Kearney Agricultural Research and Extension Center
University of California
Kearney Agricultural Research and Extension Center

Nematode overview

Ring nematode

The fatter, short nematode is ring nematode, Mesocriconema xenoplax. Note the annulations or rings circling the exterior of the nematode. The spear of this nematode is large and easy to see whereas the spear on the root knot nematode cannot be seen at this magnification. The spear indicates the head end of plant parasitic nematodes.

This nematode also feeds on a wide range of plants including legumes, but is of particular importance on several woody rooted plants including peach, apple, walnut and grape. One of the plants that will reduce populations is sudan grass. Wheat also tends to be a poor host.

The fatter size of this nematode renders it almost immobile within individual soil pore spaces. It only develops to high populations within soils of high porosity, including sands and well-structured clay loam soils. Fine sandy loam soils have a smaller pore size which limits population development.

Ring nematodes do not penetrate roots but their long spears enable nutrient extraction from deep within the root cortex. In fact, with roots of peach this nematode is known to induce a sophisticated straw-like feeding site. This nematode is capable of reaching high population levels in warm, moist soils but populations may then decline because of organisms in soil that are antagonistic to its build-up. Highest population levels of this nematode are achieved by fumigating the soil and then re-introducing the nematode.

Root lesion nematode

The nematode appearing longer and with darker contents than root knot is root lesion nematode, Pratylenchus vulnus. Take a closer look and note the short, strong spear at the head end. This nematode may extract its food from outside the root or by entering along the root and moving between cells.

It is an endoparasitic nematode and may lay its eggs in or out of woody roots.  P. vulnus prefers to feed on roots of woody perennials and it does not flourish in alfalfa, most annual crops or grasses.

This is a case where identification of “which species” is most important because there are currently six root lesion species in California that do flourish on non woody crops. Although grass-feeding root lesion species may develop to high population levels, Pratylenchus vulnus generally does not build soil population levels as high as ring or root knot nematode. It develops best on walnut and rose, moderate population levels on peach and plum roots and generally not very high population levels on citrus or grape. Notable exceptions include zinfandel and Pinot Noir grape.

On dwarfing apple rootstocks this nematode can sometimes build to high population levels in the first five years then subside after trees become established. Most, but not all, selections of Pistacia terebinthus, P. atlantica and P. integerrima do not host this nematode nor root knot species.  Pratylenchus vulnus does best in loam textured soils. By contrast, Pratylenchus thornei does best in clay and clay loam soils where wheat or milo are grown and P. scribneri does best in sandy loam soils.

Root knot nematode

Now take another look at the photo. Nematodes are as different as horses and cows or butterflies and sharpshooters. In California a nematode sample from a field of alfalfa, with few exceptions, means Meloidogyne hapla whereas a field of Acala-type cotton means Meloidogyne incognita race 3. A citrus planting is where you find citrus nematode, Tylenchulus semipenetrans, not root knot.   There are only about seven other hosts for citrus nematode. Take soil samples when your plants are of low vigor or chlorotic. What you learn might be very helpful.

The smallest, shortest nematode in the photo is root knot nematode, a Meloidogyne spp. This is the most common plant parasitic nematode world wide and it is most damaging in sandier soils. The second-stage infective juvenile pictured here has about two weeks to find and penetrate the new root tip of a nearby plant.

Temperatures must be above 60 degrees or its penetration will not occur. Once there is adequate soil moisture and warm temperature one species or another of Meloidogyne will penetrate just about any plant. Successful penetration will cause a gall on most broadleaf plants and some grasses.

Interestingly, there are some species and populations of Meloidogyne that successfully set up a feeding site on one plant, however other species or populations may not. This inconsistency in their feeding ability has been of interest to nematologists for more than 50 years. Root penetration is oftentimes followed by development of a sophisticated feeding site and progress toward a round, adult female stage. About 30 days after successful penetration the adult female is producing eggs and hundreds to more than a thousand may result from each female.

Dagger nematode

Dagger nematode, currently called Xiphinema americanum, is the largest in the photo. It is common throughout California and across the U.S. but there are reasons to believe our native populations are distinct from those east of the Rocky Mountains. This nematode reproduces once per year, reportedly lives four or five years and reaches highest population levels in undisturbed areas and in zones within soil where oxygen is most plentiful.

The spear on this nematode is quite long. Beneath the microscope one can note that the support structure which enables the spear to pierce cells consists of a flange structure rather than the knob structure of the other three nematodes.

The reader should appreciate that this nematode is from different ancestors than the others pictured. This nematode has a wide host range, is more common in grassy soils and can build to high population levels in any soil texture. On woody perennials it builds to highest population levels on those crops that do not host too many other nematodes such as pistachio, grape rootstocks such as Harmony, and apple. It does not prefer to be around walnut trees and is not usually in abundance in citrus plantings.

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