Volume
4 Issue 2 May
22, 2007
Brown Root Rot of Alfalfa
Brown root rot (BRR) is a fungal disease, caused by Phoma
sclerotioides, which is associated with stand decline and
reduced yield of forage legumes such as alfalfa, red clover,
bird's-foot trefoil, alsike clover, and sweet clover.
 In
2003, brown root rot of alfalfa was reported for the first time in
Wisconsin and Minnesota. It is likely that the fungus causing this
disease has been present for many years but was not recognized. A
three-year survey showed that the pathogen is present across
Minnesota and Wisconsin, and may be more common in the Red River
Valley, St. Croix River Valley and west of Green Bay. The fungus is
slow growing and prefers cool soil temperatures (less than 60°F) for
growth.
Symptoms and management.
Symptoms of the disease may not be visible until after the third
winter because of the slow growth of the fungus and its dependence
on environmental conditions for symptoms to develop. Because the
fungus rots alfalfa roots during fall and spring, it can severely
weaken plants and contribute to winter kill.
 Symptoms
of the disease are stunted and dead plants in 2- to 3-year-old
fields with brown sunken lesions on taproots. Infected plants may
start to regrow in spring and then stop growing and die in late
spring to early summer because the tap root has rotted off. The
fungus also causes disease on other perennial forage legumes and
winter wheat. Our research shows that its inoculum increases by
using corn and soybean plant debris, but how this contributes to the
disease is not yet known. Currently, no resistant varieties are
available, although trials are in progress to determine if current
cultivars have resistance to the disease.
A free test is
available to determine if plants are infected with the pathogen or
if the pathogen is present in soil. Details for submitting samples
are available at:
http://www.tc.umn.edu/~medicago/Sampling_Procedure.doc
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