DISTRIBUTION AND SEVERITY
OF APHANOMYCES ROOT ROT
Root Rot (Aphanomyces euteiches Drechs)
(Click on the map for a larger version; see also the key )
A. euteiches, has been reported throughout North
America, Europe, Australia, and New Zealand. The
distribution of alfalfa strains per se has not
been exhaustively studied. However, race 1 alfalfa
strains have been detected in Idaho, Illinois, Indiana,
Iowa, Kentucky, Maryland, Michigan, Minnesota,
Mississippi, Nevada, New York North Carolina, Ohio,
Oklahoma, Pennsylvania, Tennessee, Virginia, Wisconsin,
and Ontario and Quebec, Canada. Race 2 alfalfa strains
have been confirmed in Idaho, Maryland, Minnesota, North
Carolina, Iowa, Tennessee, Virginia and Wisconsin (2).
Aphanomyces can cause severe stunting and death of
seedlings, and can cause a chronic disease of lateral
roots of established plants. It frequently is recovered
from fields where Phytophthora root rot and Pythium
damping off are found. A. euteiches and Phytophthora
medicaginis may cause a root disease complex.
Aphanomyces root rot is favored by warm, saturated soil
SOURCE OF INOCULUM AND SCIENTIST WITH EXPERTISE
||Craig R. Grau
Department of Plant Pathology
University of Wisconsin Madison
1630 Linden Drive
CORRELATION TO FIELD REACTION
is a good correlation between results of this test and
visual root scores, plant vigor and forage yield in
naturally infested fields (4).
There are two recognized races of A. euteiches.
Isolates are known which may belong to additional races,
as yet not defined.
PLANT GROWTH OPTIONS AND
RANGE OF CONDITIONS
Controlled environmental conditions (20 to 24 C) are optimum
for separation of resistant and susceptible reactions.
However, warmer conditions (28 to 32 C) are more favorable for
pathogen activity, and will give a more severe test. A more
qualitative (dead or alive) reaction occurs at temperatures
equal to, or greater than, 28 C.
A. euteiches can be maintained on agar, but requires frequent
subculturing every 2-4 months. Isolates commonly lose
aggressiveness after about 1 year in culture, therefore,
isolates recently recovered from alfalfa seedlings should be
used. Use actively growing cultures for zoospore production,
as zoospore production declines for mycelial mats older than 5
days of age. Agitation will induce zoospores to encyst to a
non-motile stage allowing an accurate enumeration with a
Seedlings in classes 1 and 2 are considered resistant,
however, self-pollinated class 3 plants frequently produce
resistant progeny (3).
The seedlings assay using zoospores is the most effective and
preferred method for characterizing alfalfa populations for
reaction to Aphanomyces root rot. Older plants (6 to 12 weeks
old) may be used to screen alfalfa for reaction to A.
euteiches under controlled conditions. However the chronic
root symptoms that develop are difficult to characterize into
severity classes. Four to six week old plants can be
inoculated with A. euteiches and then clipped back, and the
amount and rate of foliage regrowth can be used to score
plants for their reaction to the pathogen.
1.Grau, C. R. 1992. Registration of WAPH-1 alfalfa germplasm
with resistance to Aphanomyces root rot. Crop Science
2.Grau, C. R., A. M. Muehlchen, J.E. Tofte, and J. E. Smith.
1991. Variability in virulence of Aphanomyces
euteiches. Plant Disease 75:1153-1156.
3.Mitchell, J. E., and C.Y. Yang. 1966. Factors affecting
growth and development of Aphanomyces euteiches.
4.Wiersma, D. W., C. R. Grau, and D. J. Undersander. 1995.
Alfalfa cultivar performance with differing levels of
resistance to Phytophthora and Aphanomyces
root rots. Journal of Production Agriculture 8:259-264.
Acknowledgments: Thanks to John Edmonds,
Jay Sandman, Sam Stratton, and Mike Velde for supplying
additional data used in the development of this test.