Many Indiana growers may have tomato transplants growing in a greenhouse for field or greenhouse/high tunnel production. The three most likely diseases are bacterial spot, bacterial speck and bacterial canker. This article describes symptoms for these diseases and some management options. While these bacterial diseases thrive in transplant production where plants are often overhead watered, these diseases are not common on tomatoes grown to maturity in greenhouses or high tunnels. This is because, for the most part, tomatoes grown to maturity in a greenhouse or high tunnel do not have the necessary leaf wetness required for these diseases. Bacterial canker is occasionally observed in greenhouse/high tunnel situations since this disease may become established in transplants and becomes systemic in plants. Once bacterial canker is systemic in the plant, it ‘spreads’ within each plant even if it does not spread from plant to plant.
Bacterial speck and spot – The symptoms produced by these two diseases are not easy to distinguish. This is particularly true for symptoms on transplants. Bacterial speck tends to produce dark brown to black round lesions. These lesions may be found throughout the leaf, but are easiest to see on leaf bottoms. Bacterial speck lesions are often surrounded by a yellow halo (chlorosis). Lesions of bacterial spot may be more brown than black and are less likely to be surrounded by a chlorotic halo, although leaves with numerous lesions may have a general chlorosis. Lesions of bacterial spot may have a shot hole appearance with the center of the lesion having fallen out. Part of the reason that bacterial spot symptoms are so diverse is that this disease is be caused by two different species of bacteria as confirmed by a survey of bacterial spot strains in Indiana funded in 2016 by a Specialty Crop Block Grant through the Indiana State Department of Agriculture (ISDA).
These lesions of bacterial speck of tomato were observed on a transplant for sale to homeowners at a retail outlet. Tomato transplants should be inspected for disease symptoms during production or at delivery.
Bacterial Canker – Symptoms on transplants can be easy to miss. Affected transplants may have what appears to be necrotic lesions along leaf margins. Older plants may become wilted or show internal stem discoloration.
Transplants that are watered overhead in a greenhouse are a near perfect environment for all of these diseases. These pathogens need overhead water for disease initiation and spread. The proximity of the transplants to each other allows easy plant-to-plant spread.
As much as possible, growers should use tomato seed tested for these bacterial diseases. Hot water or chlorine treatment of seed should also lessen the chance of seed transmission of these bacterial diseases. Such treatments may be applied by the grower or the seed company. If conducted by the grower, the Midwest Vegetable Production Guide for Commercial Growers 2017 (ID-56) has a section on seed treatments.
Conditions that favor these diseases in transplant greenhouses include prolonged leaf wetness and high relative humidity. Therefore, anything that can be done to lessen the time leaves remain wet or to reduce humidity will help to reduce these diseases. Leaf wetness can be lessened, for example, by avoiding overhead irrigation in the evening when the leaves are likely to remain moist all night. Reduce relative humidity by venting the greenhouse or high tunnel in the evening if possible.
Products that may be used to manage tomato bacterial diseases in a high tunnel or greenhouse include those with the active ingredient copper. Appropriate products may contain copper hydroxide, copper sulfate, copper oxychloride or copper soaps. Unfortunately, many strains of the bacterial spot pathogen are insensitive or resistant to copper products. That is, these pathogens may not respond to the levels of copper usually applied for bacterial diseases. Evidence of copper insensitivity of bacterial spot was observed in the survey mentioned above. Products containing mancozeb (e.g., Dithane®, Manzate®, Penncozeb®) may help to increase the amount of copper available on the leaf to combat bacterial diseases. Note that mancozeb products are fungicides and used alone will have little effect on bacterial diseases. Always check the label to make sure that these products are labeled for greenhouse use. Some copper formulations may be used in certified organic production.
Products that have the active ingredient streptomycin (streptomycin sulfate) may also be used to manage these bacterial diseases in the greenhouse or high tunnel. These products may not be used on field produced tomatoes or tomatoes grown past seedling stage. In our survey of strains of the bacteria that cause bacterial spot of tomato, we found that some strains are resistant/insensitive to streptomycin. However, streptomycin resistance is not as prevalent as copper resistance. Thus, it makes sense to treat the tomato transplants at least once with a streptomycin product. Products with streptomycin sulfate as an active ingredient include: Agri-mycin®, Firewall® and Harbour®.
Oxidate® is another product that can be effective against bacterial diseases and is labeled for greenhouse use. Normally, I am cautious about recommending the use of Oxidate® in the field. Oxidate®, which has the active ingredient hydrogen dioxide, disinfects the plant tissue with which it comes into contact. However, there is little or no residue on the leaf once Oxidate® dries. Therefore, Oxidate® may be less useful than, say, a copper product. Used in the greenhouse, however, Oxidate® may be helpful. When used on a small area, Oxidate® may be applied carefully and thoroughly. In addition, Oxidate® may be applied frequently on a relatively small area, partially making up for the lack of residue for this product. Oxidate® effectiveness will not be influenced by whether the pathogen is copper resistant or not. Oxidate® may be used in some organic certifications.
Serenade Opti® is another product which may be used to manage bacterial diseases in a greenhouse/high tunnel situation (Serenade Max® is an older name for this product). The active ingredient for this product is Bacillis subtilis strain QST 731. Although the active ingredient is a bacterium, it is not necessary that the bacterium be living. Rather, the active ingredient is a product of the bacterium that is present in Serenade Opti®. This means that the product can be mixed with copper. If used in alternation or tank mixed with some of the other products mentioned here, Serenade Opti® should be able to lessen the symptoms of tomato bacterial diseases. Serenade Opti’s® effectiveness will not be influenced by whether the pathogen is copper resistant or not. Serenade Opti® may be used in some organic certifications.
Another product that will not be influenced by whether the pathogen is copper resistant or not is AgriPhage®. This product is a microbe that parasitizes and ultimately kills the pathogens described here. If applied shortly after first symptoms are produced, AgriPhage® can slow the spread of these diseases and symptom development. AgriPhage® is very specific in which strain of pathogen is attacked. One must work closely with the manufacturer, Omnilytics, to obtain product that will be most likely to work in one’s region. AgriPhage® should not be tank mixed or contaminated with copper products. Similarly, Oxidate® will react adversely with AgriPhage®. AgriPhage® is recognized by the National Organic Program (NOP) but not by the Organic Materials Review institute (OMRI).
Many other products are labeled for use in managing bacterial diseases of tomato transplants. However, it is not always clear how effective these products are for disease control. I hope to be able to trial many of these products for efficacy over the next few years.