2 Troubleshooting Corn

Corn is highly responsive to chemicals, fertilizers, and environmental conditions. It grows rapidly, and the duration of reproductive growth is brief. It follows that problems limiting the productivity of cornfields often arise suddenly and must be corrected quickly in time to preserve full yield potential. It may not be possible to correct some problems during the crop year in which they are discovered. It is imperative that such problems be accurately diagnosed in order to maintain the profitability of fields in subsequent years.
    Poor corn growth is usually caused by environmental conditions or inadequate fertility programs. Most situations must be remedied before the crop is 18 inches high. Anything that appears abnormal in a field should be quickly investigated and systematically diagnosed. Diagnosing problems in cornfields is frequently a difficult task requiring objective thinking, considerable knowledge, and some experience. However, a careful, logical approach to each situation coupled with soil, tissue, and nematode analyses will usually yield the reason for poor corn performance.
    Modern Corn Production¹ is the most comprehensive reference on corn troubleshooting. Although this book is directed primarily at a midwestern audience, it contains a comprehensive listing of problems encountered in cornfields that serves to remind the corn scout of the diversity of difficult situations that may be found in cornfields.
    The most common and difficult problems to diagnose are those that develop early in the growing season. Quick diagnosis of these problems and corrective action will, in many cases, permit desirable yields to be obtained. The problems most commonly encountered early in the growing season are described in the following paragraphs.

¹Aldrich, S. R., W. 0. Scott, and R. G. Hoeft. Modern Corn Production, Third Edition. Champaign, Illinois: A & L Publications Inc., 1986.

Erratic Crop Stand

Erratic corn stands may be caused by a number of factors. An obvious pattern of damage or abnormal growth confined to a row suggests difficulties with planting equipment. Such an effect may also be caused by improper application of materials such as starter fertilizers, and soil insecticides or improper planting depth. Damage or abnormal growth patterns that run across the row suggest that soil type, weather conditions, or pests are at the root of the problem.
    Particular attention should be given to matching of seed size to planter plates and planting speed. Poor planter plate selection and excessive planting speed account for a large percentage of erratic stand complaints. Improper planting depth and highly variable soils in a single field also frequently combine with dry weather to produce erratic stands.
    Selection of varieties that flex ear size in response to low populations or erratic stands will assist growers in dealing with erratic stands. Cultivation(s) will probably be required to offset increased weed pressures arising from erratic stands.
 

Low pH

Low pH is the most frequently encountered problem in North Carolina cornfields (Figure 2). Symptoms of low pH are purpling of the leaves, yellow leaf edges, and, in severe cases, interveinal striping of leaves. Plants will always be uneven in height in a low pH field.
    The symptoms of low pH corn should not be confused with those caused by a phosphorus deficiency. Phosphorus deficiencies are seen in the spring and caused by cold, wet soil. They are usually temporary and frequently variety specific. Phosphorus-deficient corn will grow at a more uniform height than low-pH corn and will not have yellow leaf edges.
    Low pH problems are best corrected with limestone applications prior to planting. The effects of low pH discovered after planting cannot be completely offset, although a broadcast application of dolomitic limestone over the crop followed by cultivation may provide a marginal solution to the problem. Cultivation is also an effective tool for speeding the recovery of corn from temporary phosphorous deficiencies in cold, wet springs, particularly after packing rains.
 

Twisted Plants

A common symptom observed in cornfields soon after the crop emerges is the appearance of twisted plants, resulting when leaves fail to unroll properly. This symptom is characteristic of several problems:

1. Wireworm infestation.
2. Billbug injury.
3. Poor emergence due to soil crusting.
4. Alachlor (Lasso) or metolachlor (Dual) injury.
5. Phorate (Thimet) or fonofos (Dyfonate) injury.

The source of the problem is easily determined if a systematic approach is employed.
    If wireworms are causing plants to be twisted, the insects are usually found in the soil. Their characteristic entry hole will always be found in the plant just below the soil surface.
    Billbug activity may generally be detected by three parallel, elliptical holes in expanded leaves. Moreover, the field history will usually reveal poor crop rotation or possibly prior billbug problems in adjacent fields. Either type of insect injury may be accompanied by plant desiccation.
    Crusting injury is distinguished from billbug and wireworm infestations by the absence of plant tissue damage. The soil will be crusted and plants unnaturally thick just below the soil surface.
    The symptoms of alachlor or metolachlor herbicide injury are similar to those for crusting injury. Plants will be free of insect-damaged tissue and will exhibit an enlarged area just below the soil surface. Generally, fields in which alachlor or metolachlor injury occurs will have cold, sandy soils and a history of receiving high herbicide rates and excessive rainfall. Severe twisting in larger plants (8 to 15 inches in height) is usually indicative of injury from 2,4-D application or possibly carryover from other herbicides.
    Thimet or Dyfonate injury can be similar to the chemical injury mentioned above, except that one of these insecticides was probably used as an in-furrow treatment.
 

Nutrition

Nutritional problems are frequently observed in corn and are easily diagnosed. Where possible, soil and tissue tests should always be employed to confirm suspected nutritional deficiencies. Too often, problems observed in corn are attributed to nutritional deficiencies when, in fact, the symptoms are a reflection of other problems. Nematodes, for example, may cause nitrogen and sulfur deficiency symptoms. Only an examination of plant roots and confirmation by nematode analysis will adequately diagnose a nematode problem. Dinitroaniline herbicide carryover or misapplication, which can prune corn roots, may also cause symptoms of a nutritional problem.
    Nitrogen (Figure 3) is a mobile nutrient; deficiencies are most often observed on sandy soils following long periods of heavy rainfall or when all the nitrogen provided to the crop is applied before planting. Deficiency symptoms include thin, spindly stalks and yellow-orange V shapes on leaves. The color becomes more intense as the problem develops and leaves eventually begin to die at the tips. Symptoms are most obvious on lower leaves; the plants "fire up," with deficiency symptoms rapidly moving from lower to upper leaves with time.
    A phosphorus (Figure 4) deficiency is characterized by narrow, bluish green leaves. The leaves eventually turn reddish purple, the color appearing first at the tips of upper leaves. Symptoms are most obvious early in the crop season and are often temporary due to cold, wet soil conditions.
    If the edges of lower leaves, starting at the tips, turn yellow and die, the plant is likely suffering from a potassium deficiency. Plants are generally short and stalks weak. Kernels will be thin, chaffy, and dull in appearance with deeper-than- normal dents. Excessive lodging is often an expression of a potassium shortage. High nitrogen-to-potassium ratios en- courage stalk rot diseases that weaken stalks, causing them to fall and thus increase harvest losses.
    Sulfur deficiency (Figure 5) is becoming more frequent as a result of the removal of incidental sulfur from the fertilizer manufacturing process. It is observed most often on deep, sandy soils, especially after wet winters. A sulfur shortage is characterized by yellow, stunted plants, often with interveinal chlorosis. Symptoms will be less apparent where the underlying soil is clayey because sulfur has accumulated in the subsoil. Sulfur problems may be quickly corrected with the application of a sulfate sulfur source.
    Magnesium problems (Figure 6) are most often related to low pH (that is, a lack of dolomitic limestone). Accordingly, the preplant application of dolomitic limestone is the best corrective measure. Symptoms of magnesium deficiency include yellow streaking between veins of lower leaves. The leaves eventually become reddish purple; tips and edges may die in severe cases. The corn will usually be very uneven in height. Magnesium deficiencies may be addressed after emergence in three ways:

1. Dolomitic limestone may be broadcast over the top of small corn and cultivated to achieve a marginal solution.

2. Magnesium in soluble form may be applied in the row at a rate of 25 pounds per acre when soil pH is adequate.

3. A foliar spray of 20 pounds of magnesium sulfate (Epsom salts) in 100 gallons of water per acre will supply magnesium to the crop without burning leaves.

    Damage caused by manganese deficiency (Figure 7) is rare in corn but does occur in North Carolina on soils extremely low in manganese. Damage is most often found where soil pH is excessively high as a result of improper liming. Symptoms appear first on the youngest leaves. The leaves become pale with interveinal chlorosis. Stalks appear stunted as a result of shortened internodes. Preventive treatment should include placing 3 to 5 pounds of actual manganese per acre with an acid-forming fertilizer in a band near the corn row. Preplant broadcast applications should range from 10 pounds of actual manganese per acre on mineral soils to 20 pounds per acre on organic soils. Corrective action after emergence-generally the application of I to 2 pounds per acre of manganese sulfate-may also be necessary.
    Copper problems (Figure 8) are encountered most frequently on acid organic soils and on highly leached sandy soils. Emerging leaves may yellow. Leaf distortion and plant stunting follow. The confirming symptom is the cracking of leaf edges in tandem with the tight rolling of terminal leaves. Plants will die soon after emergence if the deficiency is severe. Copper deficiencies are easily prevented by preplant soil applications ranging from 2 pounds of actual copper per acre on mineral soils to 6 pounds per acre actual copper on organic soils. A single application is normally effective for many years. Band and foliar copper applications should be discouraged because of the potential for copper toxicity. Moreover, foliar applications are not always effective after visual symptoms of copper deficiency have been observed.
    A zinc deficiency (Figure 9) is usually characterized by broad, white bands on older leaves, a symptom that is also evidence of billbug injury. A zinc problem, however, is easily distinguished from billbug damage. A mild zinc deficiency may cause interveinal chlorosis and shortened internodes. Plants will be stunted. For a clear symptom of zinc deficiency, split and examine the plants. There will be dark bands at the internodes.