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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 Tomato with root knot nematodes

Clean tomato root, left. Root-knot nematode infected tomato root, right.

Symptoms

  • Plants can be less vigorous, stunted, chlorotic and can wilt easily in the heat of summer, grow more slowly and produce less and smaller fruits.
  • The root-knot nematode has a very wide host range (more than 2,000 hosts) and thus can reproduce on most plants. Broccoli and cauliflower are two nonhosts.
  • 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. Gently pull or dig up roots and look for galls or knots on the roots. These galls vary in size from a few centimeters to up to an inch in diameter. If found, note which parts of the garden are infected. Care should be taken not to spread the nematode from infested areas to noninfested 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. Please fill out a Nematode Soil Sample Submission Form and select plant-parasitic nematode identification test.

Sources

How did it get there?

  • Root-knot nematodes predominately are introduced into gardens by infected soil or plant roots, infected vegetable transplants, dirty garden tools (including tillers or tractors) with infested soil, muddy boots, vehicle tires, or improperly or incompletely composted material. Thousands of root-knot nematode eggs or worms might be present in a tablespoon of soil.

Prevention

Can root-knot nematode infestation be controlled in a garden? Yes, but not easily.

  • 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 nematodes.
  • 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 can 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. Winter cover crops of rye or canola (Humus or Dwarf Essex) when plowed 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 to establish larger root systems before the root-knot nematode becomes active.
  • Planting nematode resistant plants (VFN) is a good way of controlling root-knot nematodes, 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 to feed on. The fallow section must be kept free of weeds and old roots to be effective. Covering the fallow section with black plastic may help.
  • Soil solarization is another way to reduce root-knot nematode populations. Soil temperatures that reach 125 degrees F for 30 minutes can kill root-knot nematodes. The garden section to be treated should first be tilled and moistened. Then it should be covered with clear plastic (2 to 4 millimeters) for the months of June to August.
  • There are some biofumigant plants, such as French marigolds and mustards, whose roots produce a biochemical substance that can be toxic to and kill root-knot nematodes. However, not all mustard and marigold varieties act as a biofumigant. Some varieties that do are: bonita mixed, gypsy sunshine, scarlet Sophia, single gold, petite harmony, petite gold, tangerine, crackerjack and flor de muerto marigolds. 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, biofumigants and susceptible varieties can usually control root-knot nematodes.
  • Sometimes, if land is available, it is easier to start a fresh garden spot, and plant the infested garden with grass.
  • 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.

For more information, see G6204 Managing Nematodes in Gardens.

Updated 5/12/15