There is a silent killer (or at least "stunter") in the landscape. Do you ever see certain plants that have poor growth? Uneven stands? Chlorosis and yellowing foliage? Wilting? If so, you might have nematodes.
Many people think of some nematodes as beneficial; however, there is a great number of plant-parasitic nematodes that can harm our plants, including turf grass.
Nematodes are small, eellike worms that live in the soil and feed on plant roots. 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 they wilt readily under warm conditions even when soil moisture is sufficient. Plants also appear chlorotic, have poor growth and show thinning. In most cases the plants don't respond to fertilizer. Symptoms are most evident in hot weather, drought or low fertility.
Among the dozens of nematode species that 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. There used to be a chemical called Vapam that could be used in gardens, but that has been off the market for several years.
You can hope to only reduce the populations or at least focus on good cultural practices. For example, in ornamentals, the less stress the plants are under, the better they withstand nematode attack. Watering the plants deeply and less frequently boosts 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 turf grass, there again is 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 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 water around them at all times. Eliminating the film kills nematodes. Repeated disking in the dry summer months will bring nematodes to the surface to be killed by the sun.
Rotating your crops is invaluable in reducing root-knot nematode damage, but space might limit the ability to rotate garden sites. Planting French marigolds (Tagetes patula) in nematode-infested areas will control nematodes, provided no other root systems are present. The marigolds should be planted no more than 7 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 they are 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 also can plant a cover crop in your garden during the fall. Something such as rye (not rye grass) works well. The nematodes don't feed on cover crops and the populations are reduced.
If you suspect nematodes are affecting the plants in your garden, lawn or landscape, you can have the soil tested at any extension office to determine whether they are at damaging levels.
SID MULLIS IS THE DIRECTOR OF THE UNIVERSITY OF GEORGIA EXTENSION SERVICE OFFICE IN RICHMOND COUNTY. CALL 821-2349 OR SEND E-MAIL TO SMULLIS@UGA.EDU.