A produce farmer who has since passed away and I respected a lot, was known for saying, “My bottom line is always better on a dry year.”

I’m sure many growers would agree with him, especially those who have the ability to irrigate their crops. When farmers have the ability to control the timing and quantity of water on their crops, it is a tremendous benefit.

On low rainfall years, disease pressure is lower since many plant diseases require a period of leaf wetness or soil saturation to flourish. When we have a lot of rainfall, plant disease runs rampant, and the use of fungicides and rotten crops is very costly. While drip irrigation systems are relied upon heavily, there is no substitute for a nice, well-soaking rain from time to time.

Some produce growers who plant in low-lying areas without irrigation are seeing very dry conditions that are worrisome. Recent rains have not been significant or adequate. We are getting to the point where the moisture level is critical for many agronomic and horticultural crops.

Peak season is approaching when just about everything is being picked regularly. There are limitations to irrigation systems and some water supplies.

However, just because we’re currently experiencing dry weather does not mean I have not seen any plant disease. Plant disease can be caused by living things such as fungus, bacteria, or virus; or non-living reasons such as high salts, sunscald, etc.

I have seen all types of disease this season. One disease I want to discuss is bacterial canker of tomato. This disease can affect tomatoes in the field, high tunnel, and greenhouse. Symptoms of bacterial canker include leaf edges that turn brown with a yellow border, wilting and stems that split and form cankers, or dead areas.

Tomato fruits may develop spotting, known as birds-eye spots, that are whitish with brown centers. Entire plants may collapse and die. Bacterial canker likes warm weather and high humidity. It is caused by the pathogen Clavibacter michiganensis subsp michiganensis and is one of the most devastating diseases that can impact tomato plants in the field, high tunnel, or greenhouse.

The PSU Plant Disease Clinic has confirmed bacterial canker in field and high tunnel tomatoes for local growers this season. In one of these fields, it was clear to see that certain varieties were not as affected.

The cherry tomato and Mountain Fresh variety was not as “blighted” as the early Primo Red variety. There were symptoms on the cherry and Mountain Fresh but not as severe as Primo Red. While there are no tomato varieties known to be resistant to bacterial canker, there are differences in the way they react to disease pressure. In this local case, to manage the disease, the farmer used an application of fixed copper which was thankfully followed by warm, dry weather. The grower reported that the bacterial canker symptoms did not get worse in a week’s time.

He continues to harvest tomatoes from the field. Even though the yield is less than hoped for, it is not a total loss at this point. It will be very important not to grow tomatoes again in the same field next year.

Now this can be very challenging for high tunnel and greenhouse growers. However, I know a high tunnel grower who has overcome bacterial canker. One very important part of his strategy was to grow cucumbers in the high tunnel for a whole season before returning to tomatoes. If you’ve had bacterial canker on your farm, you know how devastating it can be especially when weather conditions are right. Switching to a different field, or different crop in the case of tunnel production, can make all the difference and save you much disappointment.

When talking about plant disease on farms, growers will often ask me, “where did this come from?” Inoculum for bacterial canker can come from infected plant debris, infected weeds, infected tomato transplants, contaminated tomato stakes and infected seeds. We know that bacteria is everywhere, good and bad bacteria. When weather conditions are right and there is a susceptible plant available, the bacteria will grow.

Once the pathogen is introduced into a production area it can easily be spread by workers. You’ve heard of fireblight, a bacterial disease found in tree fruit. Orchardists know that when pruning out fireblight-infected branches they must sanitize their shears in between every cut to avoid spreading the bacteria. The same idea holds true for bacterial canker of tomato.

Workers who are suckering or picking tomatoes in the tunnel with disease symptoms should be sure to wash their hands before moving to the field and working with the field tomatoes. To go the extra mile, a 10% bleach solution can be used to sanitize tools as you’re using them.

If bacterial canker is confirmed in your tomato greenhouse or high tunnel, remove symptomatic plants as soon as possible. I would consider placing all symptomatic plants in large trash bags to easily move them out of the production area to your trash dumpster or incinerator to limit the spread of disease. After disposing of the infected plants, wash your hands thoroughly before handling any other plants.

At the end of the season, it is important that all plant debris, stakes and twine be removed from the greenhouse floor to prevent infecting next year’s tomato crop. It is highly recommended to purchase new stakes, twine and clips for next season’s crops to avoid re-infection.

Since infected seed is a leading cause for infection, growers should consider disinfecting the seed using sodium hypochlorite (chlorine bleach) or by using a hot water seed treatment. Hot water seed treatments involve a process where water is heated and seeds are immersed and then cooled and dried prior to planting. Penn State Extension has the equipment necessary for this process. If you are interested in learning more about treating your seed, please contact me and we can discuss the pros and cons.

Other management tools for bacterial canker include fixed copper fungicides, which are very effective in preventing many fungal diseases, and can be used to provide some level of protection against bacterial canker, but fixed copper fungicides will not “cure” infected plants. The Mid-Atlantic Vegetable Production Guide also recommends Actigard 50WG plus fixed copper to reduce bacterial canker symptoms on tomato fruit.

Crop rotation, quality seed, hot water seed treatments and good sanitation practices in the greenhouse or high tunnel are the best means to prevent bacterial canker from destroying your crop.

Leah Fronk is a horticulture educator with Penn State Extension. You may reach her at the Juniata County Extension Office at (717) 436-7744 or lxf339@psu.edu.

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