Root
and Crown Rots and Virus Diseases of Alfalfa and Clover
Dean Malvick, Assistant Professor of Plant Pathology,
Department of Crop Sciences, University of Illinois at Urbana-Champaign
A goal for many
forage producers is to plant alfalfa and clover, harvest high yields on an
appropriate schedule, and retain stands for many years. Numerous obstacles, however, prevent this
ideal scenario. Of those obstacles,
diseases are among the most damaging.
Diseases affect yield, persistence, and quality of forage crops and
frequently force premature reestablishment of stands. Many disease affect roots, leaves, stems of alfalfa and red
clover from the seedling to established stages. Three diseases that are particularly damaging will be emphasized
in this presentation: Aphanomyces root rot of alfalfa, crown rots of alfalfa,
and clover yellow mosaic virus.
One of the first
diseases that often occurs after planting is Aphanomyces root rot. This disease, caused by the fungal-like
pathogen Aphanomyces euteiches causes death and poor growth of seedling
alfalfa in slowly drained fields. It
also can be a chronic disease of established plants that may result in
significant yield reduction.
Aphanomyces root rot is similar to and may occur in a complex with
Phytophthora root rot and Pythium damping off, diseases which also occur in wet
or slowly drained soils (3). Plants
infected with Aphanomyces usually are stunted and chlorotic before they
wilt and die, whereas Phytophthora and Pythium tend to kill seedlings quickly before plants become
severely chlorotic. Another clue to a
potential problem with Aphanomyces is stunting and poor nodulation of an
alfalfa cultivar that is highly resistant to Phytophthora. Aphanomyces root rot has also been related
to increased damage to alfalfa from winter root heave damage (9). Perhaps
because root rot of alfalfa that occurred in slowly drained soil was attributed
to Phytophthora, Aphanomyces root rot
was not recognized as a serious problem until the early 1980’s. Aphanomyces root rot is known to be a
widespread problem for alfalfa in Wisconsin (4). Although Aphanomyces
can infect red clover, it appears to be a minor problem on that crop (2).
Aphanomyces root
rot of alfalfa can best be managed by avoiding slowly-drained soils and by
using Aphanomyces-resistant alfalfa varieties.
Fungicides are not available for control of Aphanomyces. Phytophthora and Pythium seed
and root rots can be controlled with fungicidal seed treatments such as
Allegiance-FL, ApronXL, or Apron-FL.
These seed treatments are not effective against Aphanomyces. Alfalfa varieties rated highly resistant
(HR) or resistant (R) to Aphanomyces root rot should be planted where
slowly-drained soils occur and where Aphanomyces may be a problem (6, 8). Management
of Aphanomyces root rot clearly became more challenging when different races of
this pathogen were discovered.
Aphanomyces root
rot of alfalfa is caused by two races of Aphanomyces. One race (race 1) was initially discovered
and was used for development of most alfalfa cultivars with Aphanomyces
resistance. Another race (race 2) was
discovered that can kill and stunt cultivars that have resistance only to race
1. The frequency of race 1 and race 2
isolates were studied from 13 fields in Wisconsin (5). Aphanomyces isolates (total number = 405)
were collected from the fields. The
race of the isolates was determined by inoculation of Saranac (susceptible to
races 1 and 2) and WAPH-1 (resistant to race 1) alfalfa varieties. The populations of the two races of Aphanomyces
varied among the different Wisconsin fields.
For example, 51 percent were race 2 in a field never used for alfalfa
production and 27 percent of isolates were race 2 from a field used for pea
production, while 43 percent of all isolates from fields with a history of
alfalfa production were race 2 . Race 2
isolates appear to be widespread in Wisconsin and other Midwestern states and
this race of Aphanomyces occurs in fields with varied cropping histories.
Alfalfa
varieties with resistance only to race 1 are vulnerable to Aphanomyces root rot
in many areas due to the presence of race 2.
Of eight alfalfa cultivars evaluated for resistance to both races in a
recent study, all except those selected for R2 resistance were susceptible to
R2 isolates (5). Several commercial
alfalfa varieties are available that have resistance to both races of Aphanomyces. Race 2 isolates may represent a widespread
risk to alfalfa cultivars with resistance only to race 1.
Crown rot of
alfalfa can be as damaging as it is frustrating to manage. This disease complex occurs in nearly every
established stand of alfalfa. The older
the stand, the more crown rot that typically occurs (7). Typical
initial symptoms of crown rot include brown dead tissue in the crown and root
cortex, and this may spread to kill the entire root core and crown buds. While it is difficult to prevent crown rots,
we can gain understanding of what causes them, what factors favor development
of crown rot, and what management steps we can take to reduce or delay their
development.
Several fungi
are commonly associated in a complex with crown rot disease. They include Phoma spp., Colletototrichum
trifolii, Fusarium spp., Pythium spp, and Rhizoctonia
solani. These pathogens can also
cause other symptoms/diseases including root rots and stem infections. Thus, they may infect plants months or even
years before crown rot develops, and isolation of these pathogens from plants
does not mean that crown rot will develop.
These pathogenic
fungi may only cause crown rots under specific, stress conditions. Plant stresses that may favor crown rots
include poor fertility, low soil pH, other diseases, harvesting too often or
too late in the season, and grazing animals or wheel traffic that damages the
crowns. Because of the complex nature
of this disease, alfalfa varieties with high levels of specific resistance to
crown rots have not been developed.
Crown rot damage can be reduced by choosing and planting adapted
varieties that have resistance to multiple diseases, and by maintaining sound
management practices that take into account the stresses related to crown rots
noted above.
Viruses are
often unrecognized members of the large group of plant pathogens that infect
alfalfa and clover. One of
the most common pathogens in alfalfa is alfalfa mosaic virus (AMV). This virus is widespread in Wisconsin
alfalfa fields. Fortunately, it rarely
seems to cause any serious problems for alfalfa or red clover, but commonly
infects plants without causing any obvious symptoms, damage, or yield
loss. Another virus, clover yellow
mosaic virus, can act quite differently than AMV and can be very damaging to
red clover.
Clover yellow
mosaic virus (CYMV) has recently been associated with severe damage to red clover
in Wisconsin. Typical symptoms are a
mosaic pattern and brown streaks on infected leaves (10). Recent studies by C. Grau and A. Mondjana
(personal communication) in the Department of Plant Pathology at UW- Madison
suggest this is a widespread virus that can be quite destructive and cause
summer decline to red clover. It also
infects alfalfa and pea, but has not been reported to cause significant damage
to these crops in Wisconsin. Because
CYMV is a newly reported problem, there is still much we do not know about this
virus in the Midwest. Based on current
information, this virus may be transmitted by aphids or mechanically during
harvest. As such, it will be very difficult
to control the spread of this virus.
1. Grau
C. R. 1992. Registration of Waph-1 Alfalfa Germplasm with Resistance to
Aphanomyces Root Rot. Crop Science. 32: 287-288.
2. Grau
C. R., Muehlchen A. M., Tofte J. E.,
and Smith R. R. 1991. Variability in Virulence of Aphanomyces
euteiches. Plant Disease. 75: 1153-1156.
3. Holub
E. B., and Grau C. R. 1990. Ability of Aphanomyces
euteiches to Cause Disease of Seedling Alfalfa Compared with Phytophthora
megasperma f. sp medicaginis. Phytopathology. 80: 331-335.
4. Holub
E. B., and Grau C. R. 1990. Specificity
of Resistance to Aphanomycese euteiches in Seedling Alfalfa. Plant
Disease. 74: 164-168.
5. Malvick
D. K., and Grau C. R. 2001.
Characteristics and frequency of Aphanomyces euteiches races 1 and 2
associated with alfalfa in the Midwestern United States. Plant Dis. In-press
6. Munkvold
G. P., Carlton W. M., Brummer E. C., Meyer J. R., Undersander D. J., and Grau C. R. 2001. Virulence of Aphanomyces
euteiches isolates from Iowa and Wisconsin and benefits of resistance to A.
-euteiches in alfalfa cultivars. Plant Disease. 85: 328-333.
7. Stuteville
D. L., and Erwin D. C. e. 1990.
Compendium of Alfalfa Diseases. Second Edition. St. Paul: APS Press. 84 pp.
8. Wiersma D. W., Grau C. R., and Undersander D. J. 1995. Alfalfa Cultivar
Performance with Differing Levels of Resistance to Phytophthora and Aphanomyces
Root Rots. Journal of Production Agriculture. 8: 259-264.
9. Wiersma
D. W., Undersander D. J., and Grau C.
R. 1997. Root heave of alfalfa cultivars with differing levels of resistance to
Aphanomyces root rot. Agronomy Journal. 89: 148-150.
10. Website: Plant Viruses Online
(http://image.fs.uidaho.edu/vide/descr229.htm)