Reports on Plant Diseases | RPD No. 200 - Corn Stalk
Rots | December 1995 |
[ Symptoms ] [ Disease Development
] [ Control ]
| Stalk rots
are the most common diseases of dent corn in Illinois. These diseases reduce annual
yields by a minimum of 5 percent. Losses in certain years reach 10 to 20 percent
or more of the expected yield. Losses are due to (1) premature plant death, which
results in poor filling of ears or lightweight and poorly finished ears and (2)
harvest losses associated with stalk breakage or lodging (Fig. 1). Problems with
ear rots are usually greater where lodging occurs, particularly when the harvest
season is wet. Several fungi and bacteria can cause stalk rots. Stalk rots
result from the combined effects of more than one organism that attacks plants
approaching maturity. Therefore, the identification of a specific organism causing
the stalk rot is often difficult. The common stalk rot diseases caused by fungi
in Illinois include Gibberella stalk rot (Gibberella zeae, anamorph
Fusarium graminearum), anthracnose (Colletotrichum graminicola),
Fusarium stalk rot (chiefly Fusarium moniliforme, teleomorph Gibberella
fujikuroi), and Diplodia stalk rot (chiefly Diplodia maydis).
In hot dry seasons, charcoal stalk rot caused by Macrophomina phaseolina may be
prevalent. Pythium stalk rot (primarily Pythium aphanidermatum)
is usually rare but may occur if the weather is excessively wet during the summer
months. Nigrospora stalk rot (Nigrospora oryzae, teleomorph Khuskia
oryzae) sometimes is found in severely stressed plants nearing maturity.
Physoderma stalk rot (Physoderma zeae) is generally a minor disease
occurring in Illinois during hot, wet weather. Bacterial stalk rot (Erwinia
chrysanthemi pathovar zeae, synonym E. carotovora
pv. zeae) is not common in Illinois and rarely causes significant
damage. Occasionally, bacterial stalk rot appears before tassel emergence on random
corn plants following heavy rains. General infection may occur following flooding
or sprinkler irrigation from a surface source of water. Back
to Top
|
Figure
1.
Figure
1. Diplodia stalk rot. Left: external discoloration; center: pycnidia below
node; right: stalk split to show disintegration of tissue. |
Symptoms
Stalk rots caused by species of Gibberella,
Fusarium, and Diplodia are not usually apparent until several
weeks after silking and pollination. First, the leaves turn a dull gray-green,
similar in appearance to frost or drought damage. Then the leaves turn brown,
seemingly in a day or two. Death of the entire plant follows within 7 to 10 days
in susceptible hybrids. The lower internodes turn from green to tan, straw-colored,
or dark brown and are spongy and easily crushed. When stalks are split lengthwise,
only the vascular strands remain intact with the pith tissue disintegrated and
discolored (Fig. 2a, 3, and 4a).
Figure
2. Dark brown to black pycnidia of Diplodia maydis embedded in rind of dead
corn stalk. | GIBBERELLA
AND FUSARIUM STALK ROTS. Stalks infected with the Gibberella
fungus have a characteristic pink to reddish discoloration of the pith and vascular
strands. Fusarium stalk rot appears similar to Gibberella, except
that the discoloration of infected tissues commonly varies from whitish pink to
salmon. Rotting commonly affects the roots, crown, and lower internodes. The breakdown
and shredding of pith tissues starts at the nodes (Fig. 3) shortly after pollination
and becomes more severe as the plant matures. In addition, small, round, bluish
black perithecia (fungal fruiting bodies) form superficially on the surface of
Gibberella-infected stalks in the fall or during the following spring (Fig. 4c).
These fruiting bodies are easily scraped off with a thumbnail (Fig. 4b). |
Figure
3. Fusarium stalk showing early breakdown and shredding of lower stalk node. |
 
Figure
4. (A) Giberella stalk rot showing 3 split stalks. (B) tiny fungal fruit bodies
of Giberella Zeae. (C) close-up of round, black perithecia on surface of corn
stalk. | ANTHRACNOSE
STALK ROT OR TOP-DIEBACK. This was a minor problem before 1970, but is now
a serious disease. Unlike most other stalk rots, anthracnose, caused by the fungus,
Colletotrichum graminicola, will often destroy several internodes
of the stalk. The fungus may cause top-kill and lodging as well as infect most
plant parts including the shanks, kernels, and roots. The fungus also causes an
important leaf blight, which does not appear to be associated with the stalk rot
phase. Very susceptible hybrids may be killed before pollination. Most hybrids,
however, are killed only a week or two before normal maturity. | | A
shiny black or dark brown discoloration of the rind late in the season is typical
of anthracnose. This black discoloration usually extends up the stalks for several
internodes and may uniformly discolor the rind or give it a blotchy or speckled
appearance (Fig. 5, left). Black specks (acervuli) associated with the production
of microscopic spores by the fungus are often found on the stalk rind. When such
stalks are split, the pith is dark brown and decayed (Fig. 5, right). In some
hybrids the pith is soft, watery and easily crushed. Severely affected stalks
are likely to lodge. Sometimes portions of the stalk above the ear turn a gray-green,
die and collapse (top-kill) 4 to 6 weeks after pollination, while the lower stalk
remains green. The upper 2 or 3 leaves may turn yellow or reddish, collapse, and
drop off. This may occur just before the lower leaves begin to normally wither
and die. |
Figure 5. Anthracnose
stalk rot: (left) shiny black discoloration on stalk; (right) stalk split to show
dark internal decay. |
Back
to Top |
|
CHARCOAL ROT. This rot is most abundant
in hot dry seasons. Soil temperatures near 99 F (37 C) are most favorable for
disease development, while either low soil temperatures or high soil moisture
decrease severity. Brown, water-soaked lesions, that later turn black, appear
on the roots. As the plants mature, the fungus spreads into the lower internodes
of the stalk. Infected plants appear to ripen prematurely and the interior of
the lower internodes disintegrate. The disease is distinguished by the presence
of numerous, tiny, round to irregular, black fungal bodies (sclerotia), which
are present in large numbers along the vascular strands in the interior of shredded
and rotted stalks giving them a charred appearance (Fig. 6). Sclerotia also may
be found just beneath the stalk surface and on the roots. Kernels also are infected
and turn completely black. PYTHIUM STALK ROT. Pythium stalk rot usually
occurs during extended hot (optimum 90 F or 32 C), wet weather from late June
to September. The disease is most common in river-bottom fields where air and
soil drainage is poor and the humidity is high. Usually, only the internode just
above the soil line is soft and brown. The interior of the stalk appears water-soaked.
Plants often collapse, and may become twisted and distorted, but they remain green
and turgid for several weeks because the vascular bundles remain intact (Fig.
7). Positive diagnosis can be made if microscopic examination reveals the round,
thick-walled oospores of Pythium aphanidermatum in diseased tissue.
NIGROSPORA STALK ROT. This minor, often secondary rot appears as
superficial, dark gray to black lesions, with a slight bluish cast, on the lower
internodes. The discoloration may be seen as many separate, tiny to large, irregular
blotches. Nigrospora is not a serious disease and only occurs where plants are
weakened or predisposed by poor fertility, other diseases, or injury. PHYSODERMA
STALK ROT. This stalk rot first appears as small, round, yellowish spots near
the base of the leaf blade, often in bands. Later the lesions turn chocolate-brown
to red-brown and merge to form large, irregular angular blotches. Infection at
the nodes under the leaf sheaths cause similar, water-soaked lesions on the stalk
(Fig. 8) which may lodge from a black decay. |
Figure
6. Charcoal rot. Note sclerotia on surface (left); fibers within the stalk
(right).
Figure
7. Pythium stalk rot. Affected plants collapse but remain green for several
weeks.
Figure
8. Physoderma stalk rot. (Left) Brow blotches on leaf and sheath; (right)
black and decayed, lodged stalk. | Back
to Top
Figure
9. Bacterial stalk
rot caused by Erwinia chrysanthemi p. zeae. Affected stalks collapse
and twiste. | BACTERIAL
STALK ROT. This uncommon disease usually occurs about midseason in hot, damp
weather as a tan to dark brown, water-soaked, soft or slimy disintegration of
pith tissues at a single internode. Affected stalks suddenly collapse and are
usually twisted (Fig. 9). The tips of the uppermost leaves often wilt, followed
by a slimy soft rot at the base of the whorl. The decay spreads rapidly downward
until the affected plants collapse. Lodged plants usually have a foul odor. The
development of bacterial soft rot, like Pythium stalk rot, is favored by high
temperatures (85 to 95 F, 29 to 35 C) and poor air circulation. General infection
may occur following flooding or where corn is sprinkler-irrigated from a surface
source of water, such as a river, farm pond, or lake. The causal
bacterium lives as a saprophyte on plant debris in the soil. The organism also
may be seedborne. Infection occurs when the bacteria are blown or splashed onto
the plants followed by penetration through natural openings (stomates and hydathodes)
or wounds made by hail or other injuries. | Development of stalk rots is generally favored by an early
season environment that encourages kernel set and by a late season environment
that is stressful. Post-flowering stresses may include an excess or lack of moisture;
nutrient deficiency or imbalance; excessive cloudiness; insect, nematode, hail,
or other mechanical injury to the stalks or roots; loss of effective leaf area
due to a foliar disease; and an excessive plant population. Extended periods of
very dry or wet weather prior to pollination, followed by an abrupt change for
several weeks after silking, favor the development of most stalk rot fungi. High
yields often are associated with stalk rot problems. Plants can over commit to
yield when environments are ideal through the pollination period and stress occurs
later. In a stressful environment, the large number of kernels places a high demand
on the plant for sugars. If photosynthesis, which produces sugars, is reduced
because of stress, the kernels draw sugars from stalk tissue and deprive the roots
of adequate nutrients. Cloudy weather following pollination is the most common
stress factor because photosynthesis is reduced by 40 to 50 percent as compared
to bright cloudless days. High nitrogen (N) levels combined with a low level
of potassium (K) may also increase stalk rot. However, high rates of nitrogen,
balanced with adequate to high levels of potash (K20) and phosphorus
(P), do not increase the potential for stalk rot. Adequate to high
levels of nitrogen followed by a loss of available nitrogen by denitrification
or leaching may dramatically increase the incidence of stalk rot. The severity
of stalk rot has been reported to decrease with increasing N rates applied in
either spring or autumn as anhydrous ammonia. Injury to roots, stalks, or
leaves by diseases, insects, nematodes, hail, or equipment also can increase the
incidence of stalk rot. Injuries created by European corn borers, corn rootworms,
and other insects often provide avenues of entry for stalk rot pathogens. Observations
indicate that the incidence of stalk rot is increased by nematode damage. High
populations of root-lesion (Pratylenchus spp.) and spiral (Helicotylenchus
spp.) nematodes have been associated with increased stalk rot. Hail injury may
predispose plants to stalk rot primarily because the effective leaf area is reduced.
The loss of photosynthetic leaf tissue because of diseases such as northern corn
leaf blight, southern corn leaf blight, anthracnose leaf blight, Stewart's leaf
blight, and yellow leaf blight may also increase the incidence and severity of
stalk rot. Back to Top Stalk rots cannot be entirely controlled. Damage can be reduced through
the conscientious use of an integrated control program. The following practices
will reduce harvest losses: (1) plant well-adapted, disease-resistant hybrids;
(2) practice balanced soil fertility; (3) control insects; (4) plant the recommended
plant population; (5) avoid stress through proper irrigation, soil management,
and foliar disease and weed control; (6) practice regular field scouting; and
(7) harvest when the crop is mature (30 percent grain moisture) to prevent losses
from lodging. - Corn growers should select hybrids that have stalk rot
and leaf disease resistance, good standability, and high yield potential. Full-season
hybrids are generally more resistant than those that mature early in a given area.
Resistance to the fungi that cause stalk rots helps prevent losses from premature
plant death and lodging. Most hybrids, however, are not resistant to all stalk-rotting
organisms.
In addition to stalk rot resistance, growers should select hybrids
resistant to foliar diseases important in their area since less of leaf area can
predispose the corn to stalk rot problems. Hybrid standability is another factor
that should be considered. Hybrids with a thick rind (stalk strength) or other
characteristics that increase standability often remain standing even though the
interior of the stalk is thoroughly decayed. Corn producers should consider such
characteristics when selecting a particular hybrid and tour local hybrid strip
plots to check on the susceptibility of various hybrids to stalk rot. Up-to-date
information on the yield performance and lodging of many hybrids also is available
in the latest issue of the circular Performance of Commercial Corn Hybrids
in Illinois (check with your nearest Extension office for circulars). -
Balanced soil fertility, particularly with respect to potassium, is important,
as high level of nitrogen along with a low level of potassium can increase the
losses from stalk rot. However, a high level of nitrogen with adequate-to-high
potassium and phosphorus availability does not necessarily increase the
potential for stalk rot. Information on suggested nitrogen, potassium, and phosphorus
rates can be found in the latest Illinois Agronomy Handbook, available
at area Extension offices and revised semi-annually. Fertilizer should be applied
based on the results of soil tests.
Research has shown the importance of an
adequate supply of nitrogen throughout the season in reducing the severity of
stalk rot. High levels of nitrogen followed by a loss of nitrogen due to leaching
or denitrification may increase the severity of stalk rot dramatically. In areas
where leaching or denitrification is expected, the use of a nitrification inhibitor
or stabilizer, may help reduce stalk rot. - Control of root- and stalk-attacking
insects, such as the European corn borer and corn rootworms, is important in reducing
stalk rot losses. Corn growers should follow the cultural and chemical recommendations
of University of Illinois Extension Entomologists as given in University of Illinois
Cooperative Extension Circular 899, Insect Pest Management Guide: Field and
Forage Crops. The use of scouting procedures will reduce unnecessary application
of pesticides and increase yields because applications will then be made at the
precise time for optimum results.
- Corn growers should
plant a particular field at row widths and populations suggested for the particular
hybrid, soil type, fertility level, available soil moisture, and productivity
potential. Higher than recommended plant populations and narrow rows cause plants
to be stressed, result in spindly stalks with a reduced standability, and reduced
resistance to infection. Growers should consult seed-corn handbooks for suggestions
on planting rates for hybrids.
- Crop rotation with nongrass
crops in conjunction with a deep plowdown of corn residues (only where
soil erosion is not a problem) is beneficial in reducing anthracnose, Diplodia,
charcoal, and Physoderma stalk rots.
- Timely irrigations
(where possible) to keep the soil moist during dry periods up to 50 to 55 days
after tasseling, weed and nematode control, and other stress-reducing practices
are important in reducing stalk rot losses.
- Monitoring
(or scouting) fields on a weekly basis is the best way to determine pest levels
in a field. Corn producers should begin to scout fields for stalk rots (lodging
potential) when the corn contains 30 to 40 percent moisture. An effective scouting
procedure is to walk a zig-zag pattern through the field while pinching the stalks
below the lowest node to check for firmness. Another method is to push random
plants (a minimum of 100) about 5 inches from the vertical. If more than 10 to
15 percent of the plants lodge using this procedure, it would be beneficial to
harvest the field early to prevent potential harvest losses. The same procedure
also can be used to assess hybrids in strip plots. Growers also should check the
extent of premature plant kill when inspecting strip plots.
By regularly scouting
and rating fields for disease, insect, and weed infestation during the season,
growers can carry out timely and effective measures when they are needed.
Knowing which fields have the greatest potential for stalk and ear rots will help
reduce losses by enabling the grower to schedule these fields for an early harvest.
Back
to Top
For further information concerning
diseases of crucifers and other vegetables, contact Mohammad Babadoost, Extension
Specialist in Fruit and Vegetable Diseases, Department of Crop Sciences, University
of Illinois at Urbana-Champaign. University
of Illinois Extension provides equal opportunities in programs and employment. |
