Nematode Management in Field Crops

Plant parasitic nematodes can be highly damaging to crops and reduce yields. Aboveground symptoms of nematode damage in a field include patches of poor growth/yield, high incidence of soilborne diseases, and stunting, yellowing, wilting, or dying plants. Below ground signs and symptoms include stubby roots, root proliferation or abnormal branching, stunted roots, root rot, or the presence of galls (root-knot nematode) or cysts (soybean cyst nematode) on the roots. Approaches to management of nematodes depend on the types and numbers of nematodes present, so sampling fields for nematodes is a necessary first step in developing a management strategy.

TYPES OF SAMPLES: Soil can be submitted for either a diagnostic or a predictive nematode assay. Diagnostic assays are performed with the purpose of identifying the cause of poor growth in the current crop. Predictive nematode assays are performed to determine the risk of next year’s crop being impacted by nematodes. The processing of the two types of samples is identical, but procedures for sampling and interpretation of results differ. Instructions for collecting samples for the two types of assays are described in Table 1.

HANDLING AND SUBMITTING SAMPLES: Place 1 pint of soil from each sample in a labelled plastic bag and seal the bag tightly. Do not expose the samples to heat or freezing temperatures. Samples may be refrigerated up to a week prior to submission. Fill out the Nematode Assay Submission Form (see VCE Publication “Instructions for Sampling and Submitting Crop Nematode Samples”) with all the required information and mail or hand deliver samples to the Virginia Tech Tidewater AREC Nematode Diagnostic Lab.

INTERPRETATION OF NEMATODE ASSAY RESULTS: Diagnostic assays will determine if the damage or poor growth in a field is associated with high nematode populations, but they do not necessarily predict the risk of damage to subsequent crops. In order to make management decisions for a future crop, a predictive assay must be submitted. Predictive assay reports indicate the risk of damage (low, moderate, or high) to different crops based on the types and numbers of nematodes present in the sample. Risk categories are based on Virginia Cooperative Extension damage thresholds for nematodes, and thresholds are crop-specific. Threshold numbers for corn, cotton, peanut, and soybean are shown in Tables 2-5.

*Corn is a host for southern root-knot (Meloidogyne incognita) but not northern root-knot nematode (M. hapla). N/A = not applicable; this crop is a poor host or non-host for the indicated nematode. --- = no threshold level for this category.

*Cotton is a host for southern root-knot (M. incognita) but not northern root- knot nematode (M. hapla). N/A = not applicable; this crop is a poor host or non-host for the indicated nematode.

*Peanut is a host for northern root-knot (M. hapla) but not southern root-knot nematode (M. incognita). N/A = not applicable; this crop is a poor host or non- host for the indicated nematode. --- = no threshold level for this category.

*Soybean is a host for both northern root-knot (M. hapla) and southern root- knot nematode (M. incognita). --- = no threshold level for this category.

MANAGEMENT RECOMMENDATION ACTION CODES: Management recommendations are based on nematode thresholds and the crop that will be planted in a field. To determine the nematode management recommendations for a particular field, refer to the action codes in Tables 6-9. For example, if a predictive assay report indicates high risk for damage to soybean from cyst nematode, the action codes B, C, and E in the “high” threshold category column of Table 9 apply to the field. Management recommendations associated with each action code are the following:

A.  Nematodes will not damage the crop and no action is needed.

B.  Crop rotation to a non-host will reduce nematode populations.

C.  Planting a nematode resistant variety will minimize damage to the crop and reduce nematode populations.

D.  Nematode damage to the crop may be minimal and yield is unlikely to be impacted if growing conditions are favorable.

E.  Nematodes will likely cause crop damage and a nematicide may be profitable.

*These action codes apply only if southern root-knot nematode (M. incognita) is present in the field. N/A = not applicable; this crop is a poor host or non-host for the indicated nematode. --- = no threshold level for this category.

*These action codes apply only if southern root-knot nematode (M. incognita) is present in the field. N/A = not applicable; this crop is a poor host or non-host for the indicated nematode.

*These action codes apply only if northern root-knot nematode (M. hapla) is present in the field. N/A = not applicable; this crop is a poor host or non-host for the indicated nematode. --- = no threshold level for this category.

*These action codes apply for both northern (M. incognita) and southern root-knot nematode (M. incognita). --- = no threshold level for this category.

APPROACHES TO NEMATODE MANAGEMENT IN FIELD CROPS: Integrated approaches to nematode management that incorporate host resistance, cultural practice such as crop rotation and sanitation, and judicial use of nematicides are typically the most effective and economical. The management action codes listed above can be used to identify which approaches will be effective in a specific field based on economic thresholds. Fields should be re-sampled on a regular basis (at least every three years) to verify that management practices are reducing or maintaining crop parasitic nematode populations below economic thresholds. The extent to which a particular management practice will adequately control nematodes and minimize crop damage depends on both the field-specific nematode populations and the crop being planted. Host resistance is one of the most effective management approaches, but nematode-resistant varieties are limited to cotton and soybean varieties with resistance to southern root-knot nematode and soybean varieties with resistance to specific races of soybean cyst nematode. Crop rotation can be effective for nematodes with a narrow host range, but several nematodes can reproduce on a wide range of crops and other plant species (Table 10). Nematicides registered for field crops are somewhat limited, but recent research in Virginia has evaluated the efficacy and profitability of seed treatment and in-furrow nematicides for control of nematodes in field crops. For more information refer to the current version of the Virginia Cooperative Extension Pest Management Guide for Field Crops or the Applied Research on Field Crop Disease Control publications (pubs.ext.vt.edu).

YES = the crop is a good host for the indicated nematode; NO = the crop is a poor or non-host for the indicated nematode.