From the November/December 1998 issue of the California Rare Fruit Grower's "Fruit Gardener"

Ask the Experts
ABOUT ROOT-KNOT NEMATODES

Eunice Messner Fruit Specialist Coordinator
(revised 4 April 2000)

Question

I will be attempting to grow vegetables and fruit trees, but this summer I have found evidence of root-knot nematodes in my soil. Would you advise me as to what is resistant and how to control them.

Answer

It just so happens I have been collecting information on nematodes for years and years. So here is an organized list of what I have amassed:

Green Manures     When turned into the soil, most green manures are toxic to nematodes. Best control is seen with hairy indigo, white vetch, oilseed radish, rye, barley, corn wheat and millet crops. Chicory, alfalfa, timothy, and perennial rye grass also reduce nematode populations by up to 90%. Clover, mint, chickweed, dill, celery, peas, beans, raspberries, and apples, however, actually stimulate development.

Cool Weather.     Early spring, late fall, or winter plantings, when soil temperatures remain below 60 F, are unlikely to have nematodes.

Add Chitin.     Ground-up seafood shells contain a material called chitin - which promotes the growth of certain fungi and bactera in the soil. These microbes , in turn, feed on the 'bad' nematodes.

Turn up the Heat.     Nematode eggs and worms are killed in just 10 minutes at 120 F. Garden beds can be heat treated by soil solarization, but this requires about four to six weeks of continuously hot, sunny weather. Prepare the bed for planting, adding any manure or compost, and then smooth and level it. Moisten the soil 12 inches deep to improve heat transfer; then cover the bed with a close-fitting layer of clear plastic one to four mils thick. Seal the edges with soil to prevent heat and moisture loss.

Expose the Roots.     You can destroy large numbers of nematodes by digging up and exposing plant roots after harvest. Hot sun and drying winds will kill larvae in minutes and egg masses in several hours.

Add Organic Matter.     Thick layers of organic mulch encourages beneficial fungi that parasitize nematode eggs and larvae. The continual active breakdown of organic matter also produces anti-nematode toxins. Alfalfa is an excellent mulch that has the added benefit of growth hormones. Finished compost worked into the soil gives more benefits than any other garden product. Soft rock phosphate, a mined organic derived from fossilized animal remains, applied with compost, would also contribute to nematode resistance.

Use Resistant Cultivars.     A few species of tomatoes labeled VFN have been made resistant to nematodes. But that doesn't mean you can't plant your favorite heirloom tomato. It just means no one has put up the money to test it. Be observant and note any variety that seems resistant. The above-mentioned "N" indicates the plant is resistant to nematodes, but resistance is decreased if crop rotation is not practiced. Sweet corn, asparagus, lima beans and members of the cabbage family are not bothered by root-knot nematodes. Resistant fruit trees are cherimoya, pear, plum, cherry, apricot and apple. Apple trees get nodules on the roots that look like nematodes but it is a woolly-apple-aphid. Peach, nectarine, and almond can be made resistant by using nemaguard rootstock. There is a citrus nematode and according to U.C. research, they can be 90 to 95% eliminated by using enzymatic-acting Pent-a-Vate and fish emulsion. (To one quart of Pent-a-Vate and two and one-half gallons of water, add one pint of fish emulsion. Treats 400 sq.ft.)

Smother Them With Oil and Pent-aVate.     A liquid application of Pent-a-Vate, a yucca derived, enzyme product, applied to the soil and combined with corn oil seems to penetrate the soil and combat nematodes successfully. Use 6 oz. of corn oil to 3 oz. of Pent-a-Vate to cover 100 square feet. Apply every two months.

Antagonistic Plants.     Residues of mustard, asparagus, sweet potato skins, paw paw tree, Amaranthus tricolor, and water hyacinths may be worked in the soil.

Flower Power.     Common annual vinca was tested in India and found to kill virtually all nematodes in the soil. Nematodes are also attracted to the roots of living marigolds, but when they attack, the root releases ozone, and the ozone kills the tiny pests. There is no residual effect from or benefit to planting marigolds and then tilling them in. They must be grown like a cover crop. Susceptible plants remain resistant for two or three years.

Mycorrhizae.     These beneficial fungi attach to the roots of a plant in a protective, symbiotic manner. Threatening root-knot nematodes are enveloped and consumed by the mycorrhizae.

Another Home Remedy.     Try adding a teaspoon of sugar to your planting hole.