| PLANT CULTURE Field
Methods(3)
|
No. of Plants .... |
40 to 60 per
replication |
|
No. of Reps ...... |
3 or 4 replications |
|
Other ............... |
Plant in early May on a level
area with relatively heavy soil, but with good internal
drainage |
Greenhouse Method
|
Container ......... |
20 cm deep watertight tanks with drain
holes and a 2.5 cm diameter pipe placed vertically in one corner
to allow flooding sand from bottom of tank |
|
Medium ............ |
Washed, pasteurized sand |
|
Temp/Light ....... |
20 to 24°C (sand
temperature); 16 hour
daylength |
|
No. of Plants .... |
25 per replication |
|
No. of Reps ...... |
4 minimum |
|
Other ............... |
Inoculate with Rhizobium
meliloti Dang and fertilize |
INOCULUM CULTURE
Greenhouse Method
|
Source ............. |
Culture on V-8 juice agar in 9 cm
petri dishes |
|
Maintenance ..... |
Store cultures on V-8 juice agar at
4°C |
INOCULATION PROCEDURE
Field Method
|
Type of Inoc....... |
Select a field that is
naturally infested; or spread infested soil from several
geographic areas over the field, incorporate to a depth
of about 15 cm and grow a susceptible variety for one
year prior to using the area |
Greenhouse Method
|
Age of Plant...... |
Plant seed into sand |
|
Type of Inoc...... |
Two week old cultures |
|
Concentration.... |
Mix inoculum with sand before
planting at a rate of one petri dish per 500 cm2
surface area |
INCUBATION
Field Method
|
Plant Counts .... |
Count plants (alive +
dead) when seedlings are in the unifoliolate stage |
|
Culture ............. |
About 4 weeks after planting, irrigate
each day for 3 weeks to keep soil continuously saturated,
allow soil to dry for 1week, clip plants, and cultivate
soil. Repeat the sequence two more times. Spray for
insects as needed |
|
Row Spacing .... |
Approximately 0.3 m |
|
Age at Rating.... |
14 to 15 weeks after planting |
Greenhouse Method
|
Plant Counts .... |
Same as field method |
|
Culture ............. |
Water seedlings sparingly until they
are well established (4 weeks), plug drain holes and
water daily to raise water level surface; maintain
flooded conditions for about 4 weeks |
|
Row Spacing .... |
Approximately 3.5 cm |
|
Age at Rating.... |
8 weeks after planting |
RATING
Dig all plants retaining 25 cm or more of the
taproot. Spray roots to remove excess soil, bundle
plants, and soak roots in a tub of water. Complete
washing and rate plants indoors under uniform light.
|
1 Resistant ....... |
Roots clean no lesions; many
small rootlets on taproot |
|
2 Resistant ....... |
Small root lesions (2mm); small
rootlets absent |
|
3 Susceptible .... |
Large nongirdling root
lesion(s)
and/or branch root tips rotted off |
|
4 Susceptible .... |
Extensive lesions with ends of
large tap or lateral roots rotted off 10 cm or more below
the crown. |
|
5 Susceptible .... |
Tap and lateral roots almost
destroyed; plant alive. |
|
6 Susceptible .... |
Plants dead (calculated as loss
in stand) |
CHECK CULTIVARS
| |
Approximate Expected Reaction
(%) |
Acceptable Range of Resistance
(%) |
| Resistant |
|
|
| Agate** |
43 |
25-55 |
| Susceptible |
|
|
| Saranac** |
3 |
0-10 |
Values for resistant standards include total of l's
and 2's.
DISTRIBUTION AND SEVERITY OF PHYTOPHTHORA
ROOT ROT
Phytophthora root rot, Phytophthora megasperma Drechs.
f. sp. medicaginis (3)
Click on the map above for a larger version. Se
e also the KEY.
SOURCES OF INOCULUM AND EXPERTISE
Name ...............Judy A. Thies
Address ............USDA/ARS and
Department of Plant Pathology
University of Minnesota,
495 Borlaug Hall
1991 Upper Buford Circle
St. Paul, MN 55108
Phone .............. 612-625-8240
Name. ..............Craig R. Grau
Address ............Department of Plant Pathology
University of Wisconsin - Madison
Madison, WI 53706
Phone................608-262-6289
CORRELATION TO FIELD REACTION
Field and greenhouse evaluations were
correlated (r = 0.99 and 0.95) in two tests. (3). Field
tests tend to be more precise, with greenhouse tests
useful for screening.
PATHOTYPES Isolates
of Phytophthora megasperma with different levels
of pathogenicity on alfalfa cultivars exist (2).
Therefore, it is important to use a mixture of highly
palhogenic isolates.
PLANT GROWTH OPTIONS AND RANGE OF CONDITIONS
Monitor root rot development during the season.
Symptoms can be increased or reduced by changing the
frequency and/ or amount of irrigation. Potato leafhopper
control is very important in the Midwest.
HELPFUL INFORMATION
Ratings may be expressed as an average severity index
(A.S.I.) which is most precise, or percentage of
resistant plants (3) which can be adjusted to a standard
check to compare entries between tests. The percentage of
plants adjusted to Agate is useful for comparing
cultivars tested in different years.
ALTERNATIVE METHODS
Greenhouse tests using zoospores (1) in a
method analogous to that used for Aphanomyces (4) has
been successfully used to rank lines although the percent
resistant plants is somewhat lower than field test
results.
REFERENCES
1. Irwin, J.A.G., S.A. Miller, and
D.P.
Maxwell. 1979. Alfalfa seedling resistance to Phytophthora megasperma. Phytopathology 69:1051-1055.
2. Faris, M. A. 1985. Variability and
interaction between alfalfa cultivars and isolates of Phytophthora
megasperma. Phytopathology 75:390-394.
3. Frosheiser, F. I., and D. K. Barnes. 1984.
In Standard tests to characterize pest resistance in
alfalfa cultivars. USDA Misc. Pub. No. 1434.
4. Nygard, Sharie, and Craig Grau. 1991.
In Standard
tests to characterize alfalfa. NAAIC.
|
