Southern blight

It happens to the best of us. Your garden grows so nice and then, without any warning, you turn around and notice all your healthy plants wilting and dying. Southern blight on plants is a common problem in many home gardens but it doesn’t have to be. How do you control southern blight before it takes out all of your plants? Keep reading to find out ways for controlling southern blight in gardens.

Southern blight, southern wilt, southern stem rot and southern root rot all refer to the same disease. It is caused by the soil borne fungus Sclerotium rolfsii. The disease attacks a wide range of vegetable crops and ornamental plants at or below the soil line. Southern blight on plants is most likely to occur in the summer months when the soil is warm and moist. Symptoms include discolored lower leaves, wilted foliage and plant collapse; and it usually results in the death of the plant. Upon close inspection, you may find an abundance of white hyphae or mycelia around the lower stem and roots, and in the surrounding soil. When you find the hyphae or mycelia, the best course of action is to dispose of the plant and the soil surrounding it.

On tomato and pepper, the most common symptom occurs on the lower stem where it is in contact with the soil. Initially, a brown to black lesion usually develops on the stem near the soil line. The lesion will develop rapidly and can completely girdle the stem, which will cause a sudden and permanent wilt of all above ground parts. Young plants may fall over at the soil line.

Under moist conditions, white mycelium will typically develop on stem lesions and can sometimes extend several centimeters up the stem of tomato and pepper plants. After a few days, tan to reddish-brown, spherical sclerotia (1 to 2 mm in diameter) can appear on the mat of mycelia.

The fungus can easily infect fruit that are in contact with infested soil. Lesions will initially appear as sunken and slightly yellow areas that later become water-soaked, soft, and star-shaped spots. The fruit will collapse within 3 to 4 days and white mycelium and sclerotia can fill the lesion cavity. There is no offensive odor associated with rotted fruit or at least initially.

A. rolfsii can survive and overwinter as sclerotia and on host debris in the soil for years. The fungus is highly saprophytic and can produce mycelial growth on a variety of host substrates. However, the fungus is generally restricted to the upper 2 to 3 inches of soil and will not survive at deeper depths. In most North Carolina soils, the fungus does not survive in significant numbers when a host is absent for two years or more. High temperatures (77 to 95°F), aerobic and moist conditions, and acidic soil favor disease development and fungal growth. Germination of sclerotia occurs at pH 2-5 and is inhibited at pH higher than 7. Sclerotia are spread by the movement of infested soil and plant material.

Southern blight can be difficult to manage when inoculum density is high and environmental conditions favor disease development. However, losses can be reduced by adopting the following management strategies:

  • Avoid planting in fields with a history of southern blight.
  • Rotate host crops with corn, wheat, barley, or other non-host crops to reduce inoculum levels in soil.
  • Modify planting dates to avoid conditions that favor disease development, when possible.
  • Inoculum levels can be reduced by burying infected plant debris and sclerotia via deep plowing to invert the soil.
  • Ensure the previous crop is decomposed prior to planting. This may require disking the field several times in the fall and in the spring.
  • Remove symptomatic plants and reduce or eliminate weed populations.
  • Maintain adequate soil pH for optimum plant growth. Lower soil pH will encourage disease

Fungicides

Several fungicides are labeled for use on tomato and pepper to manage southern blight. Fungicide labels are legal documents—always read and follow fungicide labels.

The fumigation of soils with broad spectrum chemicals such as chloropicrin and sodium metam sodium can reduce disease incidence, but this strategy is limited by economic considerations. Soil fumigants must be applied days to weeks prior to planting.

Fungicides:

  • Aftershock Evito 480 SC
  • Priaxor 500 SC
  • Blocker 4F2
  • Fontelis
  • Cabrio EG