No. 10/May 30, 1997
Corn Nematodes Thriving
The cool and wet weather passing through Illinois is ideal for the development of corn nematode populations. These nematodes are most common in sandy soils, although they can also be a problem in some of the areas where the heavier prairie soils predominate. Irrigation also aids in population development and survival by maintaining a better soil environment (cooler and wetter).
Corn nematodes feed by inserting a hollow feeding tube (called a stylet) into the plant roots and using a muscular pumping bulb to withdraw plant cell contents. This feeding apparatus works like a hypodermic needle attached to a bulb. The common corn nematodes remain outside of the roots and can move from one feeding site to another. Once an area of roots is severely damaged, the corn nematodes simply migrate to another area and continue to feed. Soybean cyst nematodes, on the other hand, cannot move once a feeding relationship is established.
Three genera of nematodes are often associated with corn, depending on the soil type and location in the state. In sandy soils, the dagger nematode and the needle nematode are common. In some areas, even in heavier soils, the stunt nematode is found. All three may have been established in Illinois feeding on native grasses long before corn was introduced. They need moderately wet soils and cool temperatures to thrive and reproduce.
In a study conducted with Dave Feltes, IPM Educator for the Quad Cities area, soil samples collected in May of this year prior to planting corn were found to already have above-threshold numbers for the needle nematode. This nematode can cause severe damage at population levels as low as 10 per 100 cc's of soil (about 2/3 of a styrofoam coffee cup). We have detected 11 to 25 in some plots and expect that there are many more at this time. All of the nematodes seen were juveniles and not adults, meaning that they hatched from the eggs very early in the season and should begin laying eggs very soon.
The data from these plots have also shown that corn nematodes tend to have a population peak within 6 to 8 weeks after corn planting. Unfortunately, this peak may be before the stunting and poor growth symptoms begin to show and growers recognize a problem. The last two years, populations of dagger nematode were greatest in June or early July, depending on weather. They fell rapidly after this time and were at low levels during the rest of the season. So, if you suspect corn nematode problems or want to see if they are present, sample early in the season. Both dagger and needle nematode populations seem to produce one or, at most, two generations per year. Some juveniles of the needle and both juveniles and adults of dagger nematode were detected in August and September of 1996, but at very low populations. Very few were found this late in 1995. These late populations seem to depend on the weather and lateness of the crop.
How do you manage corn nematodes? Unfortunately, there are few options compared to soybean cyst nematode (SCN) management. Crop rotation is important because corn nematodes are grass feeders. Needle nematode populations will drop rapidly with a single year of soybeans, but dagger populations may remain somewhat higher. However, two years of corn generally result in very high populations that cause extensive damage. Corn nematodes are most damaging in second- or third-year corn, not in rotations.
There are two nematicides currently labeled: Mocap and Counter. Neither has the needle nematode listed on the label. I believe that this absence is due in part to the extremely low threshold (5 nematodes per 100 cc's of soil) for the needle nematode. No company will guarantee to control pest populations down to fewer than five individuals per sampling unit. If a nematicide is used, it can still provide protection against other nematodes for which it is labeled and help reduce stress from these pests. Because these compounds are highly hazardous, read and follow all label and handling directions.
Sample soil now in fields where you suspect corn nematode problems. How do you tell nematode damage in the field? That's also difficult to do because nematode damage can resemble many other problems. Typically, plants are stunted, may be yellowish, and can wilt more rapidly than healthy corn if dry conditions occur. Nematode infestations tend to be circular to oblong areas in fields. Roots may have a "bottlebrush" appearance (similar to DNA herbicide carryover) or small necrotic (dead) areas along the roots, or the root volume may simply be smaller than expected.
Dig the plants, wash the roots, and examine them carefully for these symptoms. Collect soil samples around the root zone to a depth of about eight inches. Place soils in a plastic bag, seal the bag, and keep it in a cool place until mailing to a lab. Because corn nematodes are not in a resistant stage like SCN cysts, they will die very quickly if exposed to heat. If possible, mail on a Monday or Tuesday to a lab to ensure that the sample won't remain in a post office over the weekend.
Because population thresholds vary for corn nematodes, it is important that they be correctly identified. Typically, at least three to five different genera are found in soils. These genera must be correctly identified in order to determine the most appropriate and environmentally sound management approach. If the corn nematodes are not correctly identified, unnecessary pesticide applications or rotation systems may be suggested. Consult with local agricultural dealers or Extension workers to find labs with people who can identify corn nematodes.
H. Walker Kirby, Department of Crop Sciences, (217)333-8414