Managing Stalk Rots in Corn - Anthracnose, Gibberella and Diplodia
Crop Insights by Steve Butzen and Bill Dolezal
- High temperatures create moisture stress on corn plants, even under irrigation. This stress increases the occurrence of stalk rots.
- talk rots progress as corn plants are weakened by stress such as drought, leaf diseases, insect feeding, low fertility or compaction.
- Stalk rots cannot be entirely prevented, but their effects can be reduced by selecting resistant hybrids, maintaining balanced soil fertility, minimizing crop stress, controlling insects, managing corn residue and scouting fields for timely harvest.
- This Crop Insights discusses conditions resulting in stalk rot development, symptoms associated with the predominant stalk rots, and management practices to reduce their occurrence.
Growing conditions can position a corn crop for record yields while also creating a high risk for stalk quality problems. High yield potential along with high occurrence of leaf diseases is a combination that often leads to stalk rot development during the mid to late stages of grain fill. If stresses such as water deficits occur during grain fill, the risk of stalk quality problems is greatly increased.
Stalk rots such as anthracnose, Gibberella and Diplodia reduce corn yield by killing the plant before physiological maturity of the grain. Plant lodging also increases harvest losses and impedes harvest progress. If ears contact the ground for an extended period, grain quality may also be reduced. This article reports on the development, symptoms and management of yield-reducing stalk rots of corn.
Anthracnose is a fungal disease that infects corn, grain sorghum and small grains. The pathogen overwinters in diseased leaves and stalks and produces spores when the weather warms in the spring. Spores are spread by wind and rain; infection is favored by warm temperatures (70-80°F) and high humidity.
Anthracnose has both a leaf and a stalk phase in corn. The infection can spread from leaves to stalk, or the stalk may be infected through the roots or base of the plant.
Leaf Blight Phase
Anthracnose leaf blight is common early in the season in fields where diseased crop residue was left on the soil. Infection develops when disease spores are splashed onto seedling plants by rain.
Anthracnose Stalk Rot Disease Cycle (Colletotrichum graminicola)
Upon infection, small round to irregular water-soaked spots appear and later turn yellow and then brown with reddish-brown borders. The leaf spots may enlarge to one-half inch long and coalesce (join together). In severe cases, the leaf tips or entire leaves may turn yellow.
Anthracnose. (top) - Note black "specks" in center of lesion on seedling leaf (magnified). (Bottom) - Lesions coalesced on mature leaf.
Though anthracnose is common in seedlings, plants appear to resist the leaf blight phase during vigorous vegetative development. However, during the ear-fill stage, plants often become susceptible to the top dieback and/or stalk phase of the disease.
Anthracnose Top-Dieback Phase
Top dieback is a common symptom associated with anthracnose stalk rot. About two or three weeks after pollination, the tops of affected plants begin to appear yellow. The uppermost leaves continue to turn yellow or reddish-purple and then brown.
These symptoms are often confused with second brood corn borer damage, which can also result in the death of the upper plant due to tunneling by the insect. However, when leaves are removed, typical black anthracnose lesions can often be seen on the outside of the upper stalk, and split stalks show rotted pith tissue.
Top of corn plants dying due to anthracnose top dieback.
Anthracnose Stalk Rot Phase
The anthracnose stalk rot phase can develop from seedling infection of the plant or from spores originating from corn residue. These spores infect the stalk through the roots or the base of the plant, or through insect cavities or other wounds in the stalk.
Lesions on the stalk appear as oval or narrow vertical water-soaked areas that turn tan to reddish-brown and finally black. Shiny black blotches which often coalesce are a distinguishing characteristic of this disease. Removing the leaves and leaf sheaths from the lower stalk is the best way to inspect the plant. Splitting the stalk reveals degenerated pith tissue, often with only the vascular bundles remaining. Diseased tissue is usually dark gray to brown in color.
Shiny black blotches on outside of corn stalk are characteristic of anthracnose stalk rot.
Another stalk rot common in most corn-growing regions is Gibberella. The fungal pathogen over-winters in corn residue, then produces spores the following season. Warm, wet weather during early ear-fill is conducive to disease development. Infection occurs through the roots of the plant and spreads to the stalk as the plant is weakened by stress. Rotting generally affects the roots, crown and lower internodes.
Gibberella stalk rot can best be identified by splitting the stalk. The pith inside is disintegrated and characterized by a pink or reddish color. On the outside of the stalk, small superficial black spots (perithecia) are often evident.
Gibberella Stalk Rot Disease Cycle (Gibberella zeae)
Diplodia. Note pycnidia on corn stalk node (magnified).
Diplodia and Gibberella have a similar disease cycle and are both encouraged by warm, wet weather two to three weeks after pollination. Diplodia stalk rot may first be evident when affected plants die suddenly during mid to late ear fill. Upon examination, dark brown lesions can be found extending in either direction from the node. Small black spots (pycnidia) may develop just beneath the stalk epidermis near the nodes. The black dots are not easily removed, which distinguishes Diplodia from Gibberella.
Diplodia results in rotted stalks that are disintegrated and discolored, allowing the stalk to easily break. The discoloration is brownish in appearance, not pink like Gibberella.
Stalk Rot Development
Stalk rot development is a complex problem. Although the pathogens may be present, healthy corn plants can resist infection and spread of the disease for much of the growing season. Eventually, however, accumulated stresses and the onset of natural senescence weakens the plant and allows invasion and spread of the disease.
One of the major stresses on the corn plant is internal – the competition within the plant for limited carbohydrate supplies. After pollination, the demand for carbohydrates by developing kernels becomes extremely high. The plant gives priority to the kernels, and will cannibalize other plant tissues if necessary to meet this need.
During this period of high demand, the ability of the plant to produce new carbohydrates through photosynthesis is essential. Any stress that reduces that ability will result in the translocation of carbohydrates from root and stalk tissues to the ear. This weakens those tissues and allows root and stalk rot pathogens to invade. Stresses that reduce photosynthesis in the plant are numerous:
- Drought stress reduces the rate of photosynthesis and may also reduce effective photosynthetic area if leaves roll or wilt.
- Leaf diseases, insect feeding and hail reduce the leaf area available for photosynthesis.
- Cloudy weather reduces the sunlight available for photosynthesis.
Other stresses on the plant also increase its susceptibility to stalk rot infection, including:
- High yield potential, which increases the movement of carbohydrates to the ear from the roots and stalks. If good growing conditions during ear and kernel formation are followed by moisture stress later on, the problem is worsened.
- Low soil fertility. Potassium aids in strong stalk development; its adequate supply is critical to maintain healthy stalks. Nutrient imbalance caused by low potassium and high nitrogen increases losses from stalk rot. If nitrogen is available early but is lost before the end of the season, stalk rot may be increased dramatically.
- Insect feeding, especially corn rootworms and corn borers. These primary pests of corn provide entry ports for disease organisms. They also diminish plant function by reducing water and nutrient flow into and through the plant.
- Compaction, which limits normal root growth and water and nutrient uptake. Nematodes and soil insects also reduce normal root growth and function.
Managing Stalk Rot and Lodging
Stalk rots cannot be entirely prevented, but their effects can be reduced through good management practices. The following practices will reduce stalk rot, lodging and harvest losses (Shaner and Scott, 1998, Illinois Extension, 1995).
- Plant well-adapted, disease/stress resistant hybrids.
- Practice balanced soil fertility.
- Reduce crop stress through proper plant populations, irrigation, soil management and foliar disease and weed control.
- Control insects.
- Manage corn residue.
- Scout fields prior to crop maturity and harvest fields based on crop condition.
Hybrid differences. Hybrids are different in resistance to leaf diseases, stalk rots, drought stress and stalk lodging. Pioneer Hi-Bred International, Inc. provides ratings on these traits. Growers should select high-yielding hybrids with good standability and stress resistance. Your Pioneer representative can help you select the right hybrid for your field and growing conditions.
Soil Fertility. Test soils regularly and apply nutrients based on soil test results and yield goals. Be sure potassium levels are adequate, and manage nitrogen to prevent losses and ensure its availability throughout plant uptake.
Crop stress. Crop stress is never eliminated but can be reduced with good crop, soil and water management. Excessive plant populations increase stress and stalk lodging. Poorly spaced or "clumped" plants create a high population micro-environment similar to overplanting. Maintain planter and planter meters properly and do not exceed manufacturer’s suggested ground speed. Calibrate planter meters for optimum plant spacing and monitor rates carefully when planting.
Compaction is one of the primary causes of crop stress, and once it occurs, may take years to remedy. Avoiding compaction and maintaining soil quality are keys to reducing crop stress. Proper irrigation management is critical to minimize crop stress in arid regions.
Insects. Manage insects such as corn borer and corn rootworm to prevent plant wounds and stress. Pioneer® brand hybrids with the YieldGard*, Herculex® I or Herculex® XTRA genes have shown excellent resistance to corn borer feeding. Corn rootworm insecticides may be needed on continuous corn acres or where corn rootworms have adapted to corn-soybean rotations. This includes extended diapause areas for northern corn rootworm and areas where the western corn rootworm variant deposits eggs in soybean fields.
Corn Residue. Stalk rot pathogens overwinter in corn residue. Occurrence and intensity of stalk rots are sometimes related to the amount of inoculum present. Anthracnose is a prime example; it is more prevalent in continuous corn and notill fields. Rotation to a non-host crop such as soybeans is recommended to reduce corn residue and stalk rot. Disking or otherwise incorporating residue may also be beneficial in some fields. Reduction of stalk rots must be weighed against the advantages of soil conservation and maintaining soil carbon levels when deciding whether to till.
Scouting. Careful scouting and harvesting fields according to crop conditions can help prevent field losses due to stalk rot. Corn loss potential should be weighed just as heavily as grain moisture in deciding which fields to harvest first. Scouting fields approximately two to three weeks prior to the expected harvest date can identify fields with weak stalks predisposed to lodging. Fields with high lodging potential should be slated for early harvest.
Weak stalks can be detected by pinching the stalk at the first or second elongated internode above the ground. If the stalk collapses, this indicates advanced stages of stalk rot. Another technique is to push the plant sideways about 8-12 inches at ear level. If the stalk crimps near the base or fails to return to the vertical position, stalk rot is indicated. Check 20 plants in five areas of the field. If more than 10-15% of the stalks are rotted, that field should be scheduled for early harvest.
Butzen, S. 1999. Stalk quality problems in corn; Crop Insights 9 (18). Pioneer Hi-Bred International, Inc., Johnston, IA.
Illinois Extension Staff, 1995. Corn stalk rots. Report on Plant Disease #200. Dept. of Crop Sciences, University of Illinois. Urbana, IL.
Shaner, G. E and .D. H. Scott, 1998. Stalk Rots of Corn. Extension publication BP-59, Dept. of Botany & Plant Pathology, Purdue University. West Lafayette, IN.