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Cotton

Weighted Nematode Rating

Weighted nematode rating (WNR) is an easy and inexpensive way to rate root knot nematode damage where cotton follows cotton. This method does not require soil samples or a laboratory to extract nematodes because roots are rated in the field. Rating is based on relative intensities of root galling.

Start sampling at harvest time. Before you begin, draw a map dividing any areas in the field that differ in soil texture, cropping history, or crop injury. Further divide the areas where conditions are most uniform into smaller blocks, using a grid pattern.

Sample each block separately; blocks should be no larger than 10 acres (4 ha). Use a root lifter such as a carrot or beet lifter attached to a light tractor to easily lift cotton roots out of the soil. Record ratings on the WNR form (PDF).

How to sample:

1. Divide the field into representative blocks.
2. Select a sample of 15 to 20 root systems in a random pattern from as broad of a representation of each block as possible. Remove as much of the root system as possible with a root lifter (for example, carrot or beet lifter), keeping side roots intact.
3. Gently wash or shake the soil from the roots.
4. To sort the root systems into weighted ratings (0, 1, 3, 5, 7) use the diagram above and silhouette the root system against the sky or a white board and evaluate for gall severity.
5. Use the weighted root rating table below to record the number of roots in each weighted rating category and to calculate the WNR.

Important links

• Root galling diagram
• Weighted nematode rating form (PDF)
• Root knot nematode pest management guideline