Southern Blight of Vegetable Crops
Vegetable Disease Information Note 9 (VDIN-009)
Charles W Averre, Extension Plant Pathologist
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.
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.
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.
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
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.
diseases since dead leaves on the ground may trigger infection. Weeds
should also be controlled early in the season for the same reason.
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.
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.
Disease Information Notes Home Page
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Carolina Agricultural Chemicals Manual
with a specific problem, contact your local
North Carolina Cooperative Extension Service
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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
North Carolina Cooperative Extension Service
Distributed in furtherance of the Acts of Congress of May 8 and June
Employment and program opportunities are offered to all people regardless
race, color, national origin, sex, age, or disability. North Carolina
University at Raleigh, North Carolina A&T State University, U.S. Department
Agriculture, and local governments cooperating.
Revised Oct'99 by G. J. Holmes