Plant-parasitic nematodes are microscopic worms that feed on plants. Several nematode species are serious pathogens of cotton, reducing overall US cotton production by an estimated 4.7%. Though losses in nematode infested fields are frequently 10 to 30%, losses can be greater than 50%. Cotton plants can suffer 10 to 15 % yield loss without obvious above-ground symptoms of nematode damage. The most important nematodes in US cotton production, listed from the most widespread to the least widespread, are the southern root-knot nematode (Meloiodgyne incognita), the reniform nematode (Rotylenchulus reniformis), and the Columbia lance nematode (Hoplolaimus columbus). The damage caused by nematodes involves physical damage to the roots that impairs their ability to move water and nutrients efficiently. Some nematode species also may cause physiological changes in the plant that direct some of the plant's energy to feeding the nematode instead of making cotton fibers. Most above-ground symptoms caused by nematodes are generic expressions of damage to the roots, such as stunting, premature wilting, yellowing or other apparent nutrient deficiencies. Root-knot nematodes cause distinctive galls on the roots, making them one of the few nematode problems that can be conclusively diagnosed based on symptoms. Though damaging nematode species are widespread, they may not be present in a field or they may be present at low population levels that do not cause measurable damage. So the first step in minimizing losses to nematodes is to identify which nematodes are present in a field and at what levels. This is accomplished by submitting a soil sample (a composite sample collected from multiple sites in a field) to a qualified laboratory which will identify plant-parasitic nematodes to the genus level and count the number of each genus. To get accurate results, soil samples must be protected from drying out or getting too hot prior to processing, so samples should be sealed in plastic bags and kept out of direct sunlight and high heat. Different states, and even different labs within a state, may base nematode management recommendations on different action threshold levels, which can vary due to different extraction methods, geographic location, soil types, and many other factors. Therefore, it is usually best to use the threshold levels provided by the lab that performed the analysis.