Corn: Other Crop Problems

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| Insects and Pests of Field Crops | Diseases of Field Crops | Appendices |

Pub 811: Agronomy Guide > Corn > Other Crop Problems

Excerpt from Agronomy Guide for Field Crops
Order OMAFRA Publication 811: Agronomy Guide for Field Crops

Table of Contents

• Insects and Diseases
  • Figure 1-2, Corn Scouting Calendar
• Cold
  • Table 1-29. Estimated Risks to Grain Corn Yield and Quality From Late-Season Frost Damage
  • Table 1-30. Estimated Percentage Corn Grain Yield Loss Due to Defoliation at Various Growth Stages
• Heat
• Hail
• Flooding
• Drought
• Bird Damage

Insects and Diseases

Figure 1-2, Corn Scouting Calendar shows insects and diseases that could be causing the symptoms in the field.

Individual descriptions of insects, pests and diseases , scouting and management strategies can be found in Chapter 13, Insects and Pests of Field Crops, or Chapter 14, Diseases of Field Crops.

Cold

Early-Season Cold

Frost damage in May or June will generally have little impact on the crop, providing the growing point of the corn plant is still below the soil surface. This is the case until the young plant reaches roughly the sixth-leaf stage. On more advanced plants and/or where damage is more severe, split stalks to see if the growing point has been damaged. This procedure will require some time to make the correct recommendation. It probably takes 3-5 days following a frost to accurately determine the degree of damage, to verify the presence of healthy growing points (yellowish-white and firm) or to see new leaf growth.

Frozen leaf tissue bleaches to a straw colour several days after freezing. In some cases, it also develops a "knot," which may restrict expansion of the undamaged tissue lower in the whorl Plate 9. It often appears that clipping these knots by mowing the field aids in the plant's recovery, but this is mostly cosmetic. Tests conducted on frosted corn fields concluded that clipping appeared to help the fields "green up" but that unclipped sections of the same fields often recovered as quickly and yielded as much or more than the clipped sections.

Plate 9. Frost injury on corn in mid-June. Smaller plants can recover, but growth in larger plants may be restricted by frost-injured dead tissue.

There is very little that can be done to minimize the potential problem. Some management factors that increase the risk of frost damage to corn should temperatures fall are:

• inter-row cultivation
• side-dressing nitrogen (where soil is disturbed)
• herbicide applications
• presence of weeds
• high levels of previous crop residue

If the forecast calls for a risk of frost, consider delaying inter-row cultivation, nitrogen side-dressing or herbicide applications until warmer temperatures return. Soil disturbance at the surface introduces more air into the soil and insulates the corn plants from the heat of the soil mass, thus increasing the risk of frost damage. Similarly, crop residues and weeds act as a barrier for heat transfer from the soil to the corn plant. Dry soils are more prone to frost damage because they have a lower capacity to store heat during the day and thus less heat to transfer and protect the corn plant overnight.

Late-Season Cold

Cold temperatures during the grain-filling period in August and September may cause yield and quality losses. The extent of these losses depends on the developmental stage of the corn and the temperatures recorded.

As temperatures drop to 0°C, frost damage first occurs to the leaves of the corn plants. This damage will eliminate any further photosynthesis, reduce grain filling and often have a negative effect on stalk strength. However, as long as air temperatures do not fall below -2°C, stalk tissues will remain viable and stalk constituents will be mobilized to fill the ear as much as possible. On the other hand, if temperatures fall below -2°C, both leaves and stalks may be damaged and no further photosynthesis or remobilization can occur. This will terminate grain filling, and kernel black layer will develop. Table 1-29, Estimated Risks to Grain Corn Yield and Quality From Late-Season Frost Damage, outlines the potential risks to yield and quality for grain corn experiencing different levels of frost damage.

Generally the early dent stage is the cut-off point where corn can withstand frost damage to the leaves and still produce a reasonable grain yield. This stage is characterized by having kernels showing small indentations in the crown of the kernel, at least in the lower half of the cob.

The other question regarding cold nights revolves around the corn crop's ability to continue grain filling after experiencing several cold nights without frost damage. Dr. Thys Tollenaar of the University of Guelph has conducted research that measured 50% reductions in photosynthesis and rate of grain filling due to cold nights of 2°C. However, when these plants were restored to higher temperature conditions, they resumed plant activities at rates similar to those plants that had never experienced the low temperatures. If cornfields can escape any serious frost damage during cold nights, grain filling should carry on once normal temperatures return.

In some situations, frost damage will preclude harvesting the crop as grain and will force the grower to consider harvesting it as silage. There are important issues surrounding the management of the silage crop as well. Following a frost, silage corn frozen before reaching the half milk line on the kernel may be too high in moisture to properly ensile. Ideally, in cases of frost, delay corn harvest until the entire plant reaches the desired moisture content for ensiling.

Heat

Heat stress is different from drought stress. Corn can usually tolerate temperatures as high as 38°C before injury occurs, as long as drought conditions are not present as well. Temperature and drought sensitivity varies by hybrid. Drought-tolerant hybrids may result in yield drag. They are not likely good choices for a normal growing season.

Hail

Cornfields damaged by hail may experience a reduction in leaf surface area, bruising of the stalk and ear and, in serious incidences, stalk breakage.

Plate 10. Hail damage is most harmful if defoliation occurs during tasselling.

Yield loss due to hail is dependent on the stage of the crop at the time of the hail event and the level of defoliation. Yield loss is greatest when the corn is defoliated during tasselling. Younger plants may experience a delay in growth and development due to hail, but yield loss is usually minimal. Defoliation of plants near maturity tends to cause little yield loss. Table 1-30, Estimated Percentage Corn Grain Yield Loss Due to Defoliation at Various Growth Stages, when making yield loss estimates due to hail damage. Hail damage may also provide an entry point for diseases such as smut.

Flooding

Flooding stresses the plant by cutting off the supply of oxygen to the root system. Younger corn plants die if submerged in water for more than 5 days, especially in warmer weather conditions. If air temperatures are high, death may occur in only a few days, as plant processes are sped up and the need for a supply of oxygen to the roots is high. In cooler weather, submerged plants may live for up to a week. After the 8-leaf stage of corn, plants can tolerate being submerged in water for more than 8 days but may be more susceptible to disease and may experience limited root development while under water. Yield loss due to flooding is most substantial for plants submerged immediately before and during tasselling and silking. Plants in the later vegetative growth stages (10-16 leaves) and/or during the grain filling period, suffer little yield loss to flooding.

Drought

The corn crop requires approximately 50 cm of water to produce high yields. This can be supplied over the growing season from a combination of stored water in the soil, rainfall or irrigation.

Lack of water causes the leaves to wilt and turn a greyish colour Plate 6. Corn is most susceptible to dry conditions during the tasselling-to-silking stage and may experience yield loss if under stress at this time. During the later vegetative stages of growth (V8-V14), the plant may benefit from dry conditions, as it forces the more rapid downward growth of the roots. Drought conditions during silking can reduce pollination and a lack of silk emergence, while drought after silking may cause a reduction in grain fill.

Plate 6. Moisture deficiency, or drought stress, is most critical to yield during tasselling-to-silking stages.

Bird Damage

Birds can damage emerging seedlings. However, the more serious bird damage occurs to grain in August and September Plate 11. Birds eat the kernels off the cob, causing direct yield loss, and kernel damage may result in mould growth. Bird damage can be easily confused with seedling damage caused by black cutworms or ear damage caused by grasshoppers. Noisemakers, such as Av-alarms, propane cannons, exploding shotgun shells, the Phoenix Wailer and recordings of bird distress calls may be successful deterrents if more than one technique is used and their pattern is changed frequently. If crop damage due to birds or wildlife is substantial, contact your local Ministry of Natural Resources office for control options.

Plate 11. Bird damage on corn ears.

Recommended treatments to control insects, pests and diseases can be found in OMAFRA Publication 812, Field Crop Protection Guide.