root knot nematodes in potatoes

Asked May 30, 2014, 10:07 PM EDT

My potatoes have small pimples on the skin which I am told is caused by root knot nematodes. Are the potatoes damaged this way still edible?

Walker County Texas fruits and vegetables vegetables gardens horticulture

1 Response

Your description sounds like the traditional explanation of nematode damage on potatoes. A positive identification of the problem can be determined by the Texas Plant Disease Diagnostic Lab. If it is nematodes, the potatoes are edible if peeled. “Above ground symptoms are similar to many other root diseases or environmental factors limiting water and nutrient uptake. These symptoms consist of wilting during periods of moisture stress, stunted plants, chlorotic or pale green leaves, and reduced yields. Most characteristic symptoms; however, are those occurring on underground plant parts. Infected roots swell at the point of infection and form knots or galls.” Control

  1. Crop Rotation – a three or four year rotation program with resistant crops is an effective program. Most of the cereal crops are fairly resistant. A little more detail on crop rotation for vegetable gardens can be found at:
2. Resistant or Tolerant Varieties – Some vegetable and field crop varieties have resistance to root knot nematodes, and are advertised as such. Reaction of several ornamental plants is given in a table at the end of the Shrubs section on root knot disease.
  1. Clean Summer Fallow – dry summer fallow with cultivations every 3 to 4 weeks is an effective method of reducing nematode populations. This method may be impractical in some instances.
  2. Selection of Planting Stock and Planting Sites – Select transplants free of root knots. Plant roots should be washed and carefully inspected for signs of nematode injury. Select planting sites free of nematode infestations. An indicator crop of tomatoes, okra or other susceptible plants could be grown in the area if you do not know rotation histories. Submit soil samples to the Plant Nematode Detection Laboratory for analysis.
5. There are no labelled chemical nematacides for home owner use at this time. A few other recommendations include specifically; rotation with non-susceptible vegetable crops. “Sweet corn is a poor host and is good to use in a rotation, especially in an area where root knot has done severe damage. Onions, garlic, asparagus, and shallots are also poor hosts. Cool-season crops such as cabbage, Irish potatoes, greens (turnips), radishes, and broccoli are less likely to suffer yield loss from root knot nematodes. Even though these are susceptible plants, they grow best in the cooler time of the year which is not favorable for root knot nematode development.” “Root knot resistant vegetable varieties are not plentiful. Fortunately, progress has been made in the development of root knot resistant tomatoes. The best resistance is found in the hybrid varieties that have been developed in recent years. Root knot resistant varieties are noted in seed catalogs by “N” following the variety name. Preceding the “N” are often “V” and “F.” “VFN” stands for verticillium wilt resistance, fusarium wilt resistance, and nematode resistance. Hybrid tomato seed is more expensive than open-pollinated seed, but the benefits in disease resistance alone are enough to make it worth buying.” In addition to the dry summer fallow option, planting a cool-season crop of Elbon Rye (cereal rye –not rygrass) in the garden is a helpful method of reducing the nematode population. This crop of rye needs to be planted solid, then mowed and tilled into the soil 30 days prior to planting nematode resistant spring garden vegetables. One more option includes the use of marigolds as a trap crop if done properly. “Some people think that marigolds secrete a toxic substance into the soil that kills nematodes and that planting a few marigolds around annual plants in infested soil will prevent infection. This is not true. Marigolds primarily act as a trap crop. Nematodes are able to enter their roots but are unable to complete their life cycle. The trapped nematodes die without reproducing. The type of marigold is also important. French marigolds, Tagetes patula, are more effective in controlling root knot nematodes than the African marigold, Tagetes erecta, which is also referred to as the American, Big, or Aztec marigold. To be effective, marigolds must be planted as a solid crop and grown for 90 to 120 days to effectively reduce the nematode population sufficiently to grow annual plants without treatment. Marigolds should be planted in rows no further than seven inches between each plant, so that the roots penetrate the entire soil mass to trap as many nematodes as possible. If marigolds are planted close together, they form a dense canopy which helps retard the development of weeds and grasses. Many weeds and grasses serve as hosts for root knot nematodes. If the weeds are not controlled, the marigolds may be unable to suppress the nematode population. Using marigolds in a manner other than that recommended can invite disaster. For example, planting a few marigolds here and there among tomatoes will encourage spider mites. The spider mite is one of the most difficult garden pests to control and can become nearly as serious a problem as the nematodes.”