   Root Knot Nematodes - July 7, 1999 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County Nematodes are microscopic worms that live in the soil. Some nematodes are beneficial in the garden because they attack destructive insect larvae. Other nematodes attack roots, bulbs, leaves, and flowers of plants. These nematodes are parasites that use your garden plants as hosts. They are always difficult to manage. Nematode biology (nematology) is a distinct science unto itself. As with other studies in biology, the more we learn about nematodes, the less we really know about them. The plant parasitic nematode life cycle consists of six stages: egg, four immature stages, each of which is followed by a molt (shedding of skin), and the adult stage. Many species can go from egg to egg-laying adult in as little as 21 to 28 days. In Arizona, the most common plant parasitic nematode is the root knot nematode. You may have noticed swollen portions on tomato (and other broadleaf garden plant) roots pulled up at the end of the growing season. These galls are most likely caused by root knot nematodes. On okra and fig, the galls can be up to one inch in diameter. Most often, the galls are the size of a pea and are not easily confused with other plant diseases such as crown gall. The galls are also more persistent than nitrogen-fixing nodules found on the roots of legumes (beans, peas, alfalfa, etc.). After hatching, root knot nematodes invade the root tips causing abnormally large cells to grow at the point of attack. These cells are where the nematodes feed. As feeding continues, the plant produces a gall around the infected area. The male departs the root while the female produces an egg mass that is attached on to the outside surface of the root. It is believed that root knot nematodes survive in the egg stage when crops are out of the ground. Populations then decrease by 80 to 90% in unplanted areas in winter. When winter weeds are allowed to grow, the nematodes survive the winter and come back more rapidly the following year. Root knot nematodes also prefer sandy soils in warm regions. The root damage cause by nematodes will reduce the individual plant's ability to take up water and mineral nutrients. Symptoms arising from an infestation are slow growth, stunting, wilting during the hottest part of the day, and slight yellowing of the leaves. Heavily infested crop plants may die before they mature. Woody plants can also be affected. However, the symptoms are usually slower to appear. Here, reduced vigor, slow growth, decreased size of leaves and fruit, and early fall season leaf drop may be evidence of root knot nematodes. The difficulty with woody plants is in diagnosis before they die. In either case, if a fresh sample is collected, the female nematode may still be present in the gall when it is cut open. Otherwise, the plant may be dead and nematodes gone before you can save the crop, tree, or vine. Once diagnosed, there are some management strategies that can help control root knot nematodes. First, it is a good idea to have the nematode species identified. If you have reason to suspect nematodes, your local University of Arizona Cooperative Extension office can help you locate a lab that can identify species and test the soil for population levels. Combinations of treatments have been most effective in controlling root knot nematodes. Some crop varieties and rootstocks are tolerant or resistant to root knot nematodes. For instance, tomatoes labeled VFN are resistant. Practices include dividing your garden into several sections and alternate crop species with fallow periods in these sections. Solarization can be employed to sanitize soil in infested areas. This means covering the soil with clear plastic and allowing the sun to heat the soil outside of the range of tolerance of the nematode species. French marigolds are also said to reduce nematode populations although the research evidence is lacking to support this. In orchards, certain cover crops can discourage nematodes. There are no chemical nematicides available to non-licensed home gardeners. We have much more to learn about these damaging pests. However, research is always giving us new alternatives to control them. The University of Arizona Cooperative Extension has publications and information on nematodes and other garden pests. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number. |