Damage caused by the needle nematode can become evident as early as 2 weeks after seedling emergence. Aboveground symptoms consist of scattered patches of stunted and nutrient-deficient (nitrogen, phosphorus, and potassium) plants. These patches are usually oval to oblong in shape with the most severely damaged plants in the centers and diminishing toward the edges to normal-appearing plants. These symptoms are most pronounced in late May and throughout the month of June.|
Belowground symptoms are typically most obvious on the seminal and first crown roots of seedlings. When plants are dug, the roots may appear discolored; lateral roots may be short, stubby, swollen at the tips, and lacking fine feeder roots, giving the overall root system a coarse appearance. Root symptoms often resemble DNA-herbicide carryover.
The needle nematode has been found only in soils containing 50 percent sand or more, but damage is greatest in soils containing 90 percent sand. When soil moisture is high, the nematode often occurs in abundance in the upper 4 inches of soil, causing the early damage to the root system. Populations are concentrated at lower depths as the season progresses, and the nematode may not be detected in the top 6 inches of soil by the end of the growing season; consequently, the sampling depth should be increased to 9 to 10 inches from the middle to the late part of the growing season.
Several other problems, including insect feeding, compaction, herbicide damage, and nutrient deficiency, can be confused with nematode damage. Therefore, to diagnose needle nematode problems, it is important that you obtain a good soil sample. Samples can be collected from an entire field or from patches of obvious damage. If samples are collected from problem patches, use a core sampler and take samples at the margin of the damaged area to the appropriate depths mentioned above. Do not collect the soil probes from the center of the damaged areas; the reduced root mass caused by nematode feeding can result in fewer feeding sites there and thus fewer nematodes being present. Sampling an entire field is beneficial because it provides a more realistic picture of field-wide nematode distribution than sampling only pockets or patches of damage. In this case, each sample should represent about 10 acres and consist of 12 to 24 probes or subsamples. Sampling should follow a zigzag pattern. After the subsamples are gathered and mixed, a sample of 1 quart and a handful of roots should be sealed in a sturdy plastic bag. The sample should be kept cool (less than 80°F) and submitted to the University of Illinois Plant Clinic or other appropriate diagnostic laboratory as soon as possible. Essential background information on the problem should be included on a nematode sample form. (For more information on collecting soil samples, refer to Report on Plant Disease, No. 1100, "Collecting and Shipping Soil Sample for Nematode Analysis," available from the Department of Crop Sciences, N-533 Turner Hall, 1102 S. Goodwin Ave., Urbana, IL 61801. The cost is $1.00 per copy.)
Infestations of the needle nematode along the Illinois River have been found on first-year corn that had followed a grass cover. In other areas, the nematode was recovered in substantial numbers from fields of continuous corn. Although the nematode has been found in a few fields of soybean that were rotated with corn, there is no evidence that soybean is a host. The nematode was rare or absent in soil samples from soybean and dicotyledonous vegetable crops but present in low numbers on corn and other graminaceous crops that had been rotated with those crops. The host range appears to be restricted to the grass family. Consequently, the nematode can be controlled by rotating out of grass crops to soybeans or dicotyledonous vegetable crops often grown on irrigated sands. Because most grassy weeds are hosts of the needle nematode, it is necessary to control them when the nonhost crop is used in the rotation.
Nematicides registered on corn have not been effective against the needle nematode. Therefore, it appears that the use of crop rotations may be the most logical approach. For more information on crop rotations, please contact Dale I. Edwards at (217)244-2011.--Dale I. Edwards