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Parasitic Roundworms may Infest Alfalfa

Written by Jeremiah Vardiman

Are you using resistant cultivars in your alfalfa pest management plan?

Utilizing resistant cultivars is the primary control method available for one particular pest – alfalfa stem nematodes. Alfalfa stem nematodes cause major yield losses to irrigated alfalfa fields in most of Wyoming, especially in fields with high clay soils,  or those with greater than 30 percent clay, such as the Big Horn Basin.

Dryland alfalfa producers do not have problems with this aquatic parasite because it primarily spreads by irrigation water. Planting resistant cultivars can also reduce the impacts or the likelihood of dealing with this pest.

How can one tell if an alfalfa field has alfalfa stem nematode? Stunted plants scattered throughout a field and/or areas of stunted plants are the most visual and common symptoms. This stunted symptom gives the field an uneven appearance, where healthy mature plants are adjacent to small, stunted plants.

The unevenness becomes more noticeable as the nematode infestation expands during the season. These infested plants are also predisposed to winter injury because the nematode interferes with essential nutrient storage that prepares the plant for winter. On top of the weakened state, these plants can also be vulnerable to bacterial wilt, even if it is a wilt-resistant cultivar.

Alfalfa stem nematodes are a microscopic aquatic indoparasitic round worm, which means the round worm lives inside the alfalfa plant and feeds off the nutrients produced by the plant. The nematodes are spread by three main sources – surface water from infested fields, infested hay and seed and infested soil particles attached to equipment or animals.

The nematode begins the parasitism in young, developing alfalfa stem buds near the soil surface after an irrigation or rain event. There are male and female nematodes that reproduce inside healthy alfalfa stem tissues, with a single female laying up to 500 eggs. The eggs hatch, releasing juvenile worms into the stems, buds, leaves, crowns and upper roots of alfalfa plants.

There are four developmental stages for the juvenile nematodes prior to becoming adults, with the complete cycle – from egg to adult – taking 19 to 23 days. The fourth-stage juvenile is unique in its ability to survive in dried tissue for up to 10 years and to survive Wyoming winters.

What management options are available?

Prevention is the best level of control – do not let nematodes infest the field. This can be managed by proper cultural practices of planting resistant cultivars, especially if there is a known nematode history in the area; crop rotation; proper management of cutting and irrigation; and fall burning.

For further information on resistant cultivars, refer to the University of Wyoming Extension bulletin “Guide for Selecting Alfalfa Varieties with Disease Resistance for Wyoming” and request resistant varieties from seed companies.

If alfalfa stem nematodes are suspected in a field, confirm the symptoms by a plant diagnostic lab prior to making management changes. If nematodes are present, harvest fields when the top few inches of soil are dry and irrigate once regrowth reaches six to eight inches.

Burning fields in the fall to remove infested tissue can aide in reducing infections. Pesticides, known as nematicides, are limited and only suppress the nematodes. They do not kill them. Rotating non-host crops, such as corn, barley and wheat, for two to three years can significantly deplete soil populations of alfalfa stem nematodes. However, reintroduction is always possible once planted back to alfalfa.

Making resistant cultivars a part of the pest management plan for an alfalfa field can protect against yield loss to specific diseases and pests, such as alfalfa stem nematode. The next time the alfalfa field is renovated, spend some time researching what resistant varieties are available and to what disease and pest the variety is resistant. This is time well spent.