Nematodes are microscopic, eel-like roundworms. Nematodes are found in the soil; you can scoop up a handful of soil and literally find thousands of nematodes, but you won't be able to see them.
The most troublesome species in the garden are those that live and feed within plant roots most of their lives and those that live freely in the soil and feed on plant roots. Although there are many different species of root-feeding nematodes, the most important in gardens are the root knot nematodes. Root knot nematodes attack a wide range of plants, including many common vegetables, fruit trees, and ornamentals.
Plant-feeding nematodes go through six stages: an egg stage, four immature stages, and an adult stage. Many species can develop from egg to egg-laying adult in as little as 21 to 28 days during the warm summer months. Immature stages and adult males are long, slender worms. The mature adult females of some species, such as root knot nematode, change to a swollen, pear-like shape, whereas females of other species such as lesion nematode remain slender worms. Nematodes are too small to be seen without the aid of a microscope.
It is believed that root knot nematode survives from season to season primarily as an egg in the soil. After the eggs hatch, the second stage juveniles invade roots, usually at root tips, causing some of the root cells to enlarge where the nematodes feed and develop. The male nematodes eventually leave the roots, but the females remain embedded within roots, where they lay their eggs into a jellylike mass that extends out through the root surface and into the soil.
Damage Caused By Nematodes
Root knot nematodes usually cause distinctive swellings, called galls, on the roots of affected plants. Infestations of these nematodes are fairly easy to recognize by digging up a few plants with symptoms, washing or gently tapping the soil from the roots, and examining the roots for galls. The nematodes feed and develop within the galls, which may grow to as large as 1-inch in diameter on some plants but are usually much smaller. The water- and nutrient-conducting abilities of the roots are damaged by the formation of the galls. Galls may crack or split open, especially on the roots of vegetable plants, allowing the entry of soilborne, disease-causing microorganisms.
Root knot nematode galls are true swellings and cannot be rubbed off the roots. Root knot nematodes may feed on the roots of grasses and certain legumes without causing galling.
Above ground symptoms of a root knot nematode infestation include wilting, loss of vigor, yellowing, and other symptoms similar to a lack of water or nutrients. Infested vegetable plants grow more slowly than neighboring healthy plants, beginning in early to mid-season. Plants often wilt during the hottest part of the day, even with adequate soil moisture, and leaves may turn yellow. Fewer and smaller leaves and fruits are produced, and plants heavily infested early in the season may die. Damage is most serious in warm, irrigated, sandy soils.
Root injury caused by other nematode species may produce above ground symptoms similar to those caused by root knot nematodes. However, the actual injury to the roots is more difficult to detect. Roots may be shortened or deformed with no other clues as to the source of the injury. You can confirm a nematode infestation by collecting soil and root samples and sending the material to a laboratory for positive identification of the infesting species.
While annual plants may be killed by nematodes, woody plants are rarely killed. Nematode injury to woody plants is usually less obvious and often more difficult to diagnose. Infested fruit and nut trees may have reduced growth and yields. Woody landscape plants that are heavily infested may have reduced growth and branch tip die back and may defoliate earlier than normal.
Management of nematodes is difficult. The most reliable practices are preventive, including sanitation and choice of plant varieties. Existing infestations can be reduced through fallowing, crop rotation, or soil solarization. However, these methods reduce nematodes primarily in the top foot or so of the soil, so are effective only for about a year. They are suitable primarily for annual plants or to help young woody plants establish. Once an area or crop is infested, try to minimize damage by adjusting planting and harvesting dates and irrigation or by the use of soil amendments.
Nematodes are usually introduced into new areas with infested soil or plants. Prevent nematodes from entering your garden by using only nematode-free plants purchased from reliable nurseries. To prevent the spread of nematodes, avoid moving plants and soil from infested parts of the garden. Do not allow irrigation water from around infested plants to run off, as this spreads nematodes. Nematodes may be present in soil attached to tools and equipment used elsewhere, so clean tools thoroughly before using them in your garden.
Resistant or Tolerant Varieties and Rootstocks
One of the best ways to manage nematodes is to use vegetable varieties and fruit tree rootstocks that are resistant to nematode injury. Tomato varieties with VFN after their name are resistant to most root knot nematodes, as are 'Nemaguard' rootstock used for stone fruit and almond trees and 'Harmony' and 'Freedom' rootstock used for grapes. Citrus trees growing on 'Troyer' and 'Trifoliate' rootstocks are resistant to the citrus nematode. Consider replacing severely infested plants with plant species and varieties that are more tolerant of the nematodes present. Unfortunately, resistant varieties are not available for many crops and ornamentals.
Growing a crop on which the nematode pest cannot reproduce is a good way to control some nematodes. For example, the sugarbeet cyst nematode attacks only a limited number of crops, including cole crops (broccoli, Brussels sprouts, cabbage, cauliflower) and related crops and weeds. Growing non-susceptible crops for 3 to 5 years reduces the sugarbeet cyst nematode population to a level where susceptible crops may be grown again. Unfortunately, rotation is not as easy for controlling root knot nematodes because so many vegetable crops and weeds are hosts of the pest.
However, with careful planning, rotation in combination with fallowing and solarization can reduce root knot nematode numbers. Annual crops that are useful in a rotation plan for reducing root knot nematode populations include small grains such as wheat and barley, sudangrass, and resistant tomato and bean varieties.
Fallowing is the practice of leaving the soil bare for a period of time. Fallowing for 1 year will lower root knot nematode populations enough to successfully grow a susceptible annual crop. Two years of fallow will lower nematode numbers even further. When fallowing, it is important to keep the soil moist to induce egg hatch and to control weeds on which nematodes may survive. As a result, eggs will hatch but the nematodes will die if there is nothing to feed on.
Soil Amendments and Irrigation
Various organic amendments can be added to the soil to reduce the impact of nematodes on crop plants. The amendments, which include peat, manure, and composts, are useful for increasing the water- and nutrient-holding capacity of the soil, especially sandy soils. Because plants that are water-stressed are more readily damaged by nematodes, increasing the soil's capacity to hold water can lessen the effects of nematode injury. Likewise, more frequent irrigation can help reduce the damage caused by nematodes. In either case, there will be just as many nematodes in the soil, but they will cause less damage.