Tips from Extension Experts: New Approach to Assessing Nematodes |

Tips from Extension Experts: New Approach to Assessing Nematodes

D. Bruce Bosley and Gregory L. Tylka
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Many different species of plant-parasitic nematodes occur in cornfields throughout the Midwest.

Most of these microscopic, parasitic worms must reach a damage threshold number before they cause yield loss in corn. The damage threshold numbers for most nematodes that feed on corn are 100 or more worms per 100 cm3, which is a little less than a half-cup of soil.

The exceptions are the needle and sting nematodes, which damage corn even if only one or two worms per 100 cm3 soil are present. Needle and sting nematodes only occur in soil with at least 70 percent sand and, therefore, aren’t considered a risk in finer textured soils of corn fields.

Nematode damage threshold numbers currently used for corn were established in the 1980s.

There are no combined or cumulative damage threshold numbers for cornfields infested with multiple nematode species. Damage threshold can vary depending on conditions in the field — things such as soil texture, cropping history, tillage and other factors.

Using a single number as a damage threshold for individual nematode species is an oversimplified approach to assessing the possibility of yield loss due to nematodes feeding on corn.

In September 2011, industry personnel hosted a meeting with nematologists, plant diagnosticians and agronomists from many universities across the Corn Belt in order to come up with a more comprehensive method corn nematode control. From these meetings standard recommendations for collecting samples to diagnose possible nematode damage to corn were developed, and those recommendations were formulated.

Also, a comprehensive approach was developed for assessing the risk of damage to corn caused by multiple species of nematodes, as described below.

Considering combined nematode damage and risk factors of fields

A value called the “total nematode damage risk index” is used to assess the potential damage from all plant-parasitic nematodes that feed on corn that are identified in a sample.

A “site sensitivity index” is calculated using background information provided about the field from which the sample was collected. Factors that determine if a field is more or less vulnerable to nematode damage on corn include:

• Number of years that corn has been grown

• Predominate soil texture

• Availability of irrigation

• Use of conservation tillage

• Occurrence of stand establishment and/or compaction problems

The “total nematode damage risk index” and the “site sensitivity index” values are used to assess overall likelihood or risk of damage from nematodes feeding on corn.


For those interested in reading more about what these Midwest scientists are recommending, see the journal article, at

Rocky Mountain region farmers can also contact Bruce Bosley at (970) 980-4001, or


Bruce Bosley is a cropping system specialist for Colorado State University Extension.

Gregory L. Tylka is a plant pathologist for Iowa State University.