Nematodes are very small (microscopic) worms that live within the soil. They are parasites, which must feed on other living tissues, and are aquatic animals that live in either the fluids surrounding or fluids directly in the host plant. 
There are two main types of nematodes ectoparasitic and endoparasitic. The ecoparasitic live outside the plant and can migrate from root to root. Whereas the endoparasitic can live within the host plants tissues.
 The two species of nematodes we are most concerned with are the Sting Nematodes (Belonolamus) and the Lance Nematode (Hoplolaimus galeatus). Both can cause considerable damage in turf, lawns, vegetable crops, ornamentals, etc.

Healthy root on Left and Nematode damaged root on right.

How Nematodes Effect Turf

Nematodes feeding on roots alter the overall growth of the roots and therefore the crown (the turf surface-grass). If nematode activity persist they may create dead spots in the turf, create soft unstable roots and then eventually kill the root overall. Nematodes extract much needed nutrients from the plant’s root system and places the turfgrass in extreme stress. If other stress damage, such as drought or poor soil conditions exist, nematode damage can rapidly become more pronounced and destroy a lawn, green or sports field.

Digestive secretions injected into the living cells by the nematode start a decomposing process which severely weakens the plant's immune system. This mechanical wounding and the inability of the roots to absorb water make grass plants more susceptible to other problems such as fungal and virus diseases.

Nematode Damage on Turf