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Root-knot nematodes in gardens of mid-Missouri

Root-knot nematodes are microscopic worms that can infect vegetable roots causing serious symptoms and producing galls or "knots" on the roots. The nematodes impede the flow of water and nutrients to the plant.

2008 Root-Knot Nematode Survey for Increased Awareness of a Threat to Urban Gardens in Missouri

Tomato with healthy roots

Clean root.

Tomato with root knot nematodes

Infected root.

Symptoms

  • Plants show less vigor, grow more slowly, show yellowing, wilt easily in the heat of summer, often stunted and produce less and smaller fruits.
  • The root-knot nematode has a very wide host range (over 2,000 hosts) and thus can reproduce on most plants. Broccoli and cauliflower are two non-hosts.
  • Root-knot is usually not a problem in mid-Missouri because the winter temperatures are too cold for them to survive. Warmer winter temperatures have changed this. A week of subzero temperatures with no snow on the ground could freeze the soil down deep enough to eliminate this pest.

Detection

Do I have this nematode in my garden?

  • Root-knot is easily diagnosed by simply looking at the roots of plants when cleaning up the garden in the fall. Pull or dig up roots and look for galls on the roots. These will often be up to an inch in diameter. They may be starting to deteriorate, but should be visible. If found, note which parts of the garden are most infected. Care should be taken not to spread the nematode from infested areas to non-infested areas.
  • The presence and level of infestation of root-knot can be confirmed by sending a soil sample to the Plant Nematology Laboratory, 23 Mumford Hall, Columbia, MO 65211. There is a $30 fee for this test.

Sources

How did it get there?

  • Root-knot can only enter a garden by soil or plant roots. Infected vegetable transplants, garden tools (including tillers or tractors) with infested soil, muddy boots or vehicle tires, improperly or incompletely composted material are all possible sources. Thousands of root-knot eggs or worms may be present in a tablespoon of soil.

Prevention

Can root-knot nematode infestation be controlled in a garden? Yes, it can be but it is not that easy.

  • If no galls or "knots" are found on the roots of your garden plants in the fall, great care should be taken not to introduce it. Good garden sanitation will keep the root-knot nematode out. Always clean garden equipment if it has been used in another garden. Buy garden transplants from a reputable dealer. Do not add compost or soil to your garden unless you are sure that it is not infested with root-knot.
  • If galls or "knots" are found on your plant roots, remove as much of this root material as possible from the garden. These roots should not be composted. They may be dried and burned or bagged and properly disposed of.
  • An infested garden should be kept clean of grass or garden debris to allow the cold winter temperatures to penetrate the soil. Winter tilling (if possible) could help facilitate the penetration of cold temperatures by turning over and loosening the soil. Or a winter cover crop of rye or canola (Humus or Dwarf Essex) when plowed under in March and allowed to decompose before planting has been known to produce a substance that is toxic to nematodes.
  • Since the root-knot nematode does not become active until the soil temperature reached 64 degrees F, early spring vegetables may avoid damage. Also, planting summer vegetables as early as possible will give them a good start before the root-knot nematode becomes active.
  • Planting nematode resistant plants (VFN) is a good way of controlling root-knot, especially if their numbers have not become excessive. There are many varieties of resistant tomatoes, but not many choices with other vegetables. After two years of resistant plants, the number of root-knot nematodes should be low enough to plant a susceptible variety again. A season of susceptible plants, though, will allow the root-knot population to again become high.
  • Keeping part of the garden fallow will reduce the numbers of root-knot nematodes because they will not have a host on which to feed. The fallow section must be kept free of weeds to be effective. Covering the fallow section with black plastic may help.
  • Soil solarization can control root-knot nematodes. Soil temperature that reaches 125 degrees F for 30 minutes can kill root-knot. The garden section to be treated should first be tilled and moistened well. Then it should be covered with clear plastic (2 to 4 millimeters) for the months of June to August.
  • There are some anti-nematode plants, such as French marigolds, whose roots produce a substance that can be toxic to root-knot. (Not all marigold varieties have this quality.) Some varieties that do are: bonita mixed, gypsy sunshine, scarlet Sophia, single gold, petite harmony, petite gold, tangerine, crackerjack, and flor de muerto. These marigolds should be planted seven inches apart over the entire section of the garden to be treated, and it must be kept weed free. Planting these marigolds mixed in with susceptible garden vegetables is not very effective.
  • Rotating sections of the garden between: resistant varieties, fallow, solarization, marigolds, and susceptible varieties can usually control the root-knot nematode.
  • Sometimes, if there is the availability of land, it is easier to start a fresh garden spot, and the infested one simply planted to grass. Great care must be taken not to infect this new garden space.
  • Agricultural nematicides and other chemicals or fumigants are neither recommended nor labeled for home gardens. There are some organic substances developed from plants that have been used in gardens for the control of nematodes, but often with limited success. There have also been some microbial pathogens (bacteria and fungi) developed into commercial formulations against nematodes, again with limited success. These can easily be explored on the internet for those interested.

Updated 1/6/14