by: Allen Buchinski

Introduction

Nematodes are tiny, round-bodied, unsegmented, worms. Most yards typically have billions of them in the soil, feeding on organic matter, bacteria, insects and plants. Nematodes are one of the most common of all animals, but because they usually can't be seen without a microscope, they aren't very well understood outside the scientific community of nematologists. Approximately 10% of all nematodes feed on plants, living around or in the roots. The most well known is the root knot nematode (Meloidogyne spp.), because of the distinctive galls it causes on infected roots, it's wide distribution, and the wide range of plants that it attacks (including most common vegetables, ornamentals, and fruit trees.)

A second stage juvenile cyst nematode has just hatched from an egg

There are two main types of plant parasitic nematodes; ectoparasitic and endoparasitic. The ecoparasitic type lives outside the plant, feeding on roots with the ability to move about 3 feet to find a host, depending on the soil and species. Endoparasitic types (which includes the root knot nematode), penetrate the root, then enter and live inside it. Each type goes through six development stages; starting from an egg, then four juvenile stages (molting after each one), and an adult stage. Under ideal soil temperatures (60 deg. F to 90 deg. F), most species develop into egg-laying adults within a month, however some species take as little as two weeks while others require a full year. As a technique to speed the maturation process, many undergo their first molting and enter the second stage while still in the egg.

In addition to the more well known root knot nematode, there are many others, most of them named for physical characteristics. They include: ring, dagger, sheath, stubby-root, spiral, pin, lesion, stem and bulb, and foliar. Each has it's own characteristics including adaptation to temperature ranges, soil preferences, host crops and egg laying quantities and methods.

As an example of these differences, the root knot nematode (Meloidogyne spp.) lays several hundred eggs outside its body (in a gelatinous mass either inside or outside of the root). The root knot egg and juvenile populations decline by up to 70 to 90 percent during winter, making them dependent on subsequent spring and summer conditions for survival. This compares to the cyst nematode (Heterodera spp.) that retains its eggs within its body, which then serves as protection for the eggs after the female dies. These eggs can remain viable for several years, with only a portion hatching even when suitable conditions occur. This ensures hatching in following years even next year's cycle produces no eggs.

Damage Caused by Nematodes

Nematode damage (Meloidogyne sp.) on turf grass

One of the most difficult factors in diagnosing nematode problems is the inability to view direct damage to the plant. In the case of root knot nematodes, where root galls are readily visible on the roots, the actual damage is hidden underground until the roots are dug up. In all cases, damage to the roots interferes with the plant's ability to deliver water and nutrients to above ground portions of the plant. In addition, openings created in the roots increase the plant's susceptibility to harmful bacterial and fungi organisms, sometimes creating a secondary detrimental effect on the plant. Finally, some nematodes have the ability to transmit viruses between plants.

Visible Symptoms

Illustration of root knot nematode (Hemicycliophora sp.) damage is visible on the center plant

Since you can't see nematode damage directly (without using a shovel!), you need to rely on visible symptoms including wilting during the warmest period of the day, chlorosis, stunted growth, and general lack of vigor. If damage is confined to the plant's water delivery system, you may only notice wilting on hot days, with the plant returning to normal once temperatures have cooled. Also possible are curling and twisting of leaves and stems, and either premature or delayed maturity. Ornamentals may have branch tip dieback and lose their leaves earlier than normal. And if you have a large enough area, you may also note that damage is uneven, with affected plants among healthy ones due to uneven distribution of nematodes.

In general, nematodes don't kill plants (they wouldn't have survived as long as they have if they did). But they reduce the vigor of the plant and make it more likely to be harmed by other factors. And because the symptoms are the same ones you see for "traditional" problems, a good rule of thumb is to analyze general factors such as irrigation, nutrients, and visible symptoms of disease before suspecting nematodes (unless you know there's a history of nematodes in your soil.)

Spreading Nematodes

Because nematodes live in the soil and don't travel great distances, they spread slowly (but surely) unless given assistance. It's important to be aware that they can be easily spread by moving soil directly and on tools, shoes, and plants. For example, it used to be common for California Imperial Valley growers to use excess soil from processing plants to level their fields. This practice led to the infection of many fields (see illustration).

This photo shows a star shaped pattern of under performing sugarbeets caused by fill brought in from a nematode infected source.