As northern alfalfa fields finally reach spring green-up, watch for root rot damage – older stands in wetter fields may be particularly susceptible, warn forage experts. “Dig up some plants and inspect their roots,” says Paul Peterson, University of Minnesota extension forage specialist. “If you see damage, 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, if possible.”

Snow fell on saturated soils in southeastern Minnesota last fall, and alfalfa doesn’t harden as well for winter in wet soil, Peterson says. Wet soil also provides a home for Aphanomyces root rot, which can kill or stunt seedlings. Infected established plants usually have few fine, fibrous and lateral roots and few nodules. Plants turn yellow and don’t regrow well after cutting. Plants infected last fall may suffer increased damage from root heaving. A soil test for Aphanomyces is available through the University of Wisconsin Plant Disease Diagnostics Clinic. Visit www.plantpath.wisc.edu/pddc/.

Extended snow cover this winter and early spring across much of the Midwest and Eastern U.S. may also herald brown root rot, which can be diagnosed incorrectly as winterkill. Usually observed in stands older than one production year, the disease is widespread in Minnesota and Wisconsin, according to an extensive survey by USDA-ARS and University of Minnesota scientists. In Minnesota, USDA-ARS scientists are working to identify alfalfa varieties with resistance to brown root rot. They’re also researching crop management methods to reduce the amount of the pathogen in soil.

The slow-growing fungus, also widespread in New York, Vermont, New Hampshire, Pennsylvania and Maine, can seriously affect the health and persistence of alfalfa in the Northeast, says Cornell researcher Gary Bergstrom. Growers should check for it when fields are slow to emerge in spring or appear to have winterkilled. Dig up and inspect plants in damaged areas using a shovel, water to rinse off soil and a pocketknife for slicing through roots to determine lesion depth. Brown root rot lesions are generally light to dark brown, often with a darker border. Those that girdle the upper taproot or crown kill the plant. Lesions that girdle the lower taproot or affect part of the root or crown reduce plant vigor and slow alfalfa emergence. Above-ground brown root rot symptoms are stunted or dead plants widely scattered throughout fields. Moderately to severely diseased plants may start to regrow in spring, but die before first cutting because the taproot has rotted.

“Absolute confirmation of brown root rot requires a molecular laboratory test,” Bergstrom points out. The Cornell University Plant Disease Diagnostic Clinic charges $40 per composite field sample. Visit plantclinic.cornell.edu/Default.htm to learn more and call 607-255-7850 before submitting samples. To submit a Midwestern sample, contact Debby Samac at Debby.Samac@ars.usda.gov and visit www.tc.umn.edu/~medicago/brr.htm.

Contact Peterson at peter072@umn.edu, or Bergstrom at gcb3@cornell.edu.

Sources: University of Minnesota Crop eNews and Cornell University What’s Cropping UP?.