I have several tomato plants in my home garden. They were all doing great but then two of them just turned brown and died. I know it’s not a watering issue because they all get the same amount of water. What happened? MW

We feel your pain. The Master Gardeners have a new two-acre farm where we are growing a variety of vegetables to donate to food pantries and the very same thing happened to a couple of our tomato plants. In our case, the culprit was Southern Blight which may be the issue with your tomato plants. But let’s talk about several common fungal diseases in tomatoes just to be sure.

Southern Blight

With Southern Blight, you will notice your tomato plant going from looking healthy to the entire plant wilting and turning brown rather quickly. To determine if southern blight is the issue you need to look at the base of the plant where it contacts the soil. This portion of the stem will be covered in a white mold that can even reach out into the soil (see photo).

Southern blight is a fungus that lives in the soil. It was either already in the soil where you planted your tomato, or it was in the soil of the transplant you purchased. Unfortunately, when your plants reach this point there is no saving them.

Southern blight thrives during conditions of high humidity, moist soil, and temperatures between 85 and 95 degrees. So, conditions recently have been perfect for southern blight.

Once you identify a plant affected with southern blight you should remove the plant and dispose of it. Do not place it in your compost bin. Also be sure to clean any tools you used to remove the plant with a 10% bleach solution to minimize the chance of your spreading the disease.

Southern blight is a challenge to control. Your first defense would be to rotate your crops each season. This will help minimize buildup of the disease in the soil. If you have had a problem with southern blight in the past, you can begin a fungicide program early on to help minimize the issue. Follow the directions on the label.

Septoria leaf spot is another fungal disease that can become a problem for tomatoes. Septoria leaf spot is another soil borne fungus. You will first notice it on the lower leaves of the plant as spots begin to appear. They start as small, dark spots that then develop a type of yellow halo around them. These spots soon begin to generate and release spores that will spread to the rest of the plant.

As a soil borne pathogen, it needs help migrating up and onto the leaves of the plant. This is usually accomplished through splashing water from rain or manual watering. A good layer of mulch is a good first line of defense because this will help minimize splashing soil. Also, it’s a good idea to trim off the lower leaves of your tomato plant, maybe up about a foot because septoria seems to like leaves better than stems. If you develop septoria in spite of this, remove diseased leaves as soon as you notice them. This will minimize it’s spread by eliminating those little spore factories on the leaves.

Once you find septoria on your tomato plant, you can begin treatment with a garden fungicide such as copper fungicide. Once again crop rotation helps minimize future problems with septoria.

Early blight is another fungus that can ruin your tomato plant’s day. This one also causes dark spots on the leaves but can also be seen on stems and sometimes the fruit. First you will probably notice spots on the leaves up to 1/2 and inch in diameter. These brown spots expand, revealing concentric rings that resemble a target. Leaves around these spots then turn yellow as the plant begins to defoliate. This defoliation moves from bottom to top. Sunken, dry, brown areas on the stem are also common with early blight. There is no cure for this disease so as soon as you identify it, you should remove the plant so spores produced by those lesions can’t spread to other plants. Plants that are either under stress, have a nitrogen deficiency or excessive fruit loads are more susceptible to early blight.

Once again, crop rotation helps minimize development of early blight. If you have had problems with this disease in the past, as soon as your tomatoes begin to fruit, you should begin spraying them with a fungicide on a 7 to 14 day schedule. Follow directions on the fungicide.

So, our takeaways to minimize fungal disease in your tomato are as follows.

1) Rotate your crops each season with plants from a different plant family. We have information on plant families on our website (www.tulsamastergardeners.org) under the Lawn and Garden Help section/Vegetables.

2) Apply a good layer of mulch to minimize splashing of soil borne fungi onto your plants.

3) Water in a way that minimizes splashing and wet leaves on your plants.

4) Remain vigilant. Early detection will be key to minimizing the spread of the disease if present.

5) Clean garden tools with a 10% bleach solution to minimize disease transfer.

See you in the garden!

You can get answers to all your gardening questions by calling the Tulsa Master Gardeners Help Line at 918-746-3701, dropping by our Diagnostic Center at 4116 E. 15th Street, or by emailing us at mg@tulsamastergardeners.org.

Photo: Tom Ingram