Tomato Diseases: How To Fight Southern Blight

by Matt Gibson

About Southern Blight

Tomatoes plants do really well in warm temperatures, and with a helping hand of regular irrigation and fertilization, they are prepared to produce an excellent harvest of big, juicy fruit. When warm rains follow hot, dry spells, a villainous fungus called sclerotium rolfsii, or southern blight, emerges from its hiding spot in the soil to attack your crop unmercilessly. Southern blight is not a slow moving disease. Once it has taken hold in your garden, the poor plants that are in its path don’t stand a chance. Beware, southern blight can easily wipe out an entire tomato bed overnight.

Causes And Symptoms of Southern Blight

Southern blight is a strong fungus strain that can live dormant in the soil for three to four years. It can be transferred on garden tools and pots, through unsterilized soil, and can even be brought in to the garden in the treads of your sneakers, tracked in from the nursery down the street.

Tomato plants that have been infected by southern blight are hard to miss. A day or two after a big rainstorm, you may notice your tomato plant’s leaves turning yellow as the plant starts to quickly wilt. Looking closely at the tomatoes on your plant, you will notice that the stems at the soil line show a white fungus and small, round growths that look like white or brown mustard seeds. Infected plants will most likely collapse at point of infection, or fall over dead in just a few short days.

Treatment and Control of Southern Blight

Preventing southern blight is highly recommended over attempting to treat it. Once it is established, using treatments to kill the infection is a big waste of time. Treatments that target southern blight are often ineffective, very expensive and time consuming, with less than satisfactory results. However, there are things that you can do to keep southern blight from invading your garden in the first place that are much more effective. Follow these simple rules to help prevent southern blight and minimize the risk of an infestation:

• Thoroughly wash hands and clean all home gardening equipment after each use.
• Buy plants from reputable growers that sterilize everything, such as gardening tools and reused pots.
• Do not allow any plant debris, such as dead leaves, blooms, and garden produce, to decompose in the soil.

Once southern blight has taken hold and established itself into the soil in your garden, treating the soil, not the plants, is the only way to go about treating the problem. The part of the fungus that looks like mustard seed is called sclerotia. This part is known to multiply in the soil when allowed to live on infected plants uninterrupted.

After your harvest, remove all infected plants and plant corn in the place where your tomatoes were. Always use corn when doing crop rotation in between for southern blight issues. Corn is one of the few plants that are naturally resistant to the fungus responsible for southern blight. Using corn for crop rotation will prevent the fungus from staying in your soil and multiplying while waiting for you to try your luck with tomatoes again.

Sclerotia loves humidity. The fungus prefers hot, moist environments and is more likely to be present in tomatoes that are grown in containers than tomatoes grown in garden beds. Pots seem to provide the conditions that are most suited to fungal growth. When you are ready to grow tomatoes again, ditch the containers and plant in a raised bed instead. 

When planting your tomato plants, practice wide spacing to promote better air circulation and to provide a distance barrier against the spread of fungi. Doing this will make for easier clean up of plant debris, especially with sclerotia’s first victims of the season.

Though southern blight is easy to detect, it is quite hard to eliminate once you are aware of its presence. Again, prevention is key, as it is better to keep the fungus from arriving, than to have to eradicate the disease once it has taken hold. Prevention from control methods and close inspections of the tomato stem, especially after a rainfall, and fast action to remove all infected plants and soil immediately, will help you to dull the impact of southern blight.

Common Questions and Answers About Southern Blight

Can you eat tomatoes with southern blight?

Tomatoes infected with southern blight are safe for people to eat, but they will be bitter, so it’s unlikely you will want to eat them.

Does neem oil kill southern blight?

Applying a neem oil treatment to your plants can help to reduce the growth of the fungus behind southern blight. You can make a homemade spray treatment by stirring four or five drops of dish soap and a teaspoon of neem oil into a liter of warm water. Spray both sides of the foliage on affected plants.

Does southern blight stay in soil?

Southern blight is a soilborne fungal infection. The fungus behind the disease, Sclerotium rolfsii, spends the winter in the top two or three inches of the soil as spherical sclerotia, the fungi’s resting body. It can also be found on the leaves and other debris that collect beneath affected plants. Runoff water and gardening tools, equipment, supplies, gloves, and clothing help spread the fungus throughout the garden if not properly cleaned and sanitized after each use. Sclerotium rolfsii can stay alive in infected soil for years, especially if the soil is acidic and conditions are hot and wet.

How do you treat southern blight?

The first step in treating southern blight is removal and destruction of all affected plants. Also remove the top layer of soil in areas surrounding affected plants and dispose of it along with the plant debris. These should be burned or buried and should not be included in compost other than hot compost. Sterilize gardening tools, equipment, supplies, gloves, and clothing after each time you work around plants or soil infected with southern blight using diluted bleach or rubbing alcohol.

Southern blight can be controlled with various soil amendments, including: aged compost, corn straw, cotton gin trash, neem oil, pine bark extract, and oat straw. Use of fertilizers with ammonium, calcium nitrate, and calcium sulfate is also effective in controlling southern blight. Commercial farmers treat infected soil with aerated steam heat, and similar results can be had with solarization. To use solarization against southern blight, position a clear plastic sheet over the soil. In the heat of the summer, the sun’s rays under the plastic can heat soil up to 140 degrees Fahrenheit, killing the fungi that cause southern blight.

Some gardeners call on fungicides and biopesticides to help fight southern blight as well. The first of these treatments should be applied between the end of May and beginning of June. These treatments include Azoxystrobin, Cyprodinil used along with Fludioxonil, Fluoxastrobin, Flutolanil, Pentachloronitrobenzene, Tebuconazole, and Triticonazole. These should be used according to the manufacturer’s instructions.

Preventive practices also must be used to help control southern blight, prevent its spread, and reduce the likelihood of reinfection. Gardeners should be vigilant about weeding, as weed plants can host the fungi behind southern blight. Space plants out generously and use staking where needed to promote air circulation and unobstructed sunlight for all plants in the garden. Before sowing new seeds or adding transplants to an area that has previously struggled with southern blight, perform a deep plowing to reduce the amount of fungus on the surface of the soil. (The fungus can’t survive at depths past two or three inches.) If southern blight continues to occur in one area, consider ceasing cultivation of that area for a few years.

Is southern blight harmful to humans?

Southern blight is a plant disease and is not harmful to humans. It’s even safe for people to eat tomatoes infected with southern blight, but they’re so bitter they aren’t appetizing.

What are other names for southern blight?

Southern blight is also called southern root rot, southern stem rot, and southern wilt. All these names refer to a plant disease caused by the soil borne fungus Sclerotium rolfsii.

What are the symptoms of southern blight?

The first symptoms of southern blight tend to appear during the hottest part of the year, in June, July, and August. The first signs of southern blight include leaves turning yellow or wilting. Then stems will start becoming discolored, turning brown and dying where they meet the soil. Then entire plants will die, normally scattered throughout the field or garden bed. White mushroom-like structures may appear, or near the soil line gardeners may see groups of the spherical reddish tan sclerotia, resembling mustard seeds, which are the resting body of the fungus. Some plants will have watery, sunken cankers near where the stem meets the soil. The disease normally kills trees in about a month and plants within a few days.

What causes southern blight?

Southern blight is caused by the fungus Sclerotium rolfsii. Southern blight is soilborne but also spreads with the help of runoff water and gardening supplies, tools, equipment, gloves, and clothing that are not properly sanitized after being exposed to the fungus. The disease is most prevalent when temperatures are between 80 and 95 degrees and conditions are wet and humid.

Have you lost your tomatoes due to southern blight? If you have pictures, send them our way.

Want to learn more about Southern Blight?

Alabama Cooperative Extension System covers Southern Blight on Tomatoes

The American Phytopathological Society covers Southern Blight of Tomato Crops

NC State Extension covers Southern Blight of Tomato and Pepper

University of Tennessee covers Southern Blight

Gardener’s Path covers Prevent Southern Blight on Your Tomato Plants

Gardening Know How covers Southern Blight on Tomatoes

Gardening Know How covers Controlling Southern Blight

Texas A&M AgrilLife Extension covers Southern Blight

Today’s Homeowner covers How to Use Neem Oil in the Garden

Tennessee State University covers Southern Blight Management

WTVA covers Southern Blight Poses Threat to Tomatoes