Edible Gardening Pests & Diseases

What’s Killing My Tomatoes?

bacterial wilt

There are so many possible answers when tomatoes die. The first question I ask is, “What seems to be the problem?” There could be leaves that are turning yellow and dying, or there could be rotten spots on the tomato fruit. The leaves could be curling or crinkled, or the entire plant may be wilting.

Root knot nematode / Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org.

Root knot nematode / Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org.

There are a number of diseases that make the whole plant wilt. Yes, that is plural, because there are three main possibilities, four if you count not watering the plant.

The first possible problem, and the easiest to diagnose, is root knot nematode. Nematodes are microscopic roundworms that feed on plant roots. This prevents the roots from taking up water and nutrients needed for plant growth. The first symptom is short or stunted plants. Next, the plants begin to wilt in the heat of the day and don’t recover even after being watered. By pulling up a plant and looking at the roots, you will see lots of deformed roots with bulges or knots.

Fusarium wilt / Clemson University, Bugwood.org

Fusarium wilt / Clemson University, Bugwood.org

The second problem is fungal wilts—fusarium wilt and verticillium wilt. Both of these fungal pathogens infect through the roots and then move up the stem. When it is noticed that the plants are wilting and not recovering, the stem may be cut lengthwise and a chocolate brown staining will be seen in the vascular tissue. With each of these wilts, the leaves turn yellow and fall off. With fusarium wilt, the lower leaves turn yellow then wilt and die. With verticillium wilt, the tips and edges of the lower leaves are the first to be affected, often with a characteristic “V” pattern to the leaf discoloration.

The third cause is bacterial wilt. This is the worst of the three and the hardest to overcome. The bacteria that cause this problem can stay in the soil for several years. They enter the plant through the roots and move through the vascular system into the stem of the plant, plugging up the xylem (water conducting tissue). There generally isn’t any discoloration of the leaves. It acts so quickly the plant doesn’t have time to compensate for the lack of water. When the stem is cut, the inner part of the stem will have a gray cast to it. A quick way to check for this disease is to place a cut stem in a clear glass container and wait for a minute or two. Then look for a characteristic gray streaming from the stem. This will be the bacteria cells leaving the stem.

For any of these diseases the best cultural control would be to rotate the tomato plants to a different area in the garden every year and not revisit the same location for a minimum of three years.

bacterial wilt

Bacterial wilt

A second method would be to grow a tomato that is resistant to the disease in your soil. One way to know if the tomato variety is resistant is to look on the package. Some plants will have some odd letters after the name like Better Boy VFN. The letters indicate the disease that plant is resistant to—V for verticillium, F for fusarium, and N for nematodes. There are no commercial varieties I know of that are resistant to bacterial wilt.

If there is a tomato that you must absolutely have in your garden that doesn’t have the resistance to the disease in your soil, tomato grafting is an excellent option. This makes it possible to grow any tomato variety.

Solarizing the garden soil is another option that may kill the disease. Place a sheet of clear plastic over the garden area. Cover the edges of the plastic with enough soil to keep the plastic in place. As the sun’s rays penetrate the plastic they heat the soil. The plastic keeps the heat from escaping. It will take 8 to 10 weeks to heat the soil to 160 to 180 degrees to about 4 to 6 inches deep.

There isn’t a chemical treatment that can be used to kill these diseases in the home garden. This is probably good, as any chemical that would kill these pathogens would also kill the good microbes in the soil.

Featured image – Bacterial wilt

Shawn Banks is the Consumer Horticulture Agent with NC Cooperative Extension in Johnston County. You may reach him at shawn_banks@ncsu.edu.