The root-knot nematode (RKN), Meloidogyne incognita, is the only economically important nematode pest of cotton in Arizona. The pest feeds on roots and causes the formation of galls (knots). Root damage can greatly reduce growth and yield.

RKN infections are often easy to identify by the swellings caused on roots, but the diagnosis should be confirmed by a qualified lab. Since RKN is most active in warm weather, it is best to take samples from late spring to early fall.

Significant losses to RKN are usually limited to moist, well-aerated sandy soils. When taking samples, focus on sandy areas of the field and sample moist soil. Soil mapping systems can help determine sandy areas in a field most vulnerable to infestation.

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Since nematodes are most abundant in the top foot of soil in irrigated cotton, take samples from 6–12 inches deep. In fallow fields, sample deep enough to find moist soil.

The accurate diagnosis of a RKN infestation usually requires laboratory analysis for detection and identification. Good samples are important for accurate identification and quantification.

The following steps will help get a good sample.

1 - Take soil samples near actively growing roots. Most plant-parasitic nematodes are found in the host root zone. Include small roots in the sample.

2 - Sample in late spring, summer, or early fall when soils are warm and moist since nematodes are most active in warm, moist soils.

3 - Take several sub-samples from the field of interest. Use a clean trowel, shovel, or soil corer. Mix subsamples totaling about a quart and place in a sealable plastic bag.

4 - Label the bag with location, host plant, date, and your name. Do not add water, moist paper towels, or sample label inside the bag.

5 - Place the soil from each sampling site in a separate plastic bag and mark each bag accordingly.

6 - Protect the sample bags from temperature extremes. Do not allow samples to dry out, sit in the sun (or in a hot truck), or freeze. If necessary, samples can be stored at room temperature for up to a week.

Submit samples to the University of Arizona county or area agent or send to: Extension Plant Pathology, School of Plant Sciences, University of Arizona, Forbes 303, 1140 East South Campus Drive, Tucson, Ariz. 85721.

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