Southern Blight of Vegetable Crops
Vegetable Disease Information Note 9 (VDIN-009)
Charles W Averre, Extension Plant Pathologist

Southern blight, also known as "southern wilt" and "southern stem rot" is a serious and frequent disease of many vegetable crops in Tidewater, Coastal Plain, and Piedmont areas of North Carolina. It is caused by the soil-borne fungus Sclerotium rolfsii and attackes a number of vegetable crops including bean, cantaloupe, carrot, pepper, potato, sweetpotato, tomato, watermelon and others. In addition, several field crops such as cotton, peanut, soybean, and tobacco are affected. The disease usally appears in "hot spots" in fields in early to mid-summer and continues until cooler, dryer weather prevails. Losses may vary from light and sporadic to almost toal destruction of the crop. The fungus may also decay harvested produce, especially carrots.

Symptoms

The disease is recognized by wilting and yellowing of leaves; and when the plant is pulled up, it is apparent that the lower stem and upper roots are infected. In watermelon, one or more runners may be affected; with cantaloupe, the melons are usually affected first. The edible roots of sweetpotatoes have 1/4 to 1/2 inch circular, sunken, dark gry surface spots. The stems of erect plants such as tomato, pepper, potato, and green bean are usually rotted at the soil line. The fungus causes a watery fruit rot on cantaloupe and tomato; in carrot the whole root becomes decayed. The rots are not associated with an offensive odor, at least initially. A white, moldy growth is evident on affected stem tissues and adjoining surface soil; later smooth, light tan to dark brown mustard seed-like bodies called sclerotia are evident in the mold. The sclerotia are diagnostic for the disease.

Disease Cycle

The fungus overwinters as sclerotia and in host debris in the soil. A characteristic of the fungus is that it is generally restricted to the upper 2 or 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. The fungus is more active in warm, wet weather, and it requires the presence of undecomposed crop residue to initiate infection.

Control

The control of southern blight is difficult, but losses can be reduced by following a "Total Program" for disease control over a period of several years.

Avoid problem fields.
Follow corn, small grain, or other grass crops.
Prepare the land properly. The previous crop must be well decomposed prior to planting, and this may require disking the field several times in the fall and in the spring. The previous crop litter should be buried with a moldboard plow equipped with heavy duty concave disc-type coulters to a depth below later cultivation equipment movements (8-12 inches). The crop litter should be below a 3 to 5 inch depth. None of the buried litter should ever be brought back near the soil surface during the current season by cultivation.
The concave disc-type coulter should be mounted so that the deepest part of its cut is centered above the cutting of the plow bottom. The adjusted cutting depth (about 4 to 5 inches) of the coulter should reach completely through the surface layer of soil containing shredded litter, leaving the bottom of the cut free of litter. The disc angle must be adjusted relative to the speed of the tractor; positioning the hub of the disc approximately over the point of the plow should provide the desired action. A greater angle is required for slower than for higher speeds.
Do not throw soil with debris against plant parts during the growing season.
Control foliar diseases since dead leaves on the ground may trigger infection. Weeds should also be controlled early in the season for the same reason.
Soil fumigation with methyl bromide, chloropicrin (and mixtures) and Vorlex or Vapam reduce the incidence of southern blight but must be applied days to weeks prior to planting. Fall application is preferred to spring application. Read label for specific crop, row application, use in organic soils, chisel depth and spacing, exact rates, and special uses. Vapam can also be applied with irrigation systems. Both products are more effective when used with plastic covers.
Use Terraclor 75WP (PCNB) in the transplant water (3 to 5 lb/100 gal, 0.5 pt/plant) or as a trench spray (10 lb/100 gal). The 10 and 20% dust formulations may also be applied over "V" trenches prior to transplanting at the rates of 75 lb and 35 lb per acre, respectively. These rates are for 7,300 linear feet of flat tomatoes, 10,900 feet of row for staked tomatoes, and 14,500 feet of row for peppers. Follow all label directions carefully.
Always use agricultural chemicals as stated on the label and follow all precautionary statements. The use of trade names in this publication does not imply endorsement of the products named or criticism of similar ones not mentioned.

Other Links

Plant Disease Information Notes Home Page
North Carolina Insect Notes
North Carolina Agricultural Chemicals Manual
CALS Home Page
NCCES Home Page
NCARS Home Page

Recommendations of specific chemicals are based upon information on the manufacturer's label and performance in a limited number of trials. Because environmental conditions and methods of application by growers may vary widely, performance of the chemical will not always conform to the safety and pest control standards indicated by experimental data. All recommendations for pesticide use were legal at the time of publication, but the status of registration and use patterns are subject to change by actions of state and federal regulatory agencies.