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).

1. Plant well-adapted, disease/stress resistant hybrids.
2. Practice balanced soil fertility.
3. Reduce crop stress through proper plant populations, irrigation, soil management and foliar disease and weed control.
4. Control insects.
5. Manage corn residue.
6. 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. Growers should select high-yielding hybrids with good standability and stress resistance. Your Great Lakes 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. Great Lakes hybrids with the YieldGard Corn Borer and Herculex I genes have shown excellent resistance to corn borer feeding. Corn rootworm traits or 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.

One Response

1. August 23rd, 2006 | Clint Hawks Replied:
   

   SCOUTING FOR STALK ROTS

   By Paul Vincelli

   I’ve received preliminary reports of stalk lodging problems in some parts of Western Kentucky. With near-record yields forecasted, this is not surprising. Maybe this seems paradoxical, but a bumper crop is at more risk of stalk rot than a normal crop. This is because corn plants in high-yielding fields-especially if they are crowded together in excessive populations-often fill out the grain by removing carbohydrates from the stalk. This can result in poor stalk strength.

   When stalk rot strikes, we’ll often find several stalk rot fungi attacking a severely lodged field. Stalk rot diseases are the result of opportunistic infections. What this means is that certain stress factors, such as low carbohydrate status in the stalk, predispose the plant to infection, and the first fungus to attack a particular plant is the one we find there. What this also means is that the most important thing to do from a production standpoint when stalk rots attack is to evaluate one’s cultural program and see if there are particular agronomic stresses that might be alleviated. Factors that might enhance stalk rot problems include: excessive plant population, excessive N (especially if potash levels are not also high), high N levels early in the season followed by N loss through leaching or denitrification, low stalk strength ratings of hybrids planted, and severe leaf disease or leaf damage. Ear set in a high position on the plant can also increase the risk, by making the plant top-heavy. Producers may wish to evaluate these factors to see if there are ways to reduce the risk of stalk lodging in future years.

   Producers should consider scouting corn crops for lodging potential as they approach maturity. This practice helps identify fields that should be harvested early and dried down. A simple way to scout for lodging potential is to walk the field and push plants 12-18 inches from vertical at about chest height. Stalks that don’t spring back have the potential to lodge. If 10-15% of the field shows such lodging potential, plan on harvesting the field soon after the grain is physiologically mature (development of black layer, about 30-35% grain moisture).

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