When a plant in the landscape grows poorly or dies, gardeners need to discover the cause. Among the questions they must ask include:
Any of these aboveground reasons can cause a plant to fail. When none of the usual explanations works, then the gardener must explore further – underground.
Some belowground pests are easy to see, ground squirrels and certain insects for instance, but others are only visible under a microscope. While these pests might not be easy to see, their effects on plants can be obvious.
Nematodes are microscopic worms (also called eelworms or roundworms) distantly related to insects and spiders. With more than 15,000 species, they are among the most plentiful animals on earth. There can be nearly 60,000 nematodes in a square foot of soil.
Many are parasites on animals, but about one- tenth are parasites on plant hosts. They are responsible for anywhere from 5% to 15% of all plant losses worldwide, a value of about $157 billion.
Other soil-inhabiting nematodes that do not prey on plants may eat bacteria, fungi or other nematodes, and may be important for breaking down organic matter in the soil.
While they require soil to have a film of moisture in order to move, they will die from lack of oxygen if the soil is totally saturated. They are also unable to tolerate very dry soils.
Temperature variation can likewise affect them; very high or freezing temperatures can even be fatal to nematodes.
In the soil, nematodes are slow moving, perhaps traveling less than four feet in a season, but soil disturbance and moving plants around may spread them widely. Nematodes are generally in the top foot of soil, but depending on the species and the environment, they may be as deep as several meters.
Among the most devastating of underground pests is the root-knot nematode (Meloidogyne spp.).
As the name suggests, this organism is a parasite on and in plant roots. Infected roots have galls, causing them to look as if they have been knotted (hence the name). This pest has a very wide “host range” and can attack tomatoes, peppers, carrots, onions, cactuses, roses, fruit trees and many ornamentals.
Nematodes can spread through soil, tools, rainwater, footwear and machinery. Since they are most active in warmer weather, the problem might not appear until late spring, when the damage has been done and becomes apparent.
When purchasing new plants, make sure to examine the roots before buying. If the roots look knotted and deformed, do not accept the plant.
Many indicators arise from a root-knot nematode infestation, but some of them may be confused with other problems. Short of removing a plant entirely, it is a good idea to track the symptoms. The following are symptoms of nematode infection, with other possible sources.
Above-ground symptoms may be confused with other environmental or root problems. Above ground, the plant may show some or all of the following before dying:
a. Stunting – the plant does not develop normally. It is prevented from growing, it is smaller and experiences a loss of vigor. This can be due not only to root-knot nematode, but also to viruses, other microscopic organisms, or even a nutrient deficiency.
b. Chlorosis – the leaves look pale, even yellowish. In addition to root-knot nematode, this is often due to nitrogen deficiency, herbicide damage, or poor soil drainage.
c. Mid-day wilting – as temperatures increase over the course of a day, the plant requires increased water. When nematodes colonize roots, much of the root system is unable to obtain soil water.
d. Leaf drop – with damaged roots, the plant is unable to acquire enough water. One survival method is to drop its leaves, which lose water. However, in addition to nematodes and drought, another cause of untimely leaf drop is a deficiency of the element zinc. This micronutrient is necessary for the leaf to remain securely attached to the plant.
e. Small fruit and reduced yield – since the plant is unable to obtain sufficient water and nutrients (barely enough to keep it alive), it can only divert a minimum of resources to developing fruit. With some kinds of plants (e.g. lettuce), there is a decrease in the number of healthy harvestable plants. For other plants, there may be patches of poor growth, indicating the loss of healthy root tissue. Another cause of this problem may be that too many small fruits were left developing on a tree.
In addition to their enormous host range, nematodes have many characteristics that cause them to be difficult to control.
When soil is already infected with root-knot nematodes, there is no quick remedy. Be certain to discard infected plants; do not compost them, as temperatures may not kill all the pests.
University of California Cooperative Extension recommends several actions:
Root-knot nematodes are very difficult to control or manage, since they live below ground and their symptoms may be confused with other plant problems. For a home gardener, they can be daunting, but not an impossible challenge.
Master Gardeners of Nevada
Program trains local gardeners to provide research-based horticulture information to Nevadans
Master Gardeners of Washoe County
Master Gardeners provide free, research-based horticulture information to Nevadans.
Integrated Pest Management
Integrated Pest Management program is a long-term management strategy that uses a combination of tactics to reduce pests to tolerable levels with potentially lower costs for the pest manager and minimal effect on the environment.
O’Callaghan, A., Robinson, M.L., and Haas, S., 2020, Root-Knot Nematode, Extension, University of Nevada, Reno, FS-20-24
An EEO/AA Institution. Copyright ©
2023, University of Nevada Cooperative Extension.
A partnership of Nevada counties; University of Nevada, Reno; and the U.S. Department of Agriculture