CHAMBERSBURG, Pa. — A produce farmer who has recently passed away whom 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 can 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 cost of using fungicides and losses due to rotten crops add up quickly. While drip irrigation systems are heavily relied upon, 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 this summer that are worrisome. Recent rains have not been significant and we are getting to the point where the moisture level is critical for many agronomic and horticultural crops. Peak season is approaching when nearly every produce crop is being picked regularly. Additionally, there are limitations to irrigation systems and some water sources.
However, just because we’re currently experiencing dry weather does not mean farmers are escaping plant disease entirely. Plant disease can be caused by living things such as fungi, bacteria, or viruses; or non-living reasons such as high salts, sunscald, etc. I have seen all types of disease this season. One disease I have seen quite often 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.
While there are no tomato varieties known to be resistant to bacterial canker, there are differences in the way they react to disease pressure. If disease is detected, it will be very important not to grow tomatoes again in the same field next year. This can be very challenging for high tunnel and greenhouse growers. However, I know a high tunnel grower who has seemingly 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.
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. Workers who are suckering or picking tomatoes in a tunnel with disease symptoms should be sure to wash their hands before moving to the field and working with the field tomatoes. Additionally, 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 by placing all diseased plants in large trash bags to easily move them out of the production area to your trash dumpster or incinerator. After disposing of the infected plants, wash your hands thoroughly before handling any other tomato 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. PSU Extension has the equipment necessary for this process. If you are interested in learning more about treating your seed, please contact your local Extension Educator to discuss the pros and cons. Other management tools for bacterial canker include fixed copper fungicides, which are 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. In conclusion, 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, Penn State Extension