Charles Overstreet, Xavier, Deborah
Corn is subject to attack by a number of plant-parasitic nematodes in Louisiana. These pests attack the root system of corn plants and decrease the efficiency of roots in taking up water and nutrients. The damage can be apparent to roots and above-ground portions of plants ultimately reducing crop yield. The degree of damage is related to the types and population levels of nematodes present as well as the environmental conditions present in a field. Soils that are very sandy are most likely to show the greatest amount of damage from nematodes. Additionally, soils that have other issues such as hardpans, nutrient deficiencies, or drought stress can increase the level of damage by nematodes. Young corn is usually more susceptible to damage from nematodes than older corn simply because of a much smaller root system and older corn appears to be more tolerant.
The common symptoms of severe nematode injury include wilting, yellowing, and stunting of above-ground corn plants. Nematodes can cause damage and loss of yield without noticeable symptoms making it difficult to recognize. Symptoms that can be observed to the roots would include necrotic lesions, swollen areas or galls, lack of fine roots, reduced root branching, and a stubby appearance. The stubby appearance of the roots results from attack to the growing points by the nematode.
Figure 1. Small galls that are visible on young corn roots.
Figure 2. Single egg masses of southern root-knot nematode visible on corn roots. Notice the absence of any swelling of the root system associated with the nematode.
Nematodes are never uniformly distributed in a production field and are often associated with “hot spots” or areas of noticeable damage. Many of the production fields have variability in soil texture and can range from a sandy loam to heavy clay in the same field. Some nematodes such as southern root-knot, stubby root or lance seem to prefer the sandier areas of field. Other nematodes may be found throughout most of the field with only a few detected in the heaviest soils.
The most common nematodes found in corn production in Louisiana include the southern root-knot, lance, lesion, stubby-root, spiral, ring and stunt. The southern root-knot nematode produces small galls on the roots and can be detected on the roots after about four to six weeks. This is the same nematode that is such a problem on many other crops including cotton, soybean, sweetpotato, and many vegetables and ornamentals. This same nematode produces much larger galls on other crops when compared to corn or any other grass crop such as sorghum or wheat.
Most of the nematodes that are present in corn are very small and cannot readily be identified in the field. Southern root-knot nematode can sometimes be identified by the presence of the small galls on roots. Soil samples are probably the most reliable method of identification of nematodes. Problem areas can be sampled during the growing season. Routine identification should be collected in late summer or early fall. Soil cores should be collected to a depth of six to eight inches from 15-20 sites in the area that you are collecting from. The soil should not be either excessively wet or dry at the time collected. Once collected, keep the samples cool (room temperature) until they can be sent to the Nematode Advisory Service. Follow the instructions for shipping and correct paperwork with each sample.
Once the problem has been identified as being caused by one or more of the plant-parasitic nematodes, some type of management should be taken. Rotation is one of the best methods for managing some types of nematodes. Unfortunately, our most problematic nematode, the southern root-knot, has a wide host range and attacks just about every crop planted in Louisiana with the single exception of peanut which is not a host to this nematode species. All of the corn varieties that were evaluated several years ago were found to be susceptible to the southern root-knot nematode. Soybean and cotton has some resistant varieties against this nematode and could be successfully rotated with corn to reduce populations. There are a few cultural practices that will help reduce damage. Make sure that corn is properly fertilized to eliminate any stress caused by a lack of one or more nutrients and that the soil pH is in the correct range. Water is particularly important and irrigation can reduce stress to plants and lessen the impact of nematodes. There are some nematicides available for corn including granule such as Mocap® 15% or Counter® 15G or 20G, seed treatments including Avicta® Complete Corn 250, Avicta® Duo Corn, and Poncho® VOTIVO®, and a fumigant Telone® II. The fumigant would be the best option for very high levels of the nematode prior to planting corn.