Answer: There are about 12,000 species of nematodes -- microscopic, unsegmented worms -- some of which are pests to gardeners, others of which are allies.
Pests include root-knot nematodes, which enter plant roots and stems. The presence of developing nematodes in the root stimulates the surrounding tissues to enlarge and produce the galls, which, in turn, interfere with the plant's uptake of water and nutrients. Mature female nematodes then lay hundreds of eggs on the root surface and these eggs hatch in warm soil to continue the life cycle.
To control root-knot nematodes, dig up the roots of affected plants and destroy. This removes a large proportion of the nematodes which would otherwise carry over until the following season. Also, research has shown that French marigold (Tagetes patula) are resistant to root-knot nematode because their roots secrete nematicidal chemicals. This phenomenon can be utilized for nematode control by dividing a vegetable garden into strips or blocks and planting marigolds in some part of the garden each year. Plant the marigolds closely so they develop complete coverage, then till them in at the end of the season. For long-term control of nematodes, the best approach is incorporating lots of organic matter in the soil, to encourage beneficial organisms.
Beneficial nematodes attack pest insects and don't harm plants, and they are harmless to earthworms, too. They are used to control a number of different soil pests. For best results, follow the application instructions carefully.
Q&A Library Searching Tips