Spot Resistance (Warm Temp. - Eastern Biotype)
THIS TEST IS IN THE DEVELOPMENTAL STAGE AND SHOULD BE USED CAUTIOUSLY
Source .........Infected leaf tissue
Storage.........Sealed cultures for several months or silica gel crystals for longer periods
Temp/Light .....Culture 21+1°C, 12 hour daylength; storage 4°C
Storage Life ...At least 4 years
Incubation .....Cultured on V-8 plates under cool white fluorescent lights at 20Mm -2sec-1 for approx. 10 days
Age of Plant....4 to 6 weeks (4 to 6 trifoliates); or on regrowth
Plant Counts....Prior to inoculation
Type of Inoc .... Conidial suspension with 50 to 100 ppm Tween 20 or 1 small drop per liter
Concentration .. lx104 to 5x104 spores per mL
Method .........Foliar spray until runoff
Location .......Moist chamber for 48 hours
Temperature.....20 to 23°C
Lighting .......Darkness is satisfactory for the initial 48 hours, then 16 hr daylength
Humidity .......100% R.H. for initial 48 hours
Age at Rating .... 5 to 8 weeks (about 10 days after inoculation)
Class descriptions are subjective and may be adjusted. Fewer classes may be used and still meet the objectives of many experiments.
1 Resistant ....Healthy, symptom-free top growth
2 Resistant ....Small (<lmm) brown or black lesions orpepper spots; no defoliation
3 Susceptible ..Larger (I to 3mm), discreet lesions with necrotic centers; usually no chlorosis or defoliation
4 Susceptible ..Large (>3mm), target-type lesions or necrotic areas; may have petiole lesions
5 Susceptible ..Lesions >3mm, chlorosis, defoliation
No checks are available for the Eastern (warm-temperature) biotype. However, Apalachee has been reported to have some resistance (3).
DISTRIBUTION AND SEVERITY OF STEMPHYLIUM LEAF SPOT
|SOURCE OF INOCULUM AND
Name .....K.T. Leath
CORRELATION TO FIELD REACTION
Greenhouse and field test results have been similar.
No races are known to exist, but two biotypes of S. botryosum have been reported; the warm-temperature (Eastern) biotype, and the cool-temperature (California) biotype.
CULTURE OPTIONS AND RANGE OF CONDITIONS
Stemphylium can appear variable in culture. Keep refrigerated for storage. The cool temperature biotype has shown variability for virulence among single spore subcultures.
PLANT GROWTH OPTIONS AND RANGE OF CONDITIONS
Plants may be started in the greenhouse or growth room. Four-week old regrowth has been inoculated successfully.
INOCULATION AND INCUBATION CONDITIONS
After inoculation, keep plants moist for a full 48 hours. Incubation can range from 1 to 2 weeks after inoculation.
Dry plants slowly after humidity treatment. Air-dry away from direct sunlight. Selections may be reinoculated to test for escapes.
Selection may be possible under field conditions. Field inoculations have been done.
1. Cowling, W.A., D.G. Gilchrist, and J.H. Graham. 1981. Biotypes of Stemphylium botryosum on alfalfa in North America. Phytopathology 71:679-684.
2. Hill, R.R., Jr., K.T. Leath, and K.E. Zeiders. 1972. Combining ability among four-clone alfalfa synthetics. Crop Sci. 12:627-630.
3. Lucas, L.T., T.H. Busbice, and D.S. Chamblee. 1973. Resistance to Stemphylium leafspot in new alfalfa variety. Plant Dis. Rep. 57 (11):946-948