Nematodes are just simple round worms:
Borers, Cat fleas, Banana moth, Corn earworm, Etc.
Damage they cause:
Above ground: Stunted growth, Mid-day wilting, Small fruit, Yellowing, Curling and twisting of plant, Premature or Delayed maturity, or Reduced yield.
Below ground: Galls or swelling, Stubby roots, Lesions or dark spots, Stunting, or "dirty roots".
Methods to control:
Management is difficult. Most reliable methods are preventative practices, like sanitation or choice of plant varieties. If the ground is already inffected; change your planting time to a cooler date, when nematodes are less active. Another more effective method is crop rotation