Sweet Corn Diseases
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Sweet corn is seldom seriously damaged by diseases in the home garden. Reduce many of
the potential disease problems by following proper cultural practices including:
- Using commercially grown certified and treated seed to improve stands.
- Planting varieties recommended for South Carolina (see HGIC 1308, Corn).
- Planting corn when the soil temperature is above 55 °F to reduce most seedling rots.
- Keeping the garden free of nearby weeds, which can harbor viruses.
- Removing smutted corn ears promptly from the stalks and garden area.
- Removing corn plant debris after harvest to reduce diseases caused by rusts and corn
smut.
Remember, preventing corn diseases in the garden is very important, since the rapid
growth of sweet corn makes it very difficult to spray adequately with chemicals. More
information on how to grow corn is available in the fact sheet HGIC 1308, Corn.
This disease is easy to find in the garden. Large, fleshy,
brownish galls (swellings) containing a black sooty mass of spores develop on leaves,
stems, ears or tassels. Young immature galls are white or grayish white. A fungus, Ustilago
maydis, causes this disease which occurs throughout the Southeast, especially when
temperatures are high (79 to 100 °F) and moisture is abundant.
Prevention and Treatment: Pick off and destroy infected ears and galls while they
are immature and have not yet released spores. Remove galls carefully, since spores can
readily blow to nearby plants, causing more disease. Corn smut overwinters on plant debris
in the soil, so do not put infected crop residues back into the soil. The most
susceptible plants are those grown in soils high in
nitrogen. No chemical controls are available.
RUST
Rust occurs in most home gardens on corn but is seldom a serious
problem. It is caused by various species of the fungus Puccinia and causes raised,
brick red spots primarily on upper leaf surfaces. Plant corn early, and avoid letting
plants get under stress from drought. Rust develops best when temperatures are cool (60 to
70 °F) and humid.
SEED ROTS AND SEEDLING DISEASE
Poor stands of corn can result from various species of fungi,
including Pythium, Fusarium, Diplodia and Penicillium, which
affect seed germination and seedling development. In cold soils that have a
temperature less than 55 °F seeds can decay and seedlings may die before they reach the
soil surface. In cool, wet soils seed germination and seedling development slows down,
increasing the length of time exposed to fungi in the soil. Low-quality seed also produces
seedlings that are weak and survive poorly, especially in cold soils. In warmer soils,
seedlings may emerge, but have rotted roots and stems at the ground line.
Prevention and Treatment: Control of these problems is obtained by using
high-quality seed that has been treated with protective fungicides. Seeds will be pink to
indicate they have been treated. Sweet corn should be planted on a raised bed, after the
soil temperature is above 55 °F.
VIRUS DISEASES
In South Carolina there are two main viruses that are a problem
on corn, Maize dwarf mosaic virus (MDMV) and Maize chlorotic dwarf virus
(MCDV). Symptoms of these virus diseases can easily be mistaken for other growth problems
such as nutrient deficiencies. The upper leaves of MDMV-infected plants show pale green
blotches or mottling. MCDV-infected leaves often have a pale green streaking of the upper
leaves, and often these leaves turn a reddish color. Both virus diseases generally cause
plant stunting. The viruses survive the winter on many perennial grasses, especially
Johnsongrass. In the spring, insects transmit the virus to young corn plants.
Prevention and Treatment: Control grassy weeds near corn plantings (Johnsongrass).
Corn planted at the appropriate time, and not too late in the season, is often damaged
less by viruses. There is no chemical control available.
ROOT AND STALK ROTS
Root and stalk rots are among the most destructive corn diseases.
They are caused by various species of fungi, including Fusarium, Diplodia, Pythium
and Macrophomina. Infected corn stalks fall over (lodging) and losses result from
unharvested ears and poor ear development on infected plants. Some of these
disease-causing organisms enter through the roots and move up into the stalk, while others
enter the stalk directly at the nodes. Insect damage can increase infection by wounding
the plant and allowing fungi to enter.
Prevention and Treatment: Plant in well-drained areas, because stalk rots are most
severe in poorly drained soil and where poor air movement slows drying. Do not exceed
recommended plant densities. Keep soil fertility balanced based on soil tests. All insect
controls should be carefully followed as described in HGIC 2205, Insect Pests of Sweet
Corn.
This disease is caused by the fungus Helminthosporium maydis.
Symptoms occur as leaf spots or cob rots and are fairly easy to recognize on plants. Spots
on the leaves are tan to light brown and occur first on the lower leaves of the plant.
Spots often enlarge and increase in number and can cause severe leaf blighting. On the ear
the fungus causes oblong, bleached spots which penetrate through the shuck layers and
finally into the ear .
Prevention and Treatment: Rotate corn with nongrass-type plants in the garden. Turn
under corn debris soon after harvest to promote rapid decay and destroy overwintering
fungi.
STEWART’S WILT (BACTERIAL WILT)
This disease causes death of seedlings and wilting of mature
plants. Long rectangular streaks, up to an inch wide, appear on the leaves, which later
turn brown and die. A brown cavity is often formed inside of the stalk at ground level. It
is caused by a bacterium, Erwinia stewartii, which survives the winter in flea
beetles. Healthy plants become infected when the beetles begin feeding on them.
Prevention and Treatment: There is no effective chemical control available for this
disease. Remove and destroy all infected plants immediately. Controlling the insects that
spread the disease can reduce disease levels. More information is available in HGIC 2205, Insect
Pests of Sweet Corn.
NEMATODES
Nematodes are microscopic roundworms that live in the soil and
can feed on corn roots. They can damage the root system to the point where it cannot
properly absorb water and nutrients. Above-ground symptoms include stunting and nutrient
deficiencies. Below the soil surface, affected roots appear stubby, swollen, and often
have an absence of small feeder roots. Nematode damage can only be confirmed by having
your soil analyzed. Samples can be submitted to your local county Extension office for
determination of the type and degree of infestation.
Prevention and Treatment: An effective nematode control program should include crop
rotation, sanitation and solarization. More information about controlling nematodes in the
home garden is available in HGIC 2216, Root-Knot Nematodes in the Vegetable
Garden.
Diseases and Insects of Corn in South Carolina, EC601.
Prepared by Nancy Doubrava, HGIC Information Specialist, and James H. Blake, Extension
Plant Pathologist, Clemson University.
This information is supplied with the understanding that no discrimination is intended
and no endorsement by the Clemson University Cooperative Extension Service is implied.
Brand and common names of pesticides are given as a convenience and are neither an
endorsement nor guarantee of the product nor a suggestion that similar products are not
effective. Use pesticides only according to the directions on the label. All
recommendations for pesticide use are for South Carolina only and were legal at the time
of publication, but the status of registration and use patterns are subject to change by
action of state and federal regulatory agencies. Follow all directions, precautions and
restrictions that are listed. (New 5/99).
The Clemson University Cooperative Extension Service
offers its programs to people of all ages, regardless of race, color, sex, religion,
national origin, disability, political beliefs, sexual orientation, marital or family
status and is an equal opportunity employer.
Clemson University Cooperating with U.S. Department of Agriculture, South Carolina
Counties, Extension Service, Clemson, South Carolina. Issued in Furtherance of Cooperative
Extension Work in Agriculture and Home Economics, Acts of May 8 and June 30, 1914
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