THE MOST COMMON GENERA OF PLANT PARASITIC NEMATODES IN CALIFORNIA GROUPED BY LIFE HISTORY PATTERNS:

This table groups the most important genera of plant-parasitic nematodes found in California according to life history. All life cycle stages of migratory ectoparasites are found within the soil. Eggs, juveniles and adults of migratory endoparasites can be found either within roots or in the soil The second stage juvenile of sedentary endoparasites is typically the migratory/infective stage which hatches from the egg. It can be found in the soil and penetrates the root to establish a feeding site. Eggs can be found either within roots or in the soil. The later juvenile stages and the adult are typically nonmotile and remain at a single feeding location.

Grouping nematodes by life history habit can be useful when considering management alternatives. For example, if one were to apply a chemical or soil amendment, the product would be more likely to control nematodes in the soil (migratory ectoparasites) than those within roots, unless the product had systemic activity. Although endoparasites may spend time in the soil at some point during their life cycle, significant numbers are found within roots.

As another example, a trap crop could be used to reduce the number of sedentary endoparasites because they lose their mobility once a feeding site is established. The idea behind use of a trap crop is that a host for the nematode is planted, nematodes enter roots, establish feeding sites, and are no longer able to leave the root. Before sufficient nematode degree days have passed for the nematode to produce eggs, the crop is tilled under and the nematodes trapped within roots die. Experimentally, effectiveness of this method of control has been limited because typically only 30-50 percent of the population enter the roots.

Nematode shape can also influence the type of extraction method used. For example, the endoparasitic cyst nematodes Heterodera produce swollen mature females attached to host roots that are visible on infested plants through direct observation. The cyst is a survival stage of the nematode that provides protection in soil for viable eggs enclosed in cysts for up to several years. The extraction of adult female stages from roots and the extraction of cysts from soil require different techniques than those required for extracting the vermiform or worm-shaped species and life stages from roots and soil. Root lesion nematodes (Pratylenchus) are also endoparasitic, but all life stages, both in soil and in roots remain vermiform. Like most nematodes, root lesion nematodes are invisible to the naked eye. They can be extracted from both soil and roots with techniques that rely on nematode motility (such as the mist chamber and Baermann funnel techniques). Ectoparasitic nematodes, such as the dagger (Xiphinema) and stubby root (Paratrichodorus and Trichodorus) species, also remain vermiform throughout their life stages outside the egg, and can be extracted from soil using the motility requiring techniques. However, because they remain outside of roots of host plants during all stages (they feed externally by inserting their stylets into roots from the root surface), they are not found within root tissue samples, but in soil in the root zone (rhizosphere) from which they may be recovered. Large or fat nematodes, such as dagger or ring nematodes, can be extracted by normal methods, but there are certain modifications that enhance the extraction efficiency of the larger nematodes.

Many nematodes have both a scientific (Genus species) name and a common name as illustrated by the examples in this table. The common names have arisen either from a morphological characteristic of the nematode or from plant damage symptoms resulting from nematode infestation.

For example, the ring nematode received its name because of the rings evident on the cuticle.

The dagger nematode and the needle nematode both have very long stylets.

Root-knot nematodes cause galls or knots to form on roots.

The sheath nematode retains one of its cuticles at molting resulting in the appearance of a sheath surrounding the body.

Feeding by stubby-root nematode causes the root tip to stop growing and appear stubby. Paratrichodorus is another important genera found in California.

Spiral nematode typically appears to be curled up when viewed microscopically.

Pin nematode is one of the shortest plant parasitic nematodes.

Feeding and migration by lesion nematode can result in visible lesions forming on roots.

Stem and bulb nematode typically affects bulb crops such as daffodil, garlic and onion, or is found within the stems of plants such as alfalfa.

Foliar nematodes are typically found within leaves of plants rather than within roots. Citrus nematode has a relatively narrow host range and was first found affecting citrus, although it is also a serious problem on other crops such as grapes.

The body of adult female cyst nematodes is typically swollen and becomes a cyst to protect eggs laid within it. The seed and leaf gall nematodes are typically found within galls that have formed around them either within seeds or on plant leaves.

The University of Nebraska has a Key for Identification of nematodes (it takes awhile to load) and descriptions of genera of plant parasitic nematodes. References: McKenry, M.V. and P. A. Roberts. 1985. Phytonematology Study Guide. University of California, Division of Agriculture and Natural Resources.

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