March, 1991                                                                               PDF Version

Common Leaf Spot Resistance
Pseudopeziza medicaginis (Lib.) Sacc.
Kenneth Leath

PLANT CULTURE

Greenhouse

Container ..............    Flats or tool carts, 31x62x7.5cm deep
Media ................... Not critical
Temp/Light ............ 18 to 24C; daylength not critical
No. of Plants ......... 20 to 25 per replication
No. of Reps ........... 4 minimum
Other .................... Inoculate with Rhizobium meliloti Dang and fertilize as needed. No insecticide
within 1 week of inoculation
  
INOCULUM CULTURE
  
Source .................... Infected leaves
Storage.................... 6 months at 4C; longer on silica gel
  
INOCULATION PROCEDURE
Age of plant ............. 6 to 8 weeks
Type of inoc............. Sporulating oatmeal agar plate cultures;21 to 30 days old
Temp/Light .............. 19 to 23C; dark
Method ................... Cultures inverted 30 to 60 cm above
plants, approximately one culture per 900cm2 of plant material, in place until 10 spores per cm2 collected on trap slides, usually 15 to 24 hours; plates repositioned periodically to increase uniformity of deposition; plants are sprayed with water after plates are removed
Time of Inoc............. Usually 24 to 48 hours
Conditions ............... Saturated RH; 19 to 21C; dark
  
INCUBATION
  
Duration .................. 72 hours at 100% RH in mist chamber
Location................... After initial 72 hour period, move from moist conditions to greenhouse, allow leaves to dry slowly out of direct sunlight
Measurement .......... Type and size of leaf spot, usually 14 days after inoculation
  
RATING
  
1 Resistant.............. No spots
2 Resistant ............. Barely visible pepper spots, 1 mm diam or less
3 Susceptible .......... Spots >1 mm, no chlorosis
4 Susceptible .......... Spots >2 mm, chlorosis or defoliation
5 Susceptible .......... Spots >3 mm, chlorosis, defoliation, with or without fruiting structure (apothecia) in center of lesion.
  
CHECK CULTIVARS
  
  Approximate Expected Resistance (%) Acceptable Range of Reaction (%)
Resistant    
MSA-CW3An3** 60 40-70
Ramsey** 60 40-70
Susceptible    
Ranger** 30 10-35
Moapa** 0-10 0-10

DISTRIBUTION AND SEVERITY OF
COMMON LEAF SPOT

Common Leafspot Pseudopeziza medicaginis Lib. Sacc.(Click on the map above for a larger version. 
See also the
KEY)

 

SOURCE OF INOCULUM AND SCIENTIST WITH EXPERTISE
  
Name ..................... K.T. Leath
Address................... USDA-ARS
U.S. Regional Pasture Research Lab
University Park, PA 16802
Phone ..................... 814-863-0945
  
CORRELATION TO FIELD REACTION

Correlation to field results is good; no exceptions reported.
  
RACES
  

No races are known.

CULTURE OPTIONS AND RANGE OF CONDITIONS

Cultures should be sealed to prevent drying. Considerable care should be taken to prevent contamination of cultures, as P. medicaginis is a poor competitor.

PLANT GROWTH OPTIONS AND RANGE OF CONDITIONS

Vigorous plants are desirable. Use of lightweight potting mix best if plants are to be pulled during scoring.

INOCULATION CONDITIONS AND RANGE OF CONDITIONS

Temperatures from 15 to 25C are acceptable. Light during infection not required.

HELPFUL INFORMATION

Plants may be cut back at scoring and regrowth used for different disease evaluation.

Isolation of fungus can be made by suspending diseased leaves over water agar plates and transferring trapped spores to oatmeal agar. Fungus usually does not kill stems or plants.

ALTERNATIVE METHODS

The greenhouse method is most dependable and can be done year round.

A field method has been used and described by Frosheiser(1).

REFERENCES

     1. Frosheiser, F.I. 1984. In Standard tests to characterize pest resistance in alfalfa. USDA Misc. Pub. No. 1434.
         p. 22.

     2. Leath, K.T., and R.R. Hill, Jr. 1984. In Standard tests to characterize pest resistance in alfalfa. USDA
         Misc.Pub. No. 1434. p. 21.

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