Detective Work Helpful In Finding Cause Of Missing Corn

May 21, 2003   10(10):78-79

Joe Lauer, Corn Agronomist

Scout corn early to evaluate stand and check for potential problems. Take some time to walk each field individually looking for soil compaction, crusting, diseases, insects, weeds and stand establishment problems. Windshield observations do not qualify as field scouting. A simple drive-by will not be sufficient, since only large-scale disasters show up at 55 miles per hour.

When the corn population is economically unacceptable, the earlier a replant decision can be made, the better. Check corn populations through out the field. Problems always seem to develop farthest away from the road. Before entering the field, look over the entire field for areas that appear different. Check those particularly well when scouting.

Scout fields in a W pattern, checking plant population in several parts of the field. Look for problem patterns. Is the problem throughout the field? Is there a pattern to missing corn? Are individual rows missing or emerging erratically? In areas without corn, determine whether or not seed was planted. Check planting depth. Counting rows can show which planter row was malfunctioning.

Field areas with pattern problems

Poor stands or seedling vigor are sometimes due to compaction and other field preparation problems. Patterns relating to wheels or tillage equipment will be visible. Misapplication of herbicide, herbicide carryover, and excessive nitrogen or potassium fertilizer in contact with the seed are identifiable problems. Fertilizer salt effects or fertilizer burn often cause seeds to turn brown or black and very firm. Roots will be nubbed off with black or brown tips. Ammonia toxicity field symptoms are long or short strips of damaged plants in rows scattered throughout the field.

Be on the lookout for soil crusting. Soils that work up very fine have greater crusting potential. If crusting is suspected, and the crop has not yet emerged, use a rotary hoe, but be careful the operating depth of the spoons does not dig out sprouted and emerged plants.

Field areas with random problems

Stand losses from insects and plant disease do not follow straight lines, and damage usually doesn't occur throughout the entire field. Localized or spotty areas with a poor stand provide a clue that the stand problem is disease- or insect-related. Examine entire plants as well as the appearance and location of problem areas in the field.

Several fungi, including Pythium and Fusarium, can cause pre-emergence and post-emergence "damping off" of corn. A rotted, discolored or water-soaked appearance of the seed or portions of the seedling indicates the presence of a pathogen. Seeds and seedlings are brown, soft and mushy and may be difficult to find because of rapid decomposition. Corn seedling diseases are often associated with cold, wet soils such as are often found in low areas. Diseases can be secondary to damage caused by other factors.

Damage form several insects tend to be worse when cool, wet soil conditions slow corn germination and emergence. Look for damage of embryonic tissue on the kernel tip, or for holes in the kernel. Damaged tissue will quickly turn brown and rot. Field history can provide clues to the species causing stand reduction. For example, if a poor stand is associated with heavy manure application or other high organic matter conditions, then suspect seed corn maggot. A recent history of sod implicates wireworms or white grubs. These problems cannot be corrected in-season.

However, several species of cutworms can be controlled effectively with insecticide rescue treatments. Early detection is important, as most species will feed on leaves before they are large enough to cut plants. Knowing the species, the stage of cutworm development, and the amount of damage will help determine the appropriate insecticide treatment. Later planted fields with lots of crop residue and weed growth have the greatest potential for cutworm damage.

Modern seed corn has better early season vigor than previous hybrids. Once in a while, seed lots have problems with poor germination and emergence. Suspect this problem only if planting, insect and disease problems have been eliminated.

Each field has its own set of circumstances, but it usually comes down to two situations when making a replant decision: the remaining stand and the likely replant date. Ultimately this decision hinges on which situation will maximize net income. Don't replant a cornfield without first considering the calendar date and the plant distribution throughout the field. Consider the nature of the problem if the remaining stand dictates replanting. For example, a seed treatment may be appropriate when planting back into an insect problem. Keep records of where stand loss occurs, as problems tend to repeat themselves in future years.

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