Many people think of nematodes as beneficial; however, a great number of plant parasitic nematodes can do harm to our plants. Most longtime vegetable growers are familiar with nematodes. All soil has a certain number of damaging nematodes; we just hope that the levels are low enough so that they don't cause plant problems.
Nematodes are microscopic, eellike worms that live in the soil and feed on the roots of plants. In the process of feeding, they might cause the roots to become knotted and galled. Most of the foliar symptoms are very similar to those caused by inadequate moisture, poor nutrition or root rot. Affected plants might be stunted, have pale or yellow-green foliage or wilt readily under hot conditions even when soil moisture is sufficient. Plants appear chlorotic, have poor growth, and show thinning. In most cases, plants don't respond to fertilizer. Symptoms are most evident in hot weather, drought, or low fertility.
Among the dozens of nematode species which have been associated with landscape ornamentals, vegetables, and lawns, the problems caused by root-knot nematodes are by far the most damaging. The galls on the roots make their presence obvious. Popular landscape plants that are known to be susceptible to root-knot nematodes include hollies, hibiscus, gardenias, boxwoods, roses, and figs.
Unfortunately, there is no cure for nematodes, not even in a vegetable garden. Many years ago, there was a chemical called Vapam that could be used in gardens but that has been long gone. Now you can only hope to reduce the populations or at least focus on good cultural practices. For example, in ornamentals, the less stress the plants are under, the better able they are to withstand nematode attack. Watering the plants deeply and less frequently encourages the growth of deeper roots, which can help minimize nematode problems.
EXCESSIVE NITROGEN fertilization can lead to a rapid increase in nematode populations as the plant produces succulent roots. Proper fertilization of the plants should minimize this problem. Nutrient deficiencies and soil compaction can inhibit root development and increase the plants sensitivity to nematode damage.
In turfgrass there again is really nothing you can do other than focus on the best cultural practices, including getting a soil test and keeping your nutrient levels at the optimum level. In some cases, you might need to change your grass type. For example, this summer I had a client who had centipede lawn looking bad and had high levels of ring nematodes in his soil. With ring nematodes the threshold level for St. Augustine, zoysia, and Bermuda is 500 or more (nematodes per 100 cubic centimeters of soil). For centipede, it is 150 or more. So, with levels of between 150 and 500 (or even higher), a turgrass other than centipede can tolerate those nematode levels better.
IN VEGETABLE gardens, start out by getting rid of all spent vegetable plants after harvest has ended. Numerous nematodes and eggs trapped in the root system will be eliminated from the area.
Let a portion of the garden lie fallow for the summer and occasionally till the area. Nematodes require a film of moisture around them at all times. Eliminating the film of moisture kills nematodes. Repeated disking in the hot, dry summer months will bring nematodes to the surface to be killed by the drying of the sun.
Rotating crops is invaluable in reducing root-knot nematode damage, but space might limit the ability to rotate garden sites. Planting nematode-infested areas in French marigolds (Tagetes patula) will control nematodes provided no other root systems are present. The marigolds should be planted no more than seven inches apart. The garden can be divided, with half planted to marigolds and half to vegetables. The plantings should then be reversed the next year. Other French marigolds varieties, Tangerine, Petite Harmony or Petite Gold may be used.
BUY OR USE CLEAN transplants for your garden. Nematode-free areas can be reinfested by bringing in infected transplants. Either grow your own in sterilized soil or buy plants from a reputable dealer who grows in sterilized medium.
Plant resistant varieties when possible. Good resistance is available in many tomato varieties and some other vegetable crops. Tomato varieties with the letters VFN mean the variety is resistant or tolerant to Verticillium wilt, Fusarium wilt and nematodes. Using resistant varieties is the easiest, least expensive, and most effective means of nematode control.
You can also plant a cover crop, such as rye, in your garden during the fall. The nematodes don't feed on cover crops and the populations are reduced.
If you ever suspect that nematodes are affecting the plants in your garden, lawn or landscape you can have the soil tested at any extension office to determine if they are at damaging levels.
REACH SID MULLIS, THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE FOR RICHMOND COUNTY, AT (706) 821-2349 OR SMULLIS@UGA.EDU.