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

Nematode damage (Meloidogyne sp.) on turf grass

Damage Caused by Nematodes

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.

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

Visible Symptoms

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.

Home growers are advised to watch all plants and soil entering their gardens, whether from friends or nurseries, and to introduce plants only from "reliable sources". Cleaning tools is always a good practice, but where nematodes are concerned, it's especially important when working in different areas of the garden. Remember, not knowing you have nematodes isn't the same as not having them! If you use manure to augment your soil, it should be fully composted before using, making sure it reaches temperatures high enough to kill nematodes (for example, 125 deg. F for 30 minutes, or 130 deg. F for 5 minutes).

Of less concern to the home grower, it's known that nematodes can be spread through irrigation, and cysts of the sugarbeet cyst nematode can pass unharmed through the digestive system of cattle. There's also the potential for spreading via wind, birds and other animals.

Management and Control

The UC IPM Pest Note on nematodes says, "management of nematodes is difficult." In fact, for the home gardener, where chemical control isn't an option, learning how to prevent them, and once infected, learning how to deal with them, are the only real options. The good news is there are a lot of different techniques available, many of which fall into the category of "good gardening practices", so they're easy to apply. Included are the following techniques:

• Crop rotation: if a specific species of nematode has been identified (through soil sampling and testing), a non-host crop can be selected. This is easier than it sounds, however, because you need to have successful identification of the nematodes, and then find a desirable alternative in the face of often conflicting information.
• Resistant varieties: the N in VFN tomato varieties indicates resistance to nematodes. Root knot nematodes completely avoid roots of Blenheim apricots. Citrus is a nonhost for root knot nematodes: the nematode penetrates but fails to develop further. Nemaguard rootstock is resistant to all root knot nematode species currently present in California. Root knot resistant grape varieties include Harmony and Freedom.
• Fallowing: the idea in fallowing, where an area of ground is left unplanted for a season or more, is to remove the food used by nematodes. Existing and newly hatched nematodes will starve, reducing the population enough to allow the planting of a susceptible crop. Note that you must also keep the area weed-free, or the nematodes will survive using the weeds as hosts. Other factors to consider: roots remaining in the ground after harvest will continue to provide food for nematodes. These roots need to be removed, or allowed to decompose. Cultivation of the soil can be effective to bring nematodes to the surface where exposure to the sun will kill them. There's conflicting evidence whether wet or dry fallowing is more effective.
• Date of planting and harvest: each nematode species has a minimum temperature where they're able to penetrate plant roots. Planting winter crops with this in mind can be an effective strategy in infected soil. Starting a summer crop before nematodes are active may give the plants enough time to get established and allow an adequate harvest.
• Trap/cover crops: this involves planting a known nematode host, allowing the plants to grow and attract nematodes, and then destroying the plants before egg bearing adults have developed. This requires knowledge of the nematode life cycle, and knowing that timing is affected by soil temperatures. There's been some research on the use of marigolds (Tagetes spp.) to reduce nematode populations, but results have been mixed.
• Removing plants with symptoms: effective for nematodes living inside roots, this involves digging up and discarding roots of infected plants after harvest to remove nematodes that would otherwise over winter.
• Solarization: placing clear plastic on top of the soil during warm summer months will raise soil temperatures high enough to kill nematodes in the top layers. To be effective, temperatures need to reach 125 deg. F, and because these temperatures can't practically be achieved deeper than a foot (and even then only in areas in full sun), the method is best followed with the planting of a shallow rooted crop.
• Flooding: in commercial applications, flooding has shown to be an alternative to methyl bromide as a pre-plant soil treatment in flat, low-lying areas rich in mineral soils where there are seasonally high water tables (at least 4-6 feet from the surface) and abundant water supplies.¹
• Organic controls: several hundred fungi have been shown to parasitize nematodes, however, no practical control applications have been developed. Only one bacterial parasite has been studied, and no viruses have been conclusively shown to affect nematodes. Suppliers of organic pest controls have recently started offering nematicide products including Nemagard (a "combination of plant extracts and fatty acids," whatever that means) and Nemastop ("made from the stalk of a hybrid sesame plant"), however there isn't much independent analysis on these products yet.
• Organic supplements: several sources report that the use of organic matter including manure and compost can help. However it's not clear from the literature whether there's a suppressive effect, or if the organic matter simply helps offset the damaging effects of nematodes by increasing the water delivery and nutrient capacity of the soil.
• Chemical: There are currently no chemical nematicides or soil fumigants available for home gardeners.

Summary

Plant parasitic nematodes have the potential to be a real problem for any gardener. Anyone lucky enough to be free of them is advised to keep it that way through good cultural practices, being careful of all additions to their garden. If you have them, you can learn to live with them using sound management practices. And finally, it's good to point out that there are also beneficial nematodes that you can use to help control certain garden pests, but that's a story for next time!

1. http://www.epa.gov/spdpublc/mbr/casestudies/volume3/floodng3.html

References:

• Management Of Plant Parasitic Nematodes, UC Davis Department of Nematology, http://ucdnema.ucdavis.edu/imagemap/nemmap/ent156html/204NEM/204INDEX
• Nematodes, Home & Landscape, http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7489.html
• Plan Ahead For Effective Garden Nematode Control, Ed Perry, Farm Advisor, http://ucce.ucdavis.edu/files/filelibrary/2193/227.htm
• Root Knot Nematodes, Franklin Laemmlen, Ph.D., http://cesantabarbara.ucdavis.edu/rkn.htm
• Nematodes in the Home Garden, G R Stirling, Biological Crop Protection Pty Ltd, reviewed by Julie Stanton, Queensland Horticulture Institute, http://www.dpi.qld.gov.au/horticulture/4752.html

Revised: 01-Mar-2003