PLANT CULTURE
|
Container........... |
Seedling
cavities within a water reservoir or a deep flat or tub
with a single drainage hole which is capable of being
plugged |
|
Media................ |
Coarse vermiculite or a porous soil mix
(eg.
3:2 sphagnum-based soilless mix: perlite); provide a
coarse drainage layer (eg. gravel); pure sand medium is
not desirable |
|
Temp/Light......... |
20-24C; 12-16 hr daylength |
|
No.of Plants....... |
50-70 per replication |
|
No. of Reps....... |
3 minimum |
INOCULUM CULTURE
|
Source............... |
Seedlings grown on infested soil |
|
Storage.............. |
Corn meal or V-8 juice agar |
|
Temperature....... |
4-12 C |
|
Storage Life....... |
6 months if hydrated |
INOCULATION PROCEDURE
|
Age of Plant....... |
10-12 days (when first trifoliate
begins expansion) |
|
Type of Inoc....... |
Zoospore suspension or comminuted
mycelium |
|
Production......... |
Zoospores are produced as per (2); or
9-day-old V-8 agar cultures of mycelium may be chopped in
a blender for 10 sec. |
|
Concentration..... |
Aprox. 50 zoospores (3) or 1 ml chopped
mycelium per seedling; mycelium prepared as: 1 culture (9
cm diam) in 1 L water |
|
Method.............. |
For zoospores- Presaturate the soil mix
and drench inoculum over the seedlings (4); For mycelium-
drench inoculum into shallow trench and then saturate the
soil with water (1) |
INCUBATION
|
Location............ |
Moderate
greenhouse or growth chamber |
|
Plant counts...... |
Count at full emergence (7-8 days after
seeding) |
|
Culture.............. |
Maintain flooded conditions for 2 days;
keep moist until rated |
|
Age at Rating..... |
Rate when nearly all Saranac plants are
stunted and dying, i.e. for zoospores- 10-12 days after
inoculation, for mycelium- 14 days after inoculation |
RATING
|
Resistant........... |
Vigorously growing plants with only slight to no
necrosis of tap and secondary roots; hypocotyl area sound
with slight to no chlorosis of cotyledons |
|
Susceptible........ |
Stunted or dead plants with moderate to
severe necrosis of roots, hypocotyls and cotyledons |
CHECK CULTIVARS
| |
Approximate
Expected
Resistance (%) |
Acceptable
Range of
Reaction (%) |
| Highly
Resistant |
|
|
| WAPH- 1* |
55 |
50-60 |
| Resistant |
|
|
| MNP-D1b |
46 |
38-54 |
| Agate* |
33 |
25-40 |
| Susceptible |
|
|
| Saranac |
1 |
0-5 |
*WAPH-I and/or Agate must be included for
varieties with fall dormancy rating 1-5.
bMNP-DI must be included for varieties with
fall dormancy rating 6-9.
REFERENCES 1.
Horhein, B. A., Bean, G. A., and Graham, J. H. 1983.
Greenhouse technique to evaluate alfalfa resistance to Phytophthora
megasperma f. sp. medicaginis. Plant
Disease 67:1332-1333.
2. Miller, S. A. and Maxwell, D. P. 1984. Light
microscope observations of susceptible, host resistant,
and nonhost resistant interactions of alfalfa with Phytophthora
megasperma. Can. J. Bot. 62: 109-116.
3. Nygaard, S. L. and Grau, C. R. 1989. Phytophthora
megasperma virulence to alfalfa measured using
single-isolate zoospore suspensions. Can. J. Plant Pathol. 11 :101-108.
4. Nygaard, S. L. and Grau, C. R. 1991. Aphanomyces Root
Rot Resistance. page D-2. Ln: Standard Tests to
Characterize Alfalfa Cultivars (Third Edition). North
American Alfalfa
Improvement Conference.
|