Controlling Nematodes | Archives | Aggie Horticulture

If you have nematodes in your garden sooner or later they will decrease or end production. Nematodes will severely damage all garden crops except corn, onions, garlic and nematode- resistant tomatoes. Even these nematode-tolerant crops will be adversely affected if a severe nematode population is present.

How does one know if nematodes are a problem in their garden? Above ground symptoms of nematode infestations are similar to many other root diseases or environmental factors limiting water and nutrient uptake. These symptoms consist of wilting during periods of moisture stress, stunted plants, chlorotic or pale green leaves and reduced yields. Most characteristic symptoms are those occurring on underground plant parts. Infected roots swell at the point of infection and form knots or galls. Several infections may occur along the same area resulting in large fleshy galls. The appearance of the galls will depend in part upon the plant being affected and the nematode species involved. Fast growing annuals will have a large fleshy gall and woody perennials, small hard galls. Infected roots are retarded in growth and lack fine feeder roots. Rotting of roots develop late in the season. When tubers, corns or other edible root portions are infected, small swellings or pimpling is evident on the surface.

Vapam was the most effective nematicide available. Unfortunately, the chemical control for nematodes named Vapam is no longer available. Organically, only the planting of cereal rye (Elbon) in the fall to grow during the winter will decrease nematode populations. Excessive drying of the soil during July will also help. Another possible solution may be the solid planting of French (small blooms) marigolds for 3 months in areas heavily contaminated with nematodes. The French marigold, when grown on soil infested with nematodes, suppresses the population of these nematodes and reduces the numbers found in the roots of susceptible host plants. Three compounds of an a-terthienyl type, toxic to nematodes, have been identified in root exudates from these plants. Terthienyls are released from growing roots, even without their decay, but benefits require three to four months to become clear. There is some evidence that a-terthienyl is inhibitory to some plant-pathogenic fungi too. Marigolds also function as a trap crop since larvae which penetrate the roots do not develop beyond the second larval stage and do not lay eggs.

French marigolds have never been used for a biological nematode control because Vapam was less trouble-some and more economic. Since Vapam is no longer available and since cereal rye does not survive in hot weather, use of the marigold for nematode control must now be examined. Interested gardeners who have a nematode infested area should transplant marigolds 12 inches apart in May and allow them to grow there until fall planting begins in August. Tops should then be removed and the root system tilled into the soil. Spider mites may have to be periodically controlled with Kelthane (Red Spider Mite Spray) or a preventive insecticide application of disyston (Systemic Insecticide) be applied after planting.

1. Q. Would it be advisable to grind up plants and roots infected with nematodes for the compost pile? Would this transfer the nematodes back into the soil when compost is put into the garden?

A. Be generous - - give your nematodes to the garbage man! If you could be sure that nematodes would be subjected to the inferno of a properly heating compost pile, then the procedure that you outline would be acceptable . However, considering that most gardeners do not compost, they rot, and since most compost piles never attain the inner temperature of over 120 degrees F., which they should, the nematodes would not be destroyed.

2. Q. I planted cereal rye (Elbon) in my vegetable garden last November. It grew fantastically! Maybe even too good since I am having troubles with all of the roots when I till the soil. How can I decompose these roots faster?

A. Shred the tops and till cereal rye into the soil one month before planting occurs. The one month of decomposition is necessary to avoid the root matting which you are describing. The process cannot be hurried. To give you some idea of how much organic material is added to the soil by rye, consider these facts about a cereal rye plant grown in a cubic foot of soil for four months: (1) It has 13,800,000 roots, (2) the total length of the roots is 385 miles, (3) the surface area of the roots is 2,550 square feet, (4) it has 14,000,000,000 root hairs, (5) the length of the root hairs is 6,600 miles, (6) the surface area of the root hairs is 4,320 square feet.

3. Q. This fall I planted the cereal rye (Elbon). I am interested in controlling soil nematodes and adding organic material to my garden. It is recommended to shred the cereal rye and rototill it into the soil at least 30 days before planting occurs. My problem is that the garden is too wet to work. What should I do?

A. You have two choices. You can wait until it is dry enough to shred and rototill the rye into the garden soil. The second choice is to mud in some of the early planted vegetables. These include Baccus broccoli and onions; be sure to use one-half cup of superphosphate per 10 linear feet of row beneath plants. If you want to be strictly organic, simply shred the rye to the ground with a flexible string trimmer and plant through the stubble after making a trench with a shovel. The rye will begin to re-grow but should be manageable until the heat of summer kills it. Several days before planting, I apply a glyphosate herbicide (Roundup, Weed Away, Kleanup) to the strips of rye in which I want to plant. This will avoid a later sprouting problem, does not damage transplants and ends nutrient competition from growing rye. In commercial plantings, tall rye is sprayed with glyphosate and left standing to provide a wind break. Either of these solutions to managing cereal rye will work but will not make for the prettiest garden you have ever had. This is not the ideal way but it is the only way, given a wet weather condition.

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