Early snow cover that stayed through winter into early spring across much of Minnesota could be a mixed blessing for alfalfa persistence. The positive side to consistent snow cover is the insulation it provides alfalfa crowns from cold ambient air temperatures. The negative side, however, is that extended snow cover may favor development of brown root rot.

In addition, following abundant fall rain, the snow settled on saturated or nearly-saturated soils in some areas, particularly in southeastern MN. Alfalfa does not harden as well for winter when soil is water-saturated. Saturated soils also favor ponding, ice sheet formation, heaving, and Aphanomyces root rot.

Brown root rot is most apt to be observed in stands older than one production year, because the fungus grows slowly and depends on specific environmental conditions for symptoms to develop. Root infection by the fungus is thought to occur during fall or spring. When it occurs during fall, it can severely weaken susceptible plants and contribute to winter kill. Aboveground symptoms of brown root rot are stunted or dead plants that are widely scattered throughout fields. Taproots will often have brown, sunken lesions. Moderately to severely diseased plants may start to regrow in spring but die before the first cutting is harvested because the taproot has rotted. When this happens, the remaining root system is too compromised to support plant growth.

Aphanomyces root rot typically occurs in areas that are poorly drained. This disease can kill and stunt seedlings. Established plants infected with Aphanomyces root rot usually have few fine fibrous and lateral roots and few nodules. Infected plants appear stunted, do no regrow well after cutting, and the foliage is yellow. Plants that were infected last fall with Aphanomyces root rot can suffer increased damage due to root heaving, which occurs during winter freezing and in cycles of spring freezing and thawing, especially on heavy clay soils.

So be optimistic, but periodically check spring green-up of your alfalfa stands closely, particularly older stands in wetter fields. And dig up some plants and inspect their roots. If you see damage, don’t over-react, but thoroughly consider your options. The economics of maintaining damaged alfalfa stands is different this year than most due to limited hay/haylage supplies and high commodity crop prices. Unless the stand is severely damaged (well below 40 stems per square foot), it may be wise to keep it for at least a first cutting.

For further information about alfalfa stand assessment, alfalfa root rot diagnosis, and management options, visit:

• http://www.extension.umn.edu/cropenews/2007/07MNCN21.htm
• http://www.extension.umn.edu/cropenews/2007/07MNCN14.htm
• http://learningstore.uwex.edu/pdf/A3620.pdf
• http://www.tc.umn.edu/~medicago/brr.htm
• http://pdc.umn.edu

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