University of Vermont

Brown Root Rot in Alfalfa Brown Root Rot of Alfalfa - A New Disease for Vermont?

Ann Hazelrigg, Extension Lecturer, Department of Plant and Soil Science, University of Vermont

Brown root rot of alfalfa caused by the fungus Phoma sclerotiodes, was detected during the 2003 growing season in the Miner research plots across the lake in Chazy, NY.  This disease has been reported in several Northern states and Canada and has been associated with stand decline of alfalfa and other forage plants. Unfortunately, the fungus is very well adapted to the cold temperatures common in our Vermont winters and springs.  Additionally, this pathogen not only attacks alfalfa but also infects red clover, sweet clover, white clover, bird’s foot trefoil and alsike clover.

The pathogen overwinters on crop debris and can survive many years in dry soil.  The root rotting fungus can be spread by wind, machinery or grazing animals.  Symptoms of the disease do not usually appear until 3 years after seeding. Infected plants begin growth in the spring but start to lose vigor as the time for the first cutting approaches.  The first tissues to show discoloration and rot are the feeder roots and nitrogen fixing nodules.  Symptoms vary from small circular dark lesions to large rotted taproots.  Crowns and lower stem tissues can also be infected although the disease is mainly a root pathogen.

Fruiting bodies of the fungus may be present as tiny black dots on the root surfaces.  Infection and decay take place in the late fall through early spring (most root infection likely occurs between 35-40 degrees F.) while the plants are dormant.  The fungus slows its growth during the summer’s warmer temperatures.  Winterkill, a symptom of brown root rot that is generally noticed in early spring, can vary from slight to severe.

Positive identification of brown root rot usually requires microscopic observation and fungal isolation from infected tissue.  Recently, a molecular test has been developed so researchers can determine if plants are infected within a matter of hours.

Managing the disease includes using crop rotation with non susceptible crops, providing appropriate fertility, avoiding late cuttings and selecting varieties less susceptible to winter injury.

Brown root rot may already be an undiagnosed problem in Vermont alfalfa fields.  In 2004, UVM Extension would like to do a systematic survey of fields to determine the presence of the pathogen and its potential impact on alfalfa stand decline.  If you have noticed any of the symptoms of the disease or if you have fields with severe winter damage or stand decline, please contact Ann Hazelrigg at 656-0493 or  Cornell researchers have kindly offered to help with diagnosis of Vermont alfalfa fields to see how widespread the disease may be. We will report the results of our testing in a future newsletter.

Send intact alfalfa roots with suspicious lesions to:

 Plant Diagnostic Clinic
 Hills Building
 105 Carrigan Drive, UVM
 Burlington,VT 05405

Include 8-10 plants including crown in a zip-lock plastic bag with moist  towels.  Precise location of the field where you collected plants is essential  information.

Gray, F, C. Hollingsworth, D. Koch, R. Groose, T.Heald. 2004.  Brown Root Rot of Alfalfa. Plant Sciences Timely Information No. 1, University of Wyoming.

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Last modified May 26 2004 01:29 PM

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