• Several species of nematodes are known to be parasitic on corn roots.
• Two species, needle and sting nematode, can cause significant damage to corn growing in sandy soils.
• Aboveground symptoms may include plant stunting and yellowing while below-ground, roots will be pruned, discolored and may have a bottle-brush appearance.
• No in-season rescue treatments are available but sampling can help us manage nematodes in future crops.

Several different species of plant parasitic nematodes are known to feed on corn roots. Nematode species such as Lesion (Pratylenchus spp), Dagger (Xiphinema), Lance (Hoplolaimus), Spiral (Helicotylenchus) and others can be present in corn fields and damage corn. However, Needle (Longidorus) and Sting nematodes (Belonolaimus) are two species that can cause significant damage even when present in low numbers. A 2018 survey of several counties in Illinois found that the spiral and lesion nematode were the most common nematodes found in the fields surveyed. In Iowa, fields sampled from 2000-2010 revealed that the spiral and the root-lesion nematode were the most frequently found species but not necessarily at damaging levels. Needle nematodes were the species most commonly found at damaging levels and most of these samples were from Muscatine county. The potential for damage and yield reductions will depend on the nematode species present and their population densities as well as other stressors the corn may be going through. A guideline on what may constitute damaging densities of these nematode species (thresholds) can be found in Table 1.

Table 1 (below )Suggested thresholds for various species of corn nematodes (Source: Iowa State University)

Based on their feeding habits, nematodes can be classified as ectoparasites and endoparasites. Ectoparasites live mainly in the soil but penetrate plant roots a short distance to feed. Endoparasites penetrate the roots entirely and spend most of their life cycle inside them. In general, the distribution of corn nematodes in a field is patchy and so are their symptoms. Above-ground plants may look stunted and yellow and below-ground roots may be pruned, discolored, look thick and stubby or have a bottle-brush appearance (Images 1 and 2).

Image 1 (below)  Above-ground symptoms of corn nematode damage.

Image 2  (below) Symptoms of corn nematode damage to roots.

Lesion nematode: This species is endoparasitic on corn roots and is present in many soil types although sandy soils appear to be more favorable. Symptoms, when present, include lesions on roots, root pruning, above-ground plant stunting and yellowing. These symptoms can be confused with seedling blights and root rots, nutrient deficiencies, insect feeding, drought etc., so laboratory analysis of roots and soil will be necessary for an accurate diagnosis.

Dagger nematodes: This is an ectoparasitic nematode that is fairly large in size (up to 2mm) and can be found in different soils types. These nematodes will migrate deeper in the soil profile when topsoil dries out and appear to be sensitive to soil disturbances such as tillage.

Lance nematode: This is an endoparasitic nematode that occurs in different soil types (silt loams and sandy soils) and it’s fairly large with about 1mm in size.They have a very wide host range and populations of 300-400 per 100 cc of soil have the potential to damage young plants. Lance nematodes injure the growing points of roots causing above-ground stunting of plants.

Sting nematode: This is an ectoparasite species generally found in sandy soils (70% or more sand). Sting nematodes are large (1-3 mm) and can cause severe damage in numbers as low as 1-10 per 100 cc of soil. They become active as soils warm up and move deeper in the soil profile as the crop ages. They feed from the outside of the root near the tip causing stunting, swelling, root discoloration and trimming of the lateral roots. Sting nematodes have a wide host range including soybeans and wheat so rotation to these crops will not aid in management but rotation to alfalfa can help reduce densities.

Needle nematode: This is an ectoparasitic nematode capable of reducing corn yields by 50 bu/A or more. It is a very long nematode (1/4” long) with very long stylets and typically found in sandy soils (at least 70% sand). Above-ground symptoms of needle nematode damage include patches of yellow or stunted corn seedlings early in the season. The roots of affected plants will be thick and stubby, with a bottle-brush appearance and missing or stiff hairs. These below-ground symptoms can resemble herbicide injury. Corn is the preferred host but they can also feed on other grasses. Hot and dry conditions in the summer will cause needle nematodes to move deeper into the soil profile. Levels as low as 1 nematode/ 100 cc of soil would be considered a damage threshold.

Spiral nematode: This an ectoparasite species more commonly found in clay or loamy soils. Their body curves into a spiral when at rest, which gives this species its name. Spiral nematodes feed on a wide range of crops including fruit crops, vegetables, ornamentals, forages, turfgrasses etc. Field crops such as soybeans, cotton and corn are a host for spiral nematodes. Symptoms of spiral nematodes are more subtle than those of other nematodes and it may include small root systems, root decay and stunted plants. High numbers of spiral nematodes (500-1000 cc soil) are necessary to raise threshold concerns.

Stunt nematode: It is an ectoparasite species that tends to be favored by heavier soils. However yield losses caused by this nematode species are rare. Symptoms of damage include smaller than normal root systems, mild stunting and chlorosis. They feed on a wide range of hosts including grasses, cereals, legumes, fruiting vegetables etc.

Sampling for corn nematodes.

Sending samples to a diagnostic lab is the only way to confirm nematode presence in a filed. Because some of the nematode species feeding on corn are endoparasitic (live and feed inside plant roots) while others are ectoparasitic (live in the soil), it will be necessary to submit soil and root samples. The best time to sample is when the nematodes are actively feeding on plant roots before they start migrating deeper into the soil profile and when roots are still young. Sampling early to mid-season (4-8 weeks after planting or around the V6 stage of corn) when the nematode numbers are highest, and symptoms are visible is best. Guidelines from Iowa State University and University of Illinois suggest:

• Collect soil cores that are least 12” long using a soil probe. Other resources suggest 6-8” deep.
• Collect 20 or more cores in an area representing about 10 acres. Samples should follow a zig-zag pattern.
• Collect soil cores from the root zone of symptomatic plants, place them in a sealed plastic bag and label it.
• Using a shovel, collect the root mass from 4-6 symptomatic plants taking care of conserving the integrity of the root system including seminal roots. It can be a good idea to also submit roots from healthy looking plants to have a point of reference. The above-ground growth of these plants can be cut off to facilitate shipment. Place the roots in a sealed plastic bag and label with a marker.
• Keep the samples cool and protect them from physical damage until they can be shipped.
• Rotation: Crop rotation to non-host crops can help reduce the population densities of “some” nematode species. Unfortunately, many of the nematode species mentioned above have very wide host ranges so rotation options are limited or not effective. For example, sting nematodes can feed on soybeans and wheat so rotation to these crops will not aid in management. However, alfalfa is nonhost crop of sting nematodes so rotation to this crop could be of benefit. On the other hand, needle nematodes feed mainly on corn and grasses so rotation to soybeans can be of benefit.
• Minimizing other stressors by providing adequate nutrition, avoiding herbicide injury, managing compaction, minimizing root feeding by insects and providing adequate moisture.
• Nematicides: Several nematicide seed treatments are commercially available for corn and can help protect the roots from nematode feeding. Products like Avicta, Votivo, Nemastrike, Aveo and N-Hibit among others act by either deterring nematode feeding, inducing plant defenses or interfering with nematode development.

Management

No in-season rescue options are available to reverse the damage caused by corn nematodes. However, confirming that nematodes are actually responsible for the damage observed in a corn field can help us create a management plan for future crops. Once we have identified the nematode species and densities we can implement some of the following management options:

• Rotation: Crop rotation to non-host crops can help reduce the population densities of “some” nematode species. Unfortunately, many of the nematode species mentioned above have very wide host ranges so rotation options are limited or not effective. For example, sting nematodes can feed on soybeans and wheat so rotation to these crops will not aid in management. However, alfalfa is nonhost crop of sting nematodes so rotation to this crop could be of benefit. On the other hand, needle nematodes feed mainly on corn and grasses so rotation to soybeans can be of benefit.
• Minimizing other stressors by providing adequate nutrition, avoiding herbicide injury, managing compaction, minimizing root feeding by insects and providing adequate moisture.
• Nematicides: Several nematicide seed treatments are commercially available for corn and can help protect the roots from nematode feeding. Products like Avicta, Votivo, Nemastrike, Aveo and N-Hibit among others act by either deterring nematode feeding, inducing plant defenses or interfering with nematode development.

Resources:

ICM articles Iowa State University.

University of Illinois Extension

University of Florida IFAS Extension.