Many nematodes are harmless to plants, but there are many others that cause damage. Every field in has some pathogenic nematodes feeding on the plant roots. Nematode damage is often seen when the same crop is planted in succession for a number of years. Nematode damage is intensified by environmental conditions, especially heat and lack of rainfail. Many nematodes cause more damage on sandy soils.Crops can be affected by a number of nematode species, including the following:
  Lesion nenatode: Pratylenchus hexincisus, and others
Spiral nematode: Helicotylenchus pseudorobustus
Dagger nematode: Xiphinema species
Needle nematode: Longidorus species
Lance nematode: Hoplolaimus species
Ring nematode: Criconemoides species
Sting nematode: Belonolamus species
Stubby root nematode: Paratrichodorus species
Stunt nematode: Tylenchorhynchus species
Cystoid nematode: Meloidodera species

Nematode damage can occur throughout the growing season. Above-ground symptoms of nematode damage include stunting, thin stands, premature wilting under moderate heat or drought stress, and nutrient deficiency symptoms. Since nematode numbers can vary greatly within very short distances in the field, areas affected by nematodes vary greatly in shape, size, and distribution. Symptoms are worse in soils that are sandy, dry, and infertile. Roots injured by nematodes are usually stunted, often with few fine secondary feeder roots. Root tips may be blunt and swollen. By damaging root tips as soon as they emerge, nematodes can be especially injurious to young seedlings. Even under moderate stress, nematode-damaged roots may cause young plants to die, resulting in a thin crop stands.