The lowdown on nematode damage

EILEEN WARD

Plant nematodes are small, simple worms. They are long and slender, 1/100 to 1/8 of an inch long. You need a microscope to see them clearly. They live in soil moisture or plant tissues and feed on plant cells with their hollow, needle-like stylet.

Some nematodes cause the tissues on which they feed to grow strangely, some stop the growth of roots, others kill the cells on which they feed, leaving dead tissue behind. Fungi and bacteria, that cause root rots, wilt and other plant diseases, often infect nematode damaged plants earlier and more severely than uninjured roots. Depending on the kinds of nematodes involved, the damage to ornamentals may include galls, stunting and decay of roots; roots which are darker in color than healthy roots.

Above ground symptoms are similar to those resulting from many kinds of root injury. Foliage loses its luster and wilts easier than it should. Prolonged stress from nematode injury to the root system results in yellowing and eventual loss of foliage. New flushes of growth are weak, with fewer and smaller leaves than healthy plants. This condition may persist even with extra fertilizer and water use. The pattern of damage is usually irregular, because nematodes are usually not distributed evenly in the soil.

Among the dozens of nematode species associated with landscape ornamentals, only a few cause most of the serious problems. The root-knot nematodes are by far the most important. Their easily recognized galls on roots make their presence obvious. Galls result from growth of plant tissues around juvenile nematodes that feed near the center of the root. Root-knot gall tissue is firm, without a hollow center, and is an integral part of the root. Removing a root-knot gall from a root tears the root tissue. Nodules formed on roots of many legumes because of beneficial bacteria and most other natural nodules or bumps are loosely attached to the roots and have hollow centers.

Not only is their damage the most easily recognized, but root-knot nematodes cause more serious damage to more kinds of ornamentals than other nematodes. Some of the more popular plants known to be susceptible to root-knot nematodes are many hollies, ixora, hibiscus, Barbados cherry, ti plant, most gardenias, pittosporum, boxwoods, rose and figs or ficus.

The less distinct discoloration, stunting, distortion and decay of roots caused by nematodes other than root-knot are easily confused with other root problems. Nematodes such as lesion, burrowing, and reniform probably cause more damage to landscape plants than is recognized, because their effects are not as readily recognized as those of root-knot nematodes.

If above ground symptoms suggest nematode injury, but root symptoms are not sufficiently clear, laboratory analysis of soil and roots can help. When soil moisture is good for plant growth, collect fine feeder roots and soil from several locations around the affected plants. The total volume of the sample should be about one quart. Pack the sample in a moisture-proof container such as a strong plastic bag, label it clearly and indelibly on the outside of the container and submit it promptly. Do not let the sample overheat by leaving it in the sun or over a hot exhaust system. Complete instructions and shipping materials to send samples to the Florida Cooperative Extension Service Nematode Assay Laboratory can be obtained at the Collier County Extension Office.

First, it is important to understand that there are a few plant nematodes in nearly every square foot of soil in Florida. So don’t panic. There is not an army of nematodes waiting to attack everything you plant. New ornamentals sometimes never really get started after planting. A young plant attacked by a high nematode population as soon as its roots are in the ground may decline rapidly and eventually die or remain so unattractive that it must be removed. Or sometimes, a long-established specimen or hedge gradually declines, and a great deal of root-knot nematode galling is found on the roots. The following suggestions should help minimize these cases.

1. Prepare new planting sites properly so that plants become rapidly established.

2. Use only top quality plants. No matter how perfect the planting site, a nematode infection already in the roots will do the most damage.

3. Use plants that are well adapted to their new location; soil type, shade, drainage, etc. They will be less likely to suffer environmental stress and be better able to live with a few nematodes.

4. Give plants proper care from the start. Fertilize as needed to maintain healthy growth, not to produce excessive, weak, succulent growth that invites attack by many kinds of pests above and below ground, including nematodes. Water deeply to encourage a deep root system. Frequent shallow watering causes shallow root systems. A larger root system can withstand a few nematodes better than a shallow, minimal root system.

5. Do not allow maintenance to lapse. Dry periods or pest outbreaks can weaken plants.

6. If nematodes had built up to high levels on a plant that was removed, do not replant without treating the soil to give the new plant a fighting chance. Look for an appropriate soil fumigant or try solar heat to sterilize the soil. The use of clear plastic can superheat the soil. The Collier County Extension Office has literature on this subject.

7. Keep soil around the plant root zone mulched to keep roots cool and minimize moisture loss by evaporation. Organic mulch can also contribute organic matter to the soil as it decomposes, thus enhancing the capacity of the soil to retain water and nutrients. Greater organic matter content is also associated with increased activity of natural enemies such as certain fungi, predatory nematodes, etc., that help keep populations of plant parasitic nematodes from exploding.

8. There is presently no chemical effective against root-knot or other nematodes that may be applied legally to landscape plants. Several effective nematicides that are registered for control of nematodes in ornamental plants are restricted to commercial production nurseries because of high mammalian (human) toxicity.

Eileen Ward and her husband Peter own and operate Greensward of Marco Inc., a lawn maintenance and landscaping company. Besides completing horticultural courses from the University of Florida, she has a commercial maintenance spray license and is a registered dealer in agricultural products in Florida.

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