(News article for May 15, 2020)
Tomatoes are one of our favorite vegetables and can often be grown successfully in pots, in raised beds, or in the ground. As with any plant, there are potential problems, and knowing something about what causes those problems can help us avoid them.
Wilting tomatoes are a common complaint. There are several possible causes for this (other than lack of water), but the most common are probably southern bacterial wilt and southern blight. Both of these can cause straight-up wilting and plant death without leaf spots or plant stunting.
In spite of the similarity in the names, these diseases are caused by different pathogens – one bacterial and one fungal. If southern blight is the problem, you’ll typically see white fungal growth at the base of the stem, next to the soil line, along with white-to-tan mustard-seed-sized structures called sclerotia.
In terms of what to do this year, there isn’t much difference between the two diseases. Make sure you don’t spread soil from around the bases of the diseased plants to other areas of the garden on your tools, feet, or hands, or in irrigation water.
One reason it can be useful to distinguish between the two diseases is to determine what can be planted back in that area of the garden. Both of these pathogens can survive in the soil for multiple years. If you have bacterial wilt, you’ll need to avoid replanting tomatoes, peppers, eggplants, and potatoes.
The southern blight fungus has a wider host range. If it’s southern blight, you’ll want to avoid replanting those vegetables, as well as a number of others, including snap beans, peanuts, cucumbers, watermelons, cantaloupes, pumpkins, snap beans, peanuts, and sweet potatoes. Probably the safest vegetable to plant in an area where you’ve had southern blight is sweet corn.
If you have limited garden space and want to take a chance on replanting susceptible vegetables back into the affected area within the next few years, one thing you can do to reduce the chance of problems is to solarize the soil.
This involves removing plants, preparing soil as if you were going to plant in it (smooth seedbed without clods, etc.), making sure the soil is moist, and covering the soil with clear plastic. There should be no holes in the plastic, and it should be held tightly to the ground and covered with soil at the edges. Leave the plastic on for about six weeks during the heat of the summer. The point is to get the soil hot enough for long enough to kill most disease-causing organisms.
Low soil pH and high soil moisture favor both of these diseases, so make sure your pH is within the recommended range for tomatoes (pH 6 to 7), and provide adequate drainage.
Let me know if you have questions.
Contact Mary Helen Ferguson.
Fungal growth (mycelia and sclerotia) of the southern blight pathogen, Sclerotium rolfsii, at the base of a tomato plant. (Photo by M.H. Ferguson)