Stem nematodes on alfalfa, an enemy

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

A few years ago, as we experienced a cool, wet spring in Michigan, a sample collected from an alfalfa field on campus was submitted to MSU Diagnostic Services. The plants were stunted and had very short internodes. High numbers of stem nematodes, Ditylenchus dipsaci, were recovered from the leaves and stems of these plants. For some reason, this spring made me think of that diagnosis because it was the first time in my 22-year career as a nematode diagnostician I had recovered these nematodes from alfalfa. However, D. dipsaci, is a very common nematode in our state and is a serious pathogen of some vegetables and many landscape plants. I worry it may rear its ugly head this growing season.

The alfalfa stem nematode, D. dipsaci, is regarded as the most serious nematode pathogen of this forage crop. It has been reported from all areas where alfalfa is grown. It tends to be more of a problem in fields with heavy soils, on irrigated sites, or in seasons with heavy spring rains. Red and white clovers are also hosts to D. dipsaci.

The alfalfa stem nematode is spread by infested seed, irrigation water and machinery contaminated with infested crop debris and soil. Water is necessary for new infestations because the nematodes move in water films on stems and leaves. Losses result from weakening and death of plants. Infested stands may decline in less than two years. In addition, these nematodes can render alfalfa more susceptible to bacterial wilt as well as wilts caused by Fusarium and Verticillium.

There are no control options available in established alfalfa stands for D. dipsaci. Host resistance is one practical method for controlling this nematode. Rotation to non-host crops, such as corn and small grains for two to four years, is required to reduce population densities of the alfalfa stem nematode.

It is important to identify fields infested with nematodes so they can be successfully contained. While examining alfalfa stands this spring, if any appear unthrifty and the plants have very shortened internodes, you should collect leaves and stems for nematode analyses and send them to MSU Diagnostic Services. There is a $25 fee for a nematode analysis and typically results will be available in seven to 14 days.

As an added note, stem nematodes typically occur at very low population densities in the soil. They prefer to stay in old plant tissue until they locate new hosts. Therefore, it is critical, for proper diagnoses of this particular nematode, to collect leaves and stems from actively growing plants to avoid potential false negatives that may result from only submitting soil or even roots to the lab.

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