Weed Control
A number of factors need to be considered when developing weed control programs for corn, including soil type, weeds, weeds present, crop rotation and budget. No single control program effectively handles the various weed problems that arise under different environmental conditions. Weeds are the major pest control problem in corn production in Ohio. Specific chemical weed control recommendations can be found in the Weed Control Guide, Extension Bulletin 789, available at all County Extension offices and online at CFAES publications at: estore.osu-extension.org/.
Insect Control
Many insects will feed on corn throughout the growing season, but only a handful cause economic injury necessitating control measures. We have seen a recent shift of insect control to more preventative practices (e.g., transgenics), but if the pest pressure is not present, then the benefit derived from these treatments will not outweigh the cost of treatment. In general, corn insects are controlled by three tactics. First, transgenic corn will express proteins from the bacteria Bacillus thuringiensis (Bt) which will provide season long control, but is only active for certain pests (for a complete list see: msuent.com/assets/pdf/28BtTraitTable2016.pdf). Bt varieties can protect against above-ground pests, below-ground or both, de- pending on the variety. A refuge (a certain percentage of the field containing non-Bt corn) must still be planted; the percentage and placement (either a separate or integrated, i.e., refuge-in-the-bag) varies depending on the traits present. Second, seed can be coated with insecticidal seed treatments, which offers preventative control of secondary and minor pests. However these seed treatments are short lived (the activity window is about 30 days after planting) and have also been linked to a negative impact on honey bees and other pollinators, so it is important to use them judiciously. Finally, soil or foliar-applied insecticides can be used, although the need for foliar application of insecticides has become extremely rare because of Bt corn. Information on insecticides can be found here: oardc.ohio-state.edu/ag/images/545_Final_2013(3).pdf.
The risk of injury from insects will vary largely depending on weather and region. Knowledge of the pests’ biology, field history and scouting will help limit the damage from insects and slugs. Here, we provide a summary of the major pests of corn, damage symptoms and management recommendations.
FLEA BEETLE These tiny, black beetles with long jumping legs can sometimes cause injury after mild winters. While their feeding is usually not economic, they can vector pathogens that cause Stewart’s wilt and leaf blight. Economic losses from the beetle or the pathogen are rare. Flea beetles can be controlled by insecticidal seed treatments or by foliar application (if > 3 percent of plants are wilted and dying and beetles are still active).
SEEDCORN MAGGOT The larvae of flies, these maggots will feed on corn seed or early emerging plants. The larvae are small, white and legless, resembling a grain of rice and can be found feeding on or near the young plant. Although rare, significant losses in plant stand can occur. Fields with the highest risk of seedcorn maggots have high organic matter (for example recent manure application), or green cover crops that are tilled under with corn planted five to 10 days later. Early-planted fields are also at higher risk. Adult maggots are attracted to the odor from decaying matter and lay eggs. Damage includes poor emergence, gaps in the row, or weak and stunted plants. Most insecticidal seed treatments (excluding imidacloprid) provide control, and soil-applied insecticides will also protect against damage. There are no thresholds or rescue treatments, and replanting may be needed in heavily damaged fields.
WIREWORM There are multiple species of wireworms, which are the larval form of adult click beetles. Wire- worms are long and thin, and tan or brown. Damage from wireworm resembles seedcorn maggot; in fact, they can be found in the same fields. Fields with a history of turf or pasture are most at risk from wireworm. There are no thresholds or rescue treatments, but fields with a history of wireworms may need insecticidal seed treatments and soil insecticides which will control wireworms.
GRUBS There are multiple species of grubs in Ohio, all of which are fairly large, creamy white with orange heads. While most species are of minor importance, the Asiatic garden beetle is one that has caused significant damage in sandy soils, especially in northwest Ohio. Most grubs are controlled by insecticidal seed treatments or soil insecticides, and fields with a prior history of grub infestation may consider these options. Later planted corn tends to escape most of the damage, as the grubs feed less as the season progress.
BLACK CUTWORM Adults migrate from southern regions and will tend to oviposit in fields with a heavy presence of broadleaf weeds such as chickweed and purple dead- nettle. After egg hatch, black cutworm larvae will feed on emerging corn. Smaller larvae will cause pinhole-like damage, while larger larvae can cut the plants at the base. If cut plants are observed, and corn is still below V6 stage, additional cutting may occur; for every fresh cut, an additional three to four plants could have damage. Only soil insecticides will control black cutworm as a preventative treatment―insecticidal seed treatments have little effect. However, it is difficult to predict the timing and location of when adults arrive and oviposit, which sometimes limits effectiveness of preventative treatments. Rescue treatments are very effective against cutworm, as is proper weed control that limits emergence of spring weeds. A few varieties with Bt will also provide control of black cutworm.
TRUE ARMYWORM Like black cutworm, armyworm adults fly from southern regions, but tend to lay eggs in grassy fields, such as wheat or rye cover crops. Once the wheat matures, or the cover crop is killed, larvae march like an army to feed on corn. The striped larvae tend to skeletonize corn, eating away the leaf edges but leaving the midrib. Soil insecticides and some seed treatments will control armyworm, as Bt varieties carrying the Vip3A genes (Viptera). Rescue treatments are effective, if 25 percent of the stand shows feeding injury.
SLUGS No-till fields covered in residue in combination with cool and wet springs favor slug conditions. Where slug populations are known to be a problem, use of row cleaners may reduce early stand losses. However, if slugs continue to be a problem on a yearly basis, implementation of minimum tillage will tend to reduce the overall problem. There are no thresholds for slug treatment, although the level of defoliation combined with corn growth stage will provide indications to treat. Slugs tend to feed only on leaves and not the growing point of corn, which makes damage less serious than it is on soybean. Slugs can be controlled by the use of slug bait; examples include metaldehyde and iron phosphate.
EUROPEAN CORN BORER Once the most common and important insect pest in Ohio, European corn borer numbers have declined significantly due to the use of Bt corn. There are two generations per year: the first generation attacks whorl stage corn, while the second generation at- tacks ears. If corn does not include above ground Bt traits, foliar applications may be needed if 75 percent of stand in V-stage corn shows evidence of damage. Controlling second generation is more difficult, and depends on the presence of eggs or early larvae. Late planted corn is at higher risk for second generation infestation.
WESTERN CORN ROOTWORM This beetle has replaced European corn borer as the most important insect pest of corn. The major damage is caused by the larvae, which emerge in the soil around mid-June and feed on developing corn roots. Significant lodging can occur if roots are not protected, decreasing yield. Rootworms can be controlled by Bt varieties, soil insecticides, and insecticidal seed treatments; however the latter do not perform well under high rootworm pressure. Furthermore, in the Western Corn Belt, rootworms are now resistant to a few Bt varieties, but this has not been found in Ohio. Continuous corn fields are at the highest risk for both high rootworm populations and Bt resistance, since adults tend to mate and lay eggs in corn fields. In recent history, a variant of rootworms would disperse into soybean fields and lay eggs, putting first year corn at risk; however the occurrence of this variant has not been seen in Ohio since 2009. Rootworm populations also tend to be higher in western Ohio. There are no rescue treatments available for larvae feeding, but digging and inspecting roots can provide some indication of risk the next year, if growing continuous corn. Continuous corn fields with a history of rootworm pressure may consider using Bt varieties, or a soil insecticide at planting. It is not recommended to use both, as this is not economical and places unnecessary se- lection pressure on rootworms. Rootworm adults may also feed on foliage or silks. In rare occasions, this can impact pollination and kernel set. Thresholds are set at five adult beetles per silk mass (25 total silk masses), and if silks are not brown and clipped to one-half inch length from the husk. Most clipping occurs at the edge of the field, so inspecting of entire field is necessary.
WESTERN BEAN CUTWORM First found in Ohio in 2006, this is a significant ear pest of corn (note: it does not feed on soybean). There is one generation per year, and adults emerge in mid to late June. Adult flight continues until the end of August, but peak flight is usually the third or fourth week of July. Females will lay eggs on the upper surface of the one to three top corn leaves. They prefer pre-tassel corn, so late-planted corn is always at a higher risk of infestation, especially if tasseling has not occurred by peak flight. After hatch, larvae will feed on pollen, but then enter the ear through silk or by chewing holes in the husk. Foliar applications are very effective against Western bean cutworm, and thresholds are set at 5 to 8 percent of 100 plants with egg masses or larvae. However, applications must be made before larvae enter the ear, so proper egg scouting is important. Bt varieties with Cry1F or Viptera (Vip3A) are labeled for control. However, Cry1F has been shown to not provide control in certain areas such that we do not recommend this gene for control. The highest distribution of Western bean cutworm is in the northern third of Ohio, specifically the northwest and northeast corners.
Disease Management
Major corn diseases in Ohio include leaf blights, stalk rots, ear rots and kernel rots (Table 4-18). Although some diseases can be controlled by a single practice, such as planting a resistant hybrid, most diseases require a com- bination of practices to ensure that economic damage is kept to a minimum. Once a disease has been identified, its management depends on understanding its cause(s), the factors that favor disease development, which plant parts are affected, as well as when and how the disease organisms are spread. The following is a summary of management practices to prevent yield losses in corn from diseases in Ohio.
1. Plant high-quality seed, treated with a fungicide seed treatment, in a well-prepared seedbed. Plant seed 1.5- to 2-inches deep at rates recommended by the seed company to ensure proper plant populations. When populations are excessively high, the stress caused by plant-to-plant competition may increase stalk rot and lodging.
2. Plant high-yielding hybrids with resistance to leaf blight, particularly northern corn leaf blight and gray leaf spot, ear rots, particularly Gibberella and Fusarium ear rots, and stalk rots. Several hybrids with high levels of resistance to these diseases are available. Review the level of resistance available in hybrids offered by your seed dealer before ordering seed for planting.
3. Balanced fertility is the key to vigorous, well-developed plants. High rates of nitrogen, especially when excessive in relation to potassium, favor the development of stalk rot and some leaf diseases. Use recommended levels of nitrogen, phosphorus, and potassium based on soil tests.
4. Crop rotation and destroying corn residues by tillage reduces the number of disease organisms surviving in the field. However, reduced tillage should be practiced to conserve energy and to protect soil from loss through erosion. When corn is planted after corn, especially under reduced tillage production, these disease management practices are lost. Other disease management practices, such as growing highly resistant hybrids and applying a fungicide, become essential to compensate for not having tillage and crop rotation as management options.
5. Improve soil drainage in poorly drained soils. This reduces water stress and reduces losses from seedling blights, root and stalk rots.
6. Control insects and weeds in and around fields. Insects such as rootworm, ear worm and stalk borer create wounds that serve as entry points for fungi causing stalk and ear rots. The corn flea beetle serves as the vector of Stewart’s leaf blight bacterium. Some weeds act as reservoirs for corn pathogens. In southern Ohio, eradicate Johnsongrass to eliminate the reservoir for corn viruses and their insect vectors.
7. Fungicide application may be justified in commercial corn production fields only if susceptible hybrids are grown and conditions are favorable for disease development. Popcorn and inbreds grown for seed production are generally more susceptible to leaf diseases than dent corn and should be scouted for leaf diseases regularly. Fungicides are most effective against foliar diseases when applied at tassel or silking (VT or R1); the most consistent yield responses are achieved when applications are made at VT/R1; and the highest yield responses are usually seen when fungicides are applied under disease-favorable conditions/situations. Early, pre-tassel applications or applications made in the absence of disease (when the hybrid is resistant or conditions are not favorable for disease development) often do not result in yield responses that are high enough to offset fungicide application cost.
8. Survey fields in the fall prior to harvest to determine the incidence of stalk rot. A rapid and easy technique to determine the incidence of stalk rot is the squeeze method. Grasp the base of the stalk above the brace roots and squeeze the stalk between the thumb and first two fingers. Stalks with significant rot will crush easily. Those fields with the greatest percentage of rotted stalks should be harvested first to avoid losses resulting from lodged corn.
9. Proper adjustment and operation of the combine or picker reduces harvesting losses in the field with stalk-rotted, lodged corn. Some equipment companies have attachments for the combine header to help pick up lodged corn.
10. Survey fields just prior to harvest for ear rots, particularly if in-season conditions were favorable for Gibberella ear rot development (cool, wet weather during the two to three weeks after R1). Harvest affected fields separately, at the correct moisture, and adjust the com- bine to minimize damage to the grain. Send samples for mycotoxin analysis, and dry and store grain at 13 to 14 percent moisture to minimize further mold growth and toxin contamination in storage.
11. For long-term storage, dry shelled corn to 13 to 14 percent. Ear corn to be cribbed should be dried to 20 percent moisture. Maintain cool and dry storage conditions to prevent storage molds from developing. For more information on recognizing and managing corn diseases, and on mycotoxins associated with moldy grain, visit ohioline.osu.edu/findafactsheet, where you can download disease fact sheets.