March, 1991                                                                                  PDF Version

Fusarium Wilt Resistance
Fusarium oxysporum Schlecht f. sp. medicaginis (Weimer) Snyd. & Hans. S. Nygaard and D. K. Barnes

PLANT CULTURE

Greenhouse

Container..........

Bench, pot, or flat deep enough to allow root development
Media............... Sand or soil mixture
Temp/Light........ 24 to 30C; 16+ hour daylength
No. of Plants .... 35 to 50 per replication
No. of Reps....... 3 replications minimum
Other................ Inoculate with Rhizobium meliloti Dang, encourage vigorous growth

INOCULUM CULTURE

Source..............

Infected root tissue; axenic culture
Storage............. Inoculated sterile soil cultures or solid PDA medium
Temperature.......   0C to ambient
Storage Life.......  Several years in soil tubes

INOCULATION PROCEDURE

Age of Plant...... 8 to 10 weeks old
Type of Inoc....... Microconidia in suspension
Inoc. Culture ..... Potato or nutrient broth (2 g NaNO3, 1 g KH2PO4, 0.5 g MgSO4-7H2O, 0.5 g KCI, 0.01 g FeSO4-7H2O, 0.5 g yeast extract, and 15 g sucrose per 1 L distilled water); inoculate sterile broth wlth PDA plugs and incubate on a shaker for 4 days at about 21C
Concentration ... 1.6 X 106 spores per mL or a 1:20 dilution of the shake culture
Method ............. Bare root soak
Time of Inoc....... 20 to 30 min.

INCUBATION

Location ........... Transplant to field
Plant Counts .... Approximately 2 wks after transplanting, count all (alive and dead) plants as the base count for initial stand
Culture ............. Maintain vigorous growth, control insects, clip plants once or twice.
Spacing ........... 0.15 m between plants, 1.0 m between rows.
Age at Rating.... 3 months after transplanting

RATING

Plants are removed from the field and the tap root sectioned for rating.

0 Resistant ............. No discoloration in the root
1 Resistant ............. Small dark strands in the stele
2 Susceptible .......... Small dark-brown arcs or rings in cross section of the stele
3 Susceptible ..........  Larger dark-brown areas, arcs or rings, or partial dark- brown ring in the outer stele
4 Susceptible .......... Entire outer stele dark brown, plant alive
5 Susceptible .......... Dead plant (stand loss)

CHECK CULTIVARS

  Approximate Expected Resistance (%) Acceptable Range of Reaction (%)
Resistant    
Agate** 54 45-65
Moapa 69 76 65-85
Moderately Resistant    
Narragansett 22 15-30
Susceptible    
MNGN-l ** 4 0-8

Values for resistant standards are totals of O's and l's.

DISTRIBUTION AND SEVERITY OF
FUSARIUM WILT

Fusarium wilt, Fusarium oxysporum Schlecht f. sp. medicaginis (Snyd. & Hans.)
(Click on the map above for a larger version. See also the KEY)

SOURCE OF INOCULUM AND EXPERTISE

Name ..............Judy A. Thies
Address ...........USDA-ARS
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