How Can I Control Root-Knot Nematodes?

Do you want to prevent nematodes by using proper sanitation in your garden?

Do you want to manage root-knot nematodes with resistant or tolerant varieties and rootstocks?

Do you want to prevent root-knot nematodes by fallowing and crop rotation?

Do you wish to use soil solarization to temporarily reduce nematode populations in the soil?

Do you want to reduce nematode injury to fall-planted crops by planting at the right time?

Do you want to plant nematode-suppressive marigold varieties?

Do you want to reduce the effects of nematode injury by soil amendments and irrigation?

Nematodes usually are introduced into new areas with infested soil or plants. Prevent nematodes from entering your garden by using only nematode-free plants purchased from reliable nurseries. To prevent the spread of nematodes, avoid moving plants and soil from infested parts of the garden. Don’t allow irrigation water from around infested plants to run off, as this also spreads nematodes. Nematodes can be present in soil attached to tools and equipment used elsewhere, so clean tools thoroughly before using them in your garden.

One of the best ways to manage nematodes is to use vegetable varieties and fruit tree rootstocks that are resistant to nematode injury. Tomato varieties with the code VFN (Verticillium, Fusarium, Nematodes) on the seed packet or label are resistant to common root-knot nematode species. For fruit trees and vines, Nemaguard rootstock used for stone fruit and almond trees and Harmony and Freedom rootstock used for grapes provide protection against root-knot and other nematodes. Consider replacing severely infested plants with plant species and varieties that are more tolerant of the nematodes present. Unfortunately, resistant varieties aren’t available for many crops and ornamentals.

Growing a crop on which the nematode pest can’t reproduce is a good way to control some nematodes. Unfortunately, rotation isn’t as easy for controlling root-knot nematodes, because so many vegetable crops and weeds are hosts of the pest. However, with careful planning, rotation in combination with fallowing and solarization can reduce root-knot nematode numbers. Annual crops that are useful in a rotation plan for reducing root-knot nematode populations include small grains such as wheat and barley, sudangrass, and resistant tomato and bean varieties.

You can use solarization to temporarily reduce nematode populations in the top 12 inches of soil, which allows the production of shallow-rooted annual crops and helps young woody plants become established before nematode populations increase. However, solarization won’t provide long-term protection for fruit trees, vines, and woody ornamental plants. For effective solarization, moisten the soil, then cover it with a clear, plastic tarp. Leave the tarp in place for 4 to 6 weeks during the hottest part of summer. Root-knot nematodes, including eggs, die when soil temperature exceeds 125°F for 30 minutes or 130°F for 5 minutes.

Most nematode species are active during warm summer months and can’t penetrate roots at soil temperatures below 64°F. Therefore, you can reduce nematode injury to fall-planted crops such as carrots, lettuce, spinach, and peas by waiting until soil temperatures have dropped below 64°F. Plant summer vegetables as early as possible in spring before nematodes become active.

Certain marigolds suppress root-knot nematodes. French marigolds (varieties include Nemagold, Petite Blanc, Queen Sophia, and Tangerine) are most effective. Avoid signet marigolds, T. signata or tenuifolia, because nematodes will feed and reproduce on these. Marigolds don’t work well against the northern root-knot nematode, Meloidogyne hapla, a species common in areas with cool winters. The effect of marigolds is greatest when you grow them as a solid planting for an entire season. When grown along with annual vegetables or beneath trees or vines (intercropping), nematode control usually isn’t very good.

You can add various organic amendments to the soil to reduce the effect of nematodes on crop plants. The amendments—which include peat, manure, and composts—are useful for increasing the water- and nutrient-holding capacity of the soil, especially sandy soils. Because nematodes more readily damage plants that are water-stressed, increasing the soil’s capacity to hold water can lessen the effects of nematode injury. Likewise, more frequent irrigation can help reduce nematode damage. In either case, you will have just as many nematodes in the soil, but they will cause less damage.

Currently no chemical nematicides or soil fumigants are available to home gardeners for nematode control in backyard gardens and home landscapes.