Corn nematode damage may become evident 2 to 4 weeks after seedling emergence but is most pronounced in late May and June. Above-ground symptoms usually include plant patches that are yellowed, stunted, or both. Symptoms of drought or nutrient deficiency may occur first in nematode-damaged areas during periods of water stress, high temperatures, or both. Because nematodes can damage corn without showing above-ground symptoms, they may be overlooked. A gradual decline in corn yield over a period of years may indicate a nematode problem. When plants are dug up, the roots may appear discolored; lateral roots may be short, stubby, and lacking fine feeder roots; and overall root systems may be reduced. Root symptoms caused by needle and dagger nematodes often resemble dinitroaniline herbicide injury. Table 6 lists nematodes that may be associated with corn, the types of damage they cause, and other host plants.
In Illinois, nematode damage to corn has been more severe where corn has been cropped continuously for three or more years on sandy soils. Needle, dagger, and lance nematodes are favored by these conditions. When nematodes are suspected, soil and root samples must be analyzed to confirm the problem. Report on Plant Disease No. 1100 gives detailed information on how to collect and ship soil samples for nematode analysis.
The use of crop rotations or nematicides will be of little value for corn already planted. However, an awareness of corn nematode problems is essential for planning control strategies for the next growing season. For planted corn, try, if possible, to maintain optimum growing conditions; nematode damage is greater on plants stressed by other factors. Damage can be reduced greatly by providing plants with adequate moisture (especially on sandy, irrigated soils), nutrients, and soil aeration at all times. Controlling other diseases and insects also reduces plant stress. For more information on corn nematodes, see Report on Plant Disease Nos. 1103 and 1106, available for $1.00 each from the Department of Crop Sciences, University of Illinois, N-533 Turner Hall, Urbana, IL 61801, (217)333-8375.--Dale I. Edwards