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Author: GAZAWAY PubID: ANR-0601 |
Title: | CORN DISEASES IN ALABAMA |
Pages: 12
Status: OUT OF STOCK |
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Corn Diseases in Alabama |
Corn diseases cost Alabama producers about 10 percent of their yield each year. Some producers have lost more than 50 percent of their expected yields.
Corn diseases are caused by fungi, bacteria, viruses, and nematodes (called pathogens). Often two or more of these pathogens may be involved in a disease complex, causing more damage to corn than either pathogen alone. To make discussion easier, diseases will be grouped according to pathogen type.
Fungal Diseases (Leaf Blight, Ear Rot, Stalk Rot)
Diseases caused by fungi can be extremely destructive on corn,
resulting in leaf blights, ear rots, and stalk rots. Long periods
of humid weather followed by intermittent dry periods are when
fungal diseases are most likely to begin and spread. When these
conditions are coupled with stress factors, including insufficient
light, drought, hail injury, mechanical injury, and insect borers,
fungal diseases are most damaging to corn.
Stalk rot, the most destructive phase of fungal diseases, damages
corn that has been weakened by stress. A high nitrogen-low potassium
imbalance in the soil can also make corn more susceptible to stalk
rot damage.
To minimize damage caused by fungal diseases, adhere to the following
cultural practices:
- Plant resistant cultivars or hybrids adapted to your area. Many cultivars now available have some tolerance to many fungal disease ear and stalk rots.
- Soil test and follow soil test recommendations. Avoid applying excessive levels of nitrogen and maintain an adequate level of potassium in the soil.
- Do not exceed the recommended seeding rate or plant rows too closely. During wet weather, overcrowding creates a humid environment in the canopy. This increases the length of the infectious period for pathogenic fungi. During dry weather, overcrowding hastens the onset of drought, stressing the plant and increasing stalk rot damage.
- Rotate corn with alternate non grass crops and turn under corn residues to hasten destruction of pathogenic fungi.
- Control insects with insecticides, crop rotation, or insect-tolerant cultivars.
- Harvest infected corn early.
This circular includes a brief description of the more common fungal blights, ear rot, and stalk rot diseases. Disease symptoms in the field may differ slightly from those described in the section below since corn may be infected with more than one pathogen under actual field conditions.
DIPLODIA ROT (Diplodia maydis) can develop in fields when the weather is dry early in the growing season, then turns warm and wet about 2 to 3 weeks after the plants start silking. With Diplodia rot, corn leaves turn grayish-green and then wilt and dry out. Within a short time, affected plants die. The lower internodes on the stalk of an infected plant are brown, spongy, and easily crushed. Internally, the pith is discolored and usually shredded. As Figure 1 shows, a characteristic sign of Diplodia rot is the presence of small, dark, speck-like fruiting bodies (pycnidia) which are grouped in clusters and embedded in the surface of the stalk near the nodes. Often, a white fungal growth may appear on the stalk's surface.
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Figure 1. Diplodia stalk rot. The split section of the stalk on the right shows its shredded pith. Note the tiny fruiting bodies near the node on the surface of the stalk on the right. |
Diplodia infects corn ears within 14 days of silking. The infected ear turns light brown, dries out, and rots completely. Diplodia-infected ears tend to stand upright. The inner husks are bound tightly to each other and to the corn ear by the fungus which is growing between them. Tiny fruiting bodies can be seen on the husks, cob, and sides of the kernels.
Later, infected ears show no outer signs of the disease, but when the ears are broken, a white fungal growth can be found between the kernels and at the tips of the cobs.
GIBBERELLA ROT (Gibberella
zeae) affects leaves, sheaths, ears, and stalks. Its symptoms
may easily be confused with Diplodia rot and other stalk rots.
When the plant is infected early in its development, its leaves
turn dull green, and the lower internodes on the stalk become
soft and later turn dark brown. The diseased pith inside the stalk
becomes stringy, and finally the stalk collapses.
Features that distinguish Gibberella stalk rot from Diplodia stalk
rot are the red cast of the diseased tissue in the stalk's pith
(Figure 2) and the presence of tiny, superficial black specks
(perithecia) on the surface of the stalk.
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Figure 2. Gibberella stalk rot. The split section of the stalk illustrates shredded pith with red cast. |
GIBBERELLA EAR ROT OR "RED EAR ROT" (Gibberella zeae) can be identified by the red fungal growth at the tip of the ear and between the husks and the ear. Superficial perithecia may occasionally be found on the ear husks. The Gibberella fungus rots the infected ear completely. In addition to reducing yields, Gibberella-infected corn may contain mycotoxins which are toxic to swine and to other animals.
FUSARIUM STALK ROT (Fusarium moniliforme) also occurs in Alabama and produces symptoms similar to Gibberella stalk rot except that it does not form the black speck-like fruiting bodies in the affected areas of the plant. Fusarium rot usually begins at roots and lower nodes (Figure 3). Like Gibberella stalk rot, Fusarium stalk rot causes premature ripening and stalk lodging.
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Figure 3. Fusarium stalk rot. The interior of the stalk shows shredded pith with red coloration. Note the similarity to Gibberella stalk rot symptoms. |
CHARCOAL STALK ROT (Macrophomina phaseoli) can be extremely destructive to corn following extended periods of hot, dry weather. In high temperature conditions (98 degrees F and above), this soil-borne fungus attacks the roots of corn and moves through the cortical tissue of the stalk's lower internodes. Infected tissue in the pith becomes shredded. Many black pepper-like specks can be seen on the surface of the shredded tissue and just under the epidermis, giving the tissue a charred appearance (Figure 4).
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Figure 4. Charcoal rot showing pepper-like fruiting bodies on shredded stalk tissue. |
Damage results from premature ripening and stalk lodging. Adequate soil moisture discourages charcoal rot development, so it can be controlled in irrigated fields by keeping the soil moist after the corn tassels. In non-irrigated fields, control measures previously mentioned for the other fungal diseases are effective against charcoal stalk rot development.
PYTHIUM STALK ROT (Pythium
aphaniderinatum) hits corn during extended periods of hot
(90 degrees F and above), wet weather. It is most frequently seen
in fields that have heavy soils and poor internal soil drainage.
Symptoms usually appear on corn just before or during tasseling.
Unlike most other stalk rots, Pythium can infect corn before full
blossom (anthesis). A rot develops on either the first or the
second internode above the ground line. The affected area of the
stalk is brown, soft, and water-soaked. The stalk collapses at
the infected area and falls over but still remains attached. Fallen
plants remain green and turgid weeks after they have fallen because
the vascular tissue stays intact.
Pythium stalk rot damage can be reduced by avoiding planting corn
in fields with poor soil drainage and aeration. Also, select corn
hybrids and cultivars which have better tolerance to Pythium.
ANTHRACNOSE (Colletotrichum
graminicola) causes stalk rot, top dieback, and leaf blight.
Under favorable conditions, damage can be severe, causing yield
losses up to 20 percent on susceptible cultivars.
Symptoms differ according to corn cultivar, age of corn plant
when infection occurs, and environmental conditions. Small, oval
spots first appear on leaves during all stages of development.
These spots often enlarge up to 1/2 inch
in diameter, turn tan in the center, and form reddish-brown or
yellow borders. If the disease is severe, the spots may run together,
causing the entire leaf to appear blighted (Figure 5). Symptoms
appear on the lower leaves early in the season and on the upper
leaves later in the season. Leaves infected late in the season
die within a few days, causing a top dieback (Figure 6). On the
stalk, symptoms appear after tasseling first as narrow, water-soaked
lesions. Later they turn reddish-brown and finally turn black
late in the season. Stalk lesions eventually coalesce and form
shiny, black, sunken streaks (Figure 7). Upon close examination,
tiny black fruiting bodies can be seen on the surface of the stalk.
Inside the stalk, the infected tissue in the pith is soft and
watery.
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Figure 5. Anthracnose lesions on corn leaf. | |
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Figure 6. Anthracnose top dieback on corn (left and normal corn (right). | |
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Figure 7. Anthracnose lesions on surface of stalk. |
The anthracnose fungus survives on corn residue and stubble. The fungal spores can survive in the soil, so crop rotation in addition to deep plowing may be necessary to manage this disease.
SOUTHERN CORN LEAF BLIGHT (Bipolaris
maydis) was a relatively minor leaf disease of corn in Alabama
until the late 60s and early 70s. In 1970, a new race called "race
T" appeared. It attacked corn hybrids that contained "Texas
male sterile" (TMS) cytoplasm, destroying up to 90 percent
of the total crop in some fields. The original Bipolaris maydis,
"race 0," causes little damage to either normal corn
hybrids or to hybrids with TMS cytoplasm.
Southern corn leaf blight race T attacks all parts of the corn
plant (leaves, stalks, ear husks, ears, and cobs) at all stages
of development, whereas race 0 attacks only the leaves. Race T
causes foliage blight, ear rot, and stalk rot. The degree of damage
depends upon the stage of development when infection occurs. Corn
infected early in the season can be severely damaged and suffer
substantial yield losses. Infection after corn reaches the milk
dough stage causes little damage.
Initial symptoms of race T southern corn leaf blight appear as
small yellow spots on the lower leaves. As the disease progresses,
lesions enlarge, becoming elliptical with a brown center and a
reddish-brown border (Figure 8). Later, lesions become numerous
and eventually merge to form large dead (necrotic) areas on the
leaves (Figure 9). Lesions on stalks, sheaths, and ears are much
larger and elongate more rapidly, forming large, irregular, water-soaked
areas several inches long. Internal stalk and ear rots may result
from these infections.
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Figure 8. Southern leaf blight (race T) on corn leaf. | |
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Figure 9. Southern leaf blight lesions in advanced stage. |
Southern corn leaf blight usually occurs in humid weather and warm temperatures ranging from 55 to 85 degrees F. The fungus is spread by wind-blown spores and can survive the winter on corn stubble left in the field.
The most effective means of controlling southern leaf blight is to avoid corn varieties containing TMS cytoplasm. Plowing under corn residue and stubble after harvest will help prevent disease carryover to next year's crop.
GRAY LEAF SPOT (Cercospora zea-maydis) occurs sporadically in Alabama but rarely causes significant damage except in isolated fields. Tan, rectangular lesions (0.2 to 2 inches long) occur first on the lower leaves (Figure 10). When conditions are favorable, lesions will cover and kill entire leaves, causing stalk breakage and lodging. The gray leaf spot fungus attacks barnyardgrass, Johnsongrass, and other Sorghum sp. and is usually more severe in no-till or minimum-till fields. Use of resistant hybrids is the most effective means of controlling gray leaf spot.
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Figure 10. Gray leaf spot lesions on leaf. |
COMMON RUST (Puccinia sorghi)
is found worldwide on corn. In Alabama, yield losses have exceeded
50 percent in severely infected corn fields. Common rust pustules
can be found on all parts of the corn plant, but most occur on
leaves. The presence of pustules in equal numbers on both the
upper and lower leaf surface distinguishes common rust from southern
rust, which produces most of its pustules on the upper leaf surface.
As corn matures, rust pustules turn from golden brown to almost
black.
Common rust development is favored by relatively cool temperatures
(60 to 73 degrees F) and humid conditions. Spores are blown into
Alabama from tropical regions where the rust survives the winter
months.
SOUTHERN RUST (Puccinia polyspora)
occurs primarily in the southeastern United States but is found
less frequently than common rust in Alabama. This rust can cause
considerable damage, with yield reductions in certain fields in
the Southeast as high as 45 percent. Several races of Puccinia
polyspora are known to exist.
Southern rust is difficult to distinguish from common rust in
the field, because the pustules are similar in size and shape.
However, the two diseases can be differentiated--southern rust
pustules are more abundant on the upper leaf surface than on the
lower leaf surface (Figure 11), whereas common rust pustules occur
in equal numbers on both the upper and lower leaf surfaces.
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Figure 11. Southern rust pustules on the upper leaf surface. |
Like common rust, southern rust survives the winter in the tropics and is carried north by the wind or by infected plant material. Southern rust infects corn under humid conditions later in the season as temperatures rise above 80 degrees F.
Common rust and southern rust can be managed by selecting resistant hybrids and applying fungicides when rust pustules first appear on the leaves.
COMMON SMUT (Ustilago maydis)
occurs wherever corn is grown in Alabama. Losses from this disease
have been minimal, rarely exceeding 3 to 4 percent over the state.
However, in corn fields adjacent to wheat fields, common smut
has been particularly damaging. The corn smut damage is heaviest
along the edges of corn fields where stink bugs have moved in
from the nearby wheat fields.
The amount of damage caused by corn smut depends upon the stage
of development at the time of infection, insect feeding activity,
and plant parts affected. Plants infected when young may be killed
outright. On older plants, the greatest damage results from infection
of the corn ear or the stalk above the ear.
Corn smut is easily recognized by its distinctive symptoms. The
smut fungus infects young, actively growing plant tissue and forms
large tumor-like growths or galls on leaves, stalks, tassels,
and ears (Figure 12). These galls, which range from pea-size to
5 inches in diameter, are filled with masses of black spores.
Initially, smut galls are soft with a shiny surface but become
hard and dry as they mature.
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Figure 12. Corn infected with corn smut. |
The corn smut fungus survives the winter in corn residue or
in the soil. Smut spores are spread by insects, by wind, or in
animal manure.
Dry conditions and warm temperatures ranging from 79 to 93 degrees
F are favorable for infection and development of common smut.
Plants grown in fields with high levels of nitrogen or with high
organic matter are more susceptible to common smut. Mechanical
injury resulting from hail, sand blast, feeding insects, or human
causes increases the potential for smut infection.
Corn smut damage can be reduced by adopting the following practices:
- Plant corn cultivars tolerant to common smut.
- Maintain a balanced soil fertility. Avoid applying excessive nitrogen.
- Do not fertilize with manure from animals that may have been fed smut-contaminated corn.
- Avoid injury to plants during cultivation or spray application.
- Control stink bugs and other insects with insecticides or tolerant cultivars.
- Avoid planting corn next to wheat fields.
CRAZY TOP (Sclerophthora macrospora) rarely causes significant damage to corn in the state, but it attracts the attention of many Alabama corn producers because of its unique disease symptoms. This fungus, which is a downy mildew, causes excessive tillering and twisting of the upper leaves. The most distinctive symptom is the tassel that resembles a mass of leafy structures--often referred to as "crazy top" (Figure 13).
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Figure 13. Leaf and tassel proliferation caused by crazy top. |
Crazy top occurs predominantly in low-lying areas of fields where water tends to accumulate. Spores of this downy mildew fungus are moved about the field primarily in ground and surface water, so infection is highest in parts of the field where water flows or stands.
The disease can be successfully managed by providing adequate soil drainage or by not planting in low, wet areas of the field.
Bacterial Diseases
Bacterial diseases are most destructive under warm, humid conditions.
Most corn grown in Alabama is planted early and matures before
the weather is hottest. For late corn, there is usually drier
weather in mid-summer. Probably for these reasons, bacterial diseases
are usually not a problem in corn produced in Alabama. Bacteria
are most frequently spread by human beings or animals moving through
the fields, contaminated plant material and soil, or splashing
rain. The presence of free moisture is almost always required
for these bacteria to spread and develop. Dry conditions and extremely
high temperatures, on the other hand, retard disease development.
There are a number of bacterial diseases that attack corn, but
only a few are worthy of mention.
STEWART'S BACTERIAL WILT (Erwinia
stewartii) is more of a problem on sweet corn but can cause
considerable damage to a few extremely susceptible field corn
hybrids. Infected seedlings may wilt and die. Leaves on plants
infected after tasseling may dry and die. Yields may be reduced,
and plants may become susceptible to other rots.
Initial symptoms appear on the leaves as linear, pale green streaks
with wavy margins that run the entire length of the leaf (Figure
14). The streaks soon turn dry and brown. Cavities form in the
pith of the stalk near the ground line. If the stalk or leaf is
cut near the infected areas, yellow, moist beads of ooze may be
seen flowing from the cut ends.
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Figure 14. Stewart's wilt lesions on leaves. |
The corn flea beetle (Chaetocnema pulicaria), the toothed flea beetle (Chaetocnema denticulata I11.), the adults and larvae of the twelve-spotted cucumber beetle (Diabrotica undecimpunctata howarti), and the May beetle (Phyllophaga sp.) can spread the Erwinia bacteria.
Plant nutrition influences corn's susceptibility to Stewart's bacterial wilt. High levels of ammonium nitrate and phosphorus increase the plant's susceptibility, whereas high potassium and calcium levels decrease its susceptibility. The environment also plays an important role in the incidence of bacterial wilt. Mild winters, which allow the insects to survive, increase the incidence of wilt the following growing season.
The use of resistant cultivars is the most effective means of controlling this disease. Applications of insecticides early to destroy the insect vectors will also reduce the incidence of wilt.
BACTERIAL STALK ROT (Erwinia chrysanthemi zeae) is most destructive in corn fields exposed to extremely wet conditions or flooding and to high temperatures (above 86 degrees F). The first signs of the disease occur during midseason when green plants suddenly fall over, leaving collapsed, twisted stalks. Water-soaked, slimy, and rotted tissue can be found in one or several internodes of the stalk above the ground line (Figure 15). Often a foul smell can be detected. Vascular strands may remain intact and keep the infected plants green for days after falling over. A top rot may occur in fields irrigated overhead with water from lakes or ponds. The tips of leaves wilt first, followed by a slimy, soft rot at the base of the leaf whorl. The rot moves into the stalk rapidly, causing the plant to collapse.
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Figure 15. Bacterial rot on corn stalk. |
The bacteria live in crop residue and infect corn through natural openings in the leaf of the plant during warm, humid conditions.
The best control for bacterial stalk rot is to avoid excessively wet fields and to use resistant hybrids.
Plant Viruses
More than forty virus-related diseases of corn are known worldwide, but only a few are of economic importance to Alabama corn producers. Viruses can be transmitted and spread to healthy plants by human beings or animals and through mechanical injury, fungal vectors, infected plant pollen, and insects. Insects, including aphids, and leafhoppers, are by far the most important vectors of the major viruses of corn in this state. Two of the most important corn virus diseases are discussed below.
MAIZE DWARF MOSAIC VIRUS (MDMV) can be found wherever corn is grown in the state. Losses from MDMV vary from year to year, but may exceed 30 to 40 percent under favorable conditions. MDMV infection first appears as a light green to dark green mottling on the base of the youngest leaves (Figure 16). These symptoms may develop into dark green "islands" with a light green to yellow background. Mottling may be observed on all leaves, sheaths, and corn husks that develop following MDMV infection. Corn infected early in its development may be slightly stunted.
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Figure 16. MDMV-infected (left) and MCDV-infected (right) corn leaves. |
MDMV is spread by aphids from MDMV-infected corn or grasses. More than 200 grass species host MDMV, but Johnsongrass is its principal weed host.
MAIZE CHLOROTIC DWARF VIRUS (MCDV) causes a distinctive chlorotic striping in the youngest leaves (Figure 16). MCDV retards the growth of internodes, severely stunting the plant (Figure 17). Infected leaves turn yellow or red. MCDV is especially damaging when it occurs in a mixed infection with MDMV. When mixed infections occur, corn plants may be severely stunted.
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Figure 17. MCDV-stunted corn plant in foreground. |
MCDV is transmitted by leafhoppers. Alternate hosts for MCDV include Johnsongrass, grain sorghum, Sudangrass, wheat, crabgrass, foxtail, and milo.
Using tolerant corn cultivars and controlling Johnsongrass in and around the field may be the most practical control measures for these viruses. Also, using systemic insecticides to reduce the overall leafhopper population may be helpful.
Nematodes
Several species of nematodes are known to attack and cause
damage to corn in Alabama. The lesion nematode (Pratylenchus
sp.) and root-knot nematode (Meloidogyne sp.) are the two
nematode species most widespread and damaging to corn in the state.
Other nematodes reported to cause damage include the stubby root
(Trichodorus sp.), dagger (Xiphinema sp.), and sting
(Belonolaimus sp.).
Losses in corn fields heavily infested with root-knot nematodes
or lesion nematodes may exceed 30 percent of expected yield. In
the field, nematode damage may be easily confused with other problems
such as nutrient deficiency, compacted soils, or acid soils. Aboveground
symptoms include stunting, nutrient deficiency, and unthrifty
growth. Because of the restricted mobility of nematodes, most
damage initially appears in localized areas of the field. This
feature often distinguishes nematode damage from other problems
whose signs are found generally throughout the field.
Actual nematode injury is restricted to the corn plant's root
system. A few nematode species produce distinctive symptoms on
the root system. For example, root-knot nematodes produce characteristic
galls when they feed on the roots, and stubby root nematodes kill
the root tips as they feed, causing the distinctive stubby roots
(Figure 18). Other nematodes produce no easily distinguishable
symptoms on corn roots other than a sparse root system.
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Figure 18. Stubby root nematode injury on corn roots. Note the "stubby" root tips. |
Since it is difficult to detect signs of nematodes, you should sample any field suspected of having nematodes. Nematode soil samples can be analyzed at the Plant Diagnostic Laboratory at Auburn University for $5.00 per sample. For further details on collecting and sending soil samples for nematode analysis, consult Extension publication ANR-114, "Collecting Soil And Root Samples For Nematode Analysis" (revised 1991). Nematodes can be managed in corn with granular nematicides, by land fallowing, or by crop rotation.
For more information, contact your county Extension office. Visit http://www.aces.edu/counties or look in your telephone directory under your county's name to find contact information.
Published by the Alabama Cooperative Extension System (Alabama A&M University and Auburn University), an equal opportunity educator and employer.
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