Early Warning Devices Available For Nematodes
Check soil for root-knot nematodes by planting cucumber seeds in a small pot of suspect soil. After the first true leaves appear (not the cotyledons), carefully wash the soil away from the roots. If nematodes are present, you will see small beadlike knots on the roots.
Another test: Collect soil samples at a depth of 3 inches below the surface from various sites in the garden. Mix these together and fill 4 small pots with the mix. Freeze two of the pots for 48 - 72 hours, to kill any nematodes. Then plant radishes in all 4 pots. Examine seedlings after 6 days--if the radishes (which are quite sensitive to nematodes) do better in the previously frozen soil than those in the unfrozen soil, you may have nematodes in your garden soil.
Using No Pesticides For Pest Nematodes
Trap Crops For Nematodes Castor beans ( Ricinus spp.) and French and African marigolds are excellent trap crops for nematodes.
Plant early in the season. Use resistant varieties wherever possible. Space plants so that the leaves do not touch. Avoid wetting foliage, since nematodes swim up the stems and infest the leaves through the stomata (breathing pores) only when the plants are wet.
Seaweed Extract Drench or Spray
Whether used as a foliar spray or as a soil flush, seaweed extract will reduce root-knot nematode infestations of ornamental plants. Natural hormones in the seaweed, called cytokinins, help the plants increase their resistance to nematode invasion. Fewer larvae penetrate the roots, and those that do are inhibited in their development. As a side benefit, the seaweed extract produces bigger roots and stems and better yields. Mix 1 tablespoon of liquid seaweed in 2 gallons of water.
Crops fertilized with fish emulsion suffer less nematode damage than those fertilized in other ways. It is suspected that some component of the fish oil may be toxic or offensive to the nematodes. A combination of fish emulsion and a by-product of the yucca cactus is even more effective at knocking out nematodes. In tests with citrus trees, a mixture of 70 percent fish emulsion and 30 percent yucca extract reduced root-knot and pin nematode populations by up to 90 percent.
An effective, nontoxic soil drench can be made with ordinary corn oil and an extract of the yucca plant (the latter is available commercially as Pent-A-Vate). The oil is thought to act as a nematocide. The Pent-A-Vate acts as a wetting agent, allowing the oil to penetrate the soil. To make the drench, mix one part Pent-A-Vate with ten parts water. Mix four parts of this with one part corn oil, and sprinkle it on the soil around the plants. Then water it in with a hose.
Traps and Mechanical Controls
Kelp Meal Trap - Certain soil fungi that kill root-knot nematodes will be stimulated by the presence of kelp meal in the infested soil. That stimulation will increase the fungi's lethal effectiveness against the nematodes. Add about 1 pound of meal for every 100 square feet of garden. This treatment is particularly effective if combined with the addition of compost.
Leaf Mold Compost- Extensive research at the Connecticut Agricultural Experiment Station has shown that leaf mold composts can suppress populations of harmful nematodes. Fatty acids, similar to those used as pest control agents, are produced by the compost and regulate nematode populations. For example, decomposing rye and timothy grasses have been shown to release the fatty acids that control some species of parasitic nematodes. For best results, the compost should be mixed into the garden soil. Many plant leaves are toxic to nematodes, and laboratory tests have shown pine needles to be particularly potent.
Add Organic Material - Make sure the soil has plenty of organic matter, which encourages nematode predators, such as springtails and carnivorous fungi. Rich soil may also stimulate root growth, which lessens the effect of nematode damage. Decomposing organic matter may generate nematode-toxic compounds. Soil amendments such as leaf mold, grass clippings, castor pomace (a by-product of castor oil production), and manure have been known to suppress nematodes under the right conditions.
Dipping Infested Plants - While not tested on all plants, some ornamentals in pots that are plagued by nematodes can be saved by dipping them in hot water. Begonias, for example, in small pots can be submerged for 1 minute at 120 to 121°F or for 3 minutes at 117 to 119°F. Soak infested tuberous begonia tubers while dormant in hot water at 120°F for 30 minutes. Cool and plant in clean soil. Bird's nest ferns and other fern species have been freed of nematodes by immersing them in hot water at 110°F for 10 to 15 minutes.