By Charlotte Glen,
As you remove summer vegetable plants in preparation to plant fall crops, be sure to check the roots for signs of infection by root knot nematodes. Most summer crops are susceptible to this soil dwelling, microscopic round worm. Above ground symptoms of infection include stunting, dropping of flowers and fruit, low yields, yellow leaves, frequent wilting, and poor growth. If you have observed these symptoms on any of your crops, take a close look at the roots when you pull them up.
If they are infected with root knot nematodes, the roots will appear swollen, bumpy and knotted. Please note: Beans and peas form knots on their roots as a result of infection by a beneficial microbe that helps these crops grow better. Knots caused by this microbe will be pink inside and will appear attached to the root rather than part of the root itself. Finding these knots on bean and pea roots is not a cause for concern.
Roots infected with root knot nematodes will be knotted, bumpy, and swollen.
Managing Root Knot Nematodes
Root knot nematodes can cause serious problem for most summer vegetables and are especially problematic in sandy soils. There is nothing available that will kill root knot nematodes, but they can be managed to keep levels low enough to successfully grow most vegetables.
One of the easiest ways to reduce nematode levels is to grow crops that are not susceptible to attack. These include asparagus, and cool season crops in the cabbage family, such as broccoli, kale, collards, and mustard. For some crops that are susceptible to nematode attack, resistant varieties are available. For example, while most heirloom tomatoes are highly susceptible to nematodes, many hybrid varieties of tomatoes have been developed with nematode resistance, including ‘Amelia’, ‘Celebrity’ and ‘Better Boy’.
For many other crops resistant varieties are not available. To grow these crops in nematode infested soils gardeners have to rely on other practices to manage nematode levels. A practice gaining in popularity is the use of certain cover crops to reduce nematode populations. These include hairy vetch, cereal rye, ‘Pacific Gold’ mustard, and oilseed radish. One of the most promising cover crops for nematode suppression is rapeseed, a relative of mustard and canola. When tilled into the soil, decaying leaves from this crop suppress root knot nematode levels.
Rapeseed, and other nematode supressing cover crops, are seeded in the fall, from late September through late October in southeastern North Carolina.. Seed should be broadcast across the garden. Plants grow through the winter and are tilled into the soil in March. Rapeseed crops have a high sulfur requirement, a nutrient that is commonly deficient in sandy soils. When growing rapeseed for nematode control, be sure to take a sample of your soil to your local Cooperative Extension office for testing to find out if you need to add additional sulfur. If sulfur levels are too low, the rapeseed crop will not be able to generate the organic compounds that suppress nematode populations.
Other practices that reduce root knot nematode levels in vegetable gardens include frequently tilling the soil in the spring and summer to expose nematodes to sun and air, adding compost to the soil, and soil solarization. Gardeners can solarize their soil by tilling and watering the garden then covering it with clear plastic for several weeks in summer. To keep nematode levels down, gardeners will need to employ as many of these practices as possible every season.
Other plants root knot nematodes will also attack include figs, peaches, gardenia, aucuba, Japanese holly, Japanese boxwood, roses, and dogwoods. It is much more difficult to manage nematodes around permanent plantings and often the best option for landscape beds that have root knot nematodes is to remove infected plants and replace them with species that are resistant.
Rapeseed bears golden yellow blossoms in spring.
Testing for Nematodes
Several other species of nematodes that do not produce obvious symptoms on the roots occur in our area. Symptoms that indicate nematode infection include stunting, discolored leaves, poor growth, and thin turf. To determine if nematodes other than root knot nematode are causing problems in your yard or garden, you will need to submit soil samples for testing. Early fall is an ideal time to test because that is when nematode levels are highest.
Testing for nematodes is similar to testing your soil for nutrients, except nematode testing costs $3. Samples should be collected around living plants that show symptoms of possible nematode infection. A total of 2 cups of soil should be collected from the plant’s root zone. Once the sample is collected do not allow it to dry out or get excessively hot. Boxes and forms for packaging samples are available from your local Cooperative Extension office.