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Pay attention to corn nematodes

So far this season, we’ve fought numerous challenges that could rob yield in cornfields: Insects, diseases, weeds, soil conditions, weather, fertilizer deficiencies and herbicide injury come to mind for most growers and agronomists. Not meaning to “pile it on,” I’m afraid I must add another to the list.

Corn nematodes are a problem we rarely think about, even with all the press about new seed treatments that offer some control. In the mid-1990s, I remember going to a company’s agronomy training event that talked about corn nematodes being a relatively common pest that cost us yield in many Midwest cornfields.

Information was shared about how one brand of insecticide offered some control, but the bottom line to us agronomists was corn nematodes weren’t that big a “blip” on our radar. Back then, we were more concerned about controlling waterhemp.

No longer a passing problem

Things have changed in the last decade. We are starting to get a handle on waterhemp, and corn nematodes have gone from a small “blip” to a pest we now recognize as a serious problem that we need to manage in some fields.

Changes in management practices may be one reason we are having more issues with corn nematodes. The increase in corn-on-corn production and no-till farming operations has created a more favorable environment for corn nematodes, which enjoy undisturbed or less disturbed soil.

Changes in corn rootworm control may also be allowing the nematodes to thrive. We used to apply more soil-applied organophosphate and carbamate insecticides to corn-on-corn fields for corn rootworm control.

Nematologists, including Iowa State University’s Greg Tylka, suggest these insecticide products probably helped suppress nematodes, minimizing their populations to the point that damage likely went unnoticed. However, a shift away from those products to using pyrethroid insecticides (which don’t have activity on nematodes) and the introduction of transgenic insect-resistant corn have led to questions about injury symptoms from corn nematodes.

Identify injury symptoms

So what does corn nematode injury look like on a corn plant? Above ground symptoms include thin stands, uneven plant height, stunted plants, uneven tasseling, leaf yellowing, and small ears and kernels.

Underground, common symptoms of nematode feeding on corn roots include swollen roots, lack of fine roots and lack of root branching, and necrotic lesions (black or dark-brown dead spots). Complicating the question is the fact that these symptoms are not unique to nematode feeding.

Soil sample to be sure

Because symptoms can’t be used to definitively diagnose nematode damage, the only way to accurately tell is through collection and analysis of soil and root samples. And the best time to take these samples is now, in the middle of the growing season.

The test results and other details will be considered in deciding whether corn nematodes are partially or primarily responsible for crop damage.

Collecting a good sample and providing complete background details are critical to making an accurate assessment. Information about the field history, soil type and rainfall can aid accurate judging of whether nematode numbers are sufficient to damage corn.

3 management strategies

If it is determined that corn nematodes are damaging your corn crop, three management strategies or options are currently available: plant non-host crops, use soil-applied nematicides, or use seed treatments such as Avicta and Votivo.

Unfortunately, none of these options can minimize damage to the current crop or rescue the current corn crop.

Non-host crops. If the culprit is needle nematodes, certain species of lesion nematodes or a combination of these species, growing non-host crops such as alfalfa and soybeans will reduce nematode numbers and the potential for damage to future corn crops. One or two years of growing non-host crops in a field may be sufficient to lower populations below damage thresholds.

A new ISU Crop Adviser Institute computer training module on corn nematodes offers more information. To learn more, visit www.cai.iastate.edu or email cai@iastate.edu.

Soil-applied nematicides. There are only a few nematicides currently labeled for control of corn nematodes. Since damaging population densities may occur in patches or hot spots, field-wide application of a nematicide may not be necessary or economical. The beneficial control effects also don’t usually carry over to subsequent cropping seasons.

Seed treatments. New options have arrived on the market. Avicta Complete Corn, widely available for the first time in 2010, is a combination of the Avicta seed treatment nematicide (active ingredient abamectin), a seed treatment insecticide and three seed treatment fungicides.

Votivo is a biological seed treatment containing the bacterium Bacillus firmus, and was available this season. Preliminary information on early-season protection provided by these products looks promising.

You can be sure that ISU and other land-grant universities will continue to assess their performance and work with the manufacturers on best practices for corn nematode management.

In the meantime, we can’t effectively manage a pest we don’t know we have in our field. So talk with your local supplier about developing a corn nematode sampling plan. Now is the time!

McGrath is partnership program manager of ISU’s Corn and Soybean Initiative.

This article published in the July, 2011 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2011.