Volume
5 Issue 9
July 15, 2008
Brown Root Rot of Alfalfa:
A subtle threat to forage production in Minnesota
 Brown
root rot (BRR) of alfalfa has been identified in several states in
the U.S. since 1996 (Figure
1), but it has been known as a damaging disease of alfalfa,
sweet clover, and grass crops in Canada since the 1920s. During
2003,
Phoma
sclerotioides, the pathogen
responsible for the disease, was recovered from roots of diseased
alfalfa plants in northwest Minnesota. Since that time, BRR has
been identified throughout the State.
 Termed
a snow mold, cool soil temperatures are ideal for the pathogen.
Fungal growth is most active during late fall continuing into late
spring, depending on soil temperatures. Plants are infected when
they are partially or fully dormant. Belowground symptoms of the
disease begin as small dark lesions on roots and nitrogen-fixing
nodules. Lesions enlarge, coalesce, and girdle roots of susceptible
plants (Figure
2). When the disease severity progresses, roots are severed (Figure
3) which often results in plant death. Root rot and plant death
can occur
 during
a single winter season, which provides diagnostic aboveground
symptoms the next spring. If disease severity progresses more
slowly, a gradual decline in the number of live plants results.
Often disease symptoms at this level are much less diagnostic since
some stand loss and reduced plant vigor are normally expected as
alfalfa stands age. When plants die during a winter season,
adjacent surviving plants should be dug to check for root health.
Yield reductions have not been quantified from BRR, but are expected
if fewer healthy plants contribute forage yield.
Plants also
die during Minnesota winters from non-disease, weather-related
stresses (insufficient snow cover, ice sheeting, etc.), but these
factors can easily be ruled out by a close examination of plants’
root systems. Plants killed from BRR have brown root and/or crown
lesions. In contrast, environmental stresses generally result in
roots that are much lighter in color.
Collaborative
research is ongoing between UM scientists located at the Northwest
Research and Outreach Center and St. Paul campuses. Their focus is
to identify which crop residues promote versus limit, winter
survival of the pathogen. The fungus is known to survive on various
types of organic debris which makes identifying those residues that
reduce in-field pathogen levels critically important in developing
disease management recommendations.
During
2005-07, seven crop species (alfalfa, canola, corn, oat, soybean,
spring wheat, and winter wheat) were grown in pots containing field
soil that was naturally infested with
P.
sclerotioides. A nontreated
control (fallow) was also included. After the first freeze of the
season, dried crop debris from each species was incorporated into
the top two inches of soil and pots were left outside during the
winter at two locations (St. Paul and Crookston, MN). In the
spring,
P. sclerotioides
density in soil was measured using a quantitative PCR
assay. Density of the pathogen was highest in soil from pots with
corn, soybean, and canola. Density of the pathogen following
alfalfa, winter wheat, spring wheat, and oat was similar to the
fallow treatment.
In separate experiments, colonization by
P.
sclerotioides of stems,
leaves, and roots of the same crop species listed above plus barley,
hairy vetch, and winter rye under controlled conditions was measured
using six different fungal isolates. The crop origin of the
isolate, whether from alfalfa, perennial rye, or winter wheat, did
not affect the fungus’s success at colonizing different crop
species. A high level of colonization was observed in canola roots
as well as leaf material from spring wheat, winter wheat, winter
rye, and corn. Soybean roots supported the lowest amount of
colonization. Results indicate that rotation to corn, soybean, or
canola will not reduce in-field pathogen density sufficiently.
Although colonization of spring wheat leaves was high, results
indicate that this type residue does not increase pathogen density
in soil. This may be, in part, due to rapid degradation of leaf
material by soil-borne microbial communities.
Disease
management strategies include reducing late-season stress on
plants. This includes allowing plants a sufficient period of time
to regrow and recover following the last cutting of the season.
This gives plants time to replenish their root reserves prior to the
winter dormancy period. Adequate soil nutrition should also be
provided. Rotate into a crop not known for BRR and whose residue
does not support winter survival by the pathogen. A crop rotation
using winter wheat is not recommended because
P.
sclerotioides
has been shown to be a pathogen of that crop, as
well. While resistant alfalfa varieties are not yet commercially
available, 26 varieties are being tested at four commercial
production field locations within Minnesota in soil infested with
the brown root rot pathogen. The purpose of the trials is to
determine if useful levels of resistance can be identified from
current commercial varieties.
Charla Hollingsworth,
Deborah Samac, and Paul Peterson
University of Minnesota
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