foundation. https://eorganic.org/node/749

Separate seed lots and varieties

 Since there is a possibility of seedborne disease, it is important to separate all varieties and seed from different seed lots from each other. Separate them with sufficient distance that overhead watering from one variety cannot splash to another variety or seed lot. In this way, if a disease does affect one variety, This makes it unlikely to spread to a different variety or seed lot. 

Inspect transplants

Once the seedlings have emerged, regularly inspect them for diseases and other possible problems such as fertility.  If suspicious symptoms are observed, send them to an official plant disease laboratory to get a diagnosis. 

Treat seedlings

Treat seedlings in the greenhouse starting at about the first true leaf stage and at 5- to 7-day intervals. Use a tank mix combination of copper and mancozeb. Streptomycin products such as Agri-mycin, Firewall or Harbour may be used starting at the 2-leaf stage. Do not apply streptomycin products in the field.

• Peroxide products such as Oxidate may be used in addition to the ones mentioned above. Be careful with mixing the Oxidate with other products. Remember that Oxidate has no residue: once the product has dried, it no longer has any activity against plant pathogens. Therefore, it is best to apply this product frequently. Do not substitute Oxidate for copper or any other product.
• I have thoughts on how to apply products by hand. I favor a backpack sprayer rather than a garden sprayer. See video about the use of backpack vs. garden sprayers.

Growers looking for listed organic products might consider Lifegard, Regalia and/or Serenade Opti. Some formulations of copper may be organically listed, but consult the label carefully before tank mixing copper with microbially based products. 

Be sure to check to see if these products are labeled for greenhouse use in your state.  Always check the label for information on rates as well as pre-harvest and re-entry information. 

— Daniel S. Egel, Department of Botany and Plant Pathology, Purdue University

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