Protect tomato transplants in the greenhouse from bacterial diseases

Watch for bacterial diseases on tomato transplants in the greenhouse and treat them now. Overcast and humid weather favors bacterial diseases.

Growers of tomato transplants should be scrutinizing plants carefully as they leave the greenhouse.

Bacterial speck

Bacterial speck on tomato is probably one of the easiest bacterial diseases to identify. Small, dark-brown spots occur on the leaves with each spot surrounded by a yellow “halo.” Typically, just a few plants within a flat show symptoms initially. Although bacterial speck may not produce the panic that the other bacterial diseases do, speck can result in significant yield losses if the blossoms become infected.

Bacterial speck
Bacterial speck on tomato seedlings

Bacterial spot

Bacterial spot on tomato is not as easy to identify as bacterial speck. Bacterial spot results in larger spots, or blotches, on the leaves and stems than bacterial speck. The spots may have tan centers and are a maximum of 0.25 inches in diameter. If the infection is severe, the entire leaf turns yellow and eventually drops off of the plant. In the last couple of years, growers in Michigan have experienced significant yield losses and devastating fruit spotting due to bacterial spot.

Bacterial spot
Bacterial spot on tomato foliage

Bacterial canker

Bacterial canker caused severe losses in the field last year and should not be overlooked. Symptoms include small, tan, “blister-like” lesions on the leaves. Symptoms progress to include brown streaking on the petioles and stems.

Plants showing symptoms of bacterial disease should be immediately removed from the greenhouse and destroyed. In addition, plants immediately adjacent to those showing symptoms should also be removed and destroyed. This ensures removal of plants that are contaminated with the bacteria, but not yet showing symptoms. If the affected flat is not immediately removed from the greenhouse, the disease will spread to nearby healthy transplants.

Bacterial canker infection
Bacterial canker infection in a flat of tomatoes

Unfortunately, if the disease begins in a flat that is too far from the walkway to be seen easily, the disease may go undetected until several flats are severely infected. Although epidemics may seem to appear overnight, chances are it had rather humble beginnings in just a few plants and simply progressed unnoticed for a couple of weeks. Plug sheets containing infected plugs should not be reused.

Greenhouse growers of tomato transplants have relied on streptomycin and copper products for disease control. However, there has not been uniform success in halting the spread of bacterial diseases with these products. This could be the result of the bacteria being resistant to the streptomycin or copper, improper timing, or plant coverage of the sprays. This week’s cloudy weather resulting in high humidity coupled with splashing occurring during watering will certainly give the edge to these bacteria that threaten tomatoes.

Bacterial canker on stems and leaves
Close up of bacterial canker on stems and leaves of a tomato seedling

Transplants must be removed

Removing infected transplants from the greenhouse is the most critical component of managing bacterial diseases once they’ve been introduced. Planting diseased transplants into the field ensures that the disease is established early with the greatest potential for yield and quality reduction if the environment favors the disease.

At this point, it is anyone’s guess what the outlook is for the occurrence and development of bacterial disease in the field this season. It is alarming to detect bacterial diseases in transplants. However, it is foolish to think that using infested transplants can result in a favorable field outcome.

It is also unrealistic to expect fungicides such as coppers or streptomycin to “cure” these diseases (only copper is labeled for field use). Watching tomato transplants carefully, learning to recognize disease symptoms, obtaining a solid diagnosis if problems occur, and immediate removal of diseased plants is a disease management strategy that affords a grower some degree of protection against crop failure as a result of bacteria.

Dr. Hausbeck’s work is funded in part by MSU‘s AgBioResearch.  

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