Southern Blight of the Tomato Plant: How to Manage

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Quick Answer: Southern Blight of the Tomato Plant

Southern Blight of the tomato plant is also called summer disease. It starts to show up in late spring and early summer. It attacks mature plants and shows its symptoms on leaves, stems, and fruits. The most effective way of controlling southern blight in tomato plants is by spraying fungicides. Soil solarization is also a good way to manage this disease.

This is your go-to guide to understanding, preventing, and managing Southern Blight of the tomato plant.

Let us get started!

What is Tomato Southern Blight Disease? 

Southern Blight of the tomato plant is caused by Sclerotium rolfsii. It is a soil-borne fungus. It primarily attacks the tomato plant’s leaves, stems, roots, fruits, and petioles. 

The southern blight-infected tomato plant shows stem lesions near the soil line. These lesions develop, girdling the stem and resulting in a sudden wilting of the tomato plant.

White molds of mycelia are produced and are observed on stems and fruits. Brown spherical sclerotia develop on the mycelial mat indicating the last stage of the infection. 

Sclerotia are the infecting spores of Sclerotium rolfsii. The fungus is saprophytic [depends on various hosts for nutrition]. It can use its mycelium [growth structures] for penetration. 

Southern Blight survives as sclerotia in the soil for years. It is not a deep-soil fungus and lives in the upper 2-3 inches of soil.

Sclerotia are spread by cultural practices like previously infected soil, infected garden tools such as scissors and cultivars, contaminated transplants, water splashes due to rain or overhead watering, wind, insects, infected hands, and infected seeds. 

Hyphal growth requires a temperature range of 46-104º F and sclerotia germination occurs between 81-95ºF. 

High humidity and water-saturated soil provide an extra edge to disease development.

Check out this video for Southern Blight of tomato-

How Southern Blight Harm Tomato Plants? 

Sclerotium rolfsii attacks any parts of a tomato plant that touch the soil but generally infects the plant at or just below the soil line. 

S. rolfsii produces an enzyme that decomposes the tomato plant’s outer cell layer resulting in tissue decay. It induces the growth of mycelium and the formation of sclerotia.  

Sclerotia penetrate the plant and show their initial signs on leaves. 

  • The leaf turns yellow and wilts.
  •  Lower stem girdles and woody stems show brown lesions. 
  • The fruit shows water-soaked circular spots that eventually decay.

After germination, the fungus colonizes the organic plant debris near the soil surface and then infects the parts of the tomato plant that touches the soil bed.

Check out this video for Southern Blight and other fungal diseases –

How to Confirm that Southern Blight is Troubling your Tomato Plants? 

Southern Blight of the tomato plants affects every part of the plant and can attack the plant at any stage of development.

Green tomatoes are at a higher risk of developing this infection.

Here are some signs of the infection –

Leaves, young stems, woody stems, petioles, roots, and fruits are susceptible to the southern Blight of the tomato. The fungus grows as white, fluffy mycelium on infected parts.

  • Round and white sclerotia of outgrows on the mycelium. These sclerotia are dark brown to black. 
  • Leaves show early signs of the disease. The leaves turn yellow or show discoloration. This is followed by immediate wilting of the leaf. Eventually, the leaf dies. 
  • Stems show water-soaked lesions near the soil line. This rapidly grows, turns brown, and girdles the stem completely. It causes a premature wilting of the plant. Young plants might fall on the ground. 
  • Lower stems appear rotten. The stem cortex above and below the soil line decay. A white mold (mycelium) covers the stem lesion. Mycelia are small (0.5-1 mm), white, and round.

After the disease progresses, tan to reddish-brown, spherical sclerotia of 1/16th inch size appear on the white mat of mycelia.

These white mold-type structures are found on the stem base of the tomato plant.

 White mycelium mat with Sclerotia

Older and woody stem tissue gradually girdles by lesions and eventually dies. 

White mycelium mats with round, white mustard seed-looking sclerotia are also found on the roots or an inch above the soil line. Roots develop the infection because the surface touching of the stems.

Fruits develop the infection at a later stage of disease development. The lesions observed on the bottom skin of the fruits are - soft, water-soaked, sunken, and slightly yellowish. 

The fruit collapse within 3 to 4 days of the infection and white mycelium mats and sclerotia fill the lesion cavity.

Ways to Control Southern Blight Disease on Tomato Plant 

You can control Southern blight in tomato plants by soil solarization, spraying fungicides, chemical treatments, crop rotation, and disease-free varieties. 

1. Soil Solarization

In soil solarization, infected soil is disinfected, to make it suitable for the next tomato cultivation. It requires the treatment of soil with plastic sheeting and the right amount of water. 

Solarization can be done on flat and raised areas. The plastic sheet is allowed to lie against the soil bed without any air pockets between them. This plastic sheeting absorbs the sunlight.

Before sheeting, wet the soil to at least 12 inches deep. The plastic sheeting should be 0.025 - 0.4 mm thick and must be applied to the garden soil for 4-8 weeks.

After solarization, the plastic is removed. Do not disturb the underlying soil to avoid bringing up viable weed seeds. The soil is suitable for planting tomatoes.

It is a non-chemical method and must be repeated every year.

Check out this video for soil solarization –

2. Fungicides

Soil fungicides are an effective way of controlling S. rolfsii in the tomato garden. Fungicides application should be repeated every year. It requires spraying of chemicals after every 2-4 weeks. 

Fungicides should be sprayed at the base of the plant as Mycelia and sclerotia of Sclerotium rolfsii are located at the base of an infected tomato plant.

PCNB [pentachloronitrobenzene] and Difenoconazole + benzovindiflupy are suitable fungicides for S. rolfsii control in the tomato plant. 

One should not apply fungicides indiscriminately. They can prevent the spreading of the disease but, at the same time can reduce the yield and can dangerously affect your tomatoes. 

Check out this video on how to mix and apply fungicides-

3. Can You Use More Resistant Tomato Varieties

Unfortunately, there are no commercially available seeds for the southern blight that show complete resistance to the disease. 

However, breeding lines 5635M, 5707M, 5719M, 5737M, 5876M, and 5913M of the tomato plant may become the next commercially exploited resistant varieties for the disease.

How to Prevent the Southern Blight on Tomato Plant 

Southern Blight of the tomato plant is a very destructive disease and leads to a reduced yield of tomatoes. 

Preventing the disease before it ruins your beautiful tomato is the best control method. 

Enrich your soil with organic matter and important fertilizers. It will hinder the survival of resting structures of southern blight called sclerotia. 

Deep plowing of the soil before planting the seeds will reduce the number of sclerotia in the upper layers of soil.

Raise the pH of the soil using calcium nitrate. It will retard the fungal growth in the soil. 

Check this video on raising pH –

Mulching is the best way to maintain a 2-3 inches gap between soil and the plant. Cover your soil bed with a layer of organic matter, plastic sheets, or aluminum sheets. It will act as a physical barrier for sclerotia.

Check out this video on step by step guide for mulching-

Staking and caging will provide your plant with physical support.

Before transplanting, remove all the plant debris from the ground and the soil. Perform soil solarization or deep plowing to achieve healthy soil for tomato plants.

Disinfect garden tools, hands, shoes, and other items that have infected contact from the infected plant parts.

Keep a good drainage system in your garden. 

Maintain good air circulation in the plants. Planting two tomato plants almost 2 hands away from each other is the best way to achieve well-ventilated plants. 

Remove all the weeds and infectious grasses such as amaranthus that can infect other healthy tomato plants by acting as a source of southern blight. 

Do not water your plants at night or late evening. 

Avoid sprinkle irrigation or overhead watering of the tomato plants. 

Burn all the infected plants in a burn pile. 


Can tomato plants recover from southern Blight?

No, tomato plants cannot recover from the disease. 

Inculcate some garden sanitation and cultural practices to prevent southern blight in your garden. 

Is it OK to eat tomatoes with southern blight?

Yes, you can eat tomatoes infected with Southern Blight.

There are no studies that suggest that the fungus responsible for southern Blight harm humans in any way. 

Can I reuse soil infected with Southern Blight?

Yes, you can reuse the soil after disinfecting it by the soil solarization method. 

Which fertilizer is used to control Southern Blight?

Fertilizers containing ammonium, calcium nitrate, or calcium sulfate are used to control southern Blight in tomatoes. These chemicals inhibit the germination of sclerotia in soil and make the soil hosts unfit for sclerotia survival.

Can you control Southern Blight in the tomato plant naturally?

Studies suggest that neem oil and pine bark extracts reduce the growth of the Southern Blight pathogen in the tomato plants. 


I hope this guide on the Southern Blight of the tomato plant will help you prevent and manage this infection in your garden.

Do share the article with tomato gardeners with your friends and family!