Established Cultivar Evaluation Procedures 



Few examples of continuous long-term pest resistance 

 tests are available for alfalfa. In fact, the oldest program in 

 the United States is the 31-year-old bacterial wilt test con- 

 ducted at the University of Minnesota. Many evaluation pro- 

 cedures for other types of pest resistance are more recent 

 or have been used only sporadically. For these reasons, 

 referring to these procedures as standardized evaluation 

 tests would be presumptuous of the authors. Nevertheless, 

 many of the tests can be used for cultivar evaluations. 



Established evaluation procedures were available for 20 of 

 the 36 major alfalfa pests listed in this publication. Scien- 

 tists that had experience with these tests were asked to pre- 

 pare brief descriptions of their methods. These descriptions 

 should provide useful guidelines for future evaluations. Ap- 

 proximate levels of pest resistance expected for the check 

 cultivars recommended for use with the evaluation pro- 

 cedures are given in the appendix, p. 37. 



Bacterial Wilt Resistance 



F. I. Frosheiser and D. K. Barnes 

 University of Minnesota, St. Paul 



The most reliable method of evaluating alfalfa cultivars for 

 bacterial wilt (Corynebacterium insidiosum (McCull.) H. L 

 Jens.) resistance has been the bare-root soak method of 

 inoculating the plants and then growing these plants for 3 

 to 4 months in the field. This method was developed by Cor- 

 mack and others (12) and modified by Elling and Frosheiser 

 (20). 



Bacterial wilt-infected roots, free of other diseases, are 

 selected from previous field tests as inoculum. After the 

 crown is removed, the roots are washed, ground, packed in 

 plastic bags, and stored at - 18°C (50). Bacteria will remain 

 virulent several years in continuously frozen material. About 

 one-half hour before inoculation, the infected root material 

 (50 g/L) is soaked in tap water to make a bacterial suspen- 

 sion. The ground-root material is left in the water throughout 

 the inoculation period. 



Seedling plants can be grown on any substrate before being 

 inoculated. In Minnesota they are grown in sand for at least 

 8 weeks. Plants are inoculated with Rhizobium meliloti, fer- 

 tilized and watered to promote maximum growth. Using 

 sand simplifies pulling the plants and washing the roots. 

 After the plants are pulled, they are individually separated 

 and small ones discarded. From 50 to 70 plants per replica- 

 tion are tied in a bundle. Three replications are grown for 

 each entry. 



Roots must not be allowed to dry out before inoculation. 

 Plants are inoculated by immersing the roots in the bacte- 

 rial suspension for 20 to 30 min. Upon removal, the tops of 

 the plants are cut off about 5 cm above the crown and the 



roots trimmed to about 10 to 12 cm. Several bundles are 

 wrapped together in paper towels to keep the roots moist 

 until transplanted. The inoculated plants wrapped in this 

 manner and placed in 1 to 2 cm of water can be stored for 

 several days at 2° to 4°C before transplanted. 



Inoculated plants are transplanted to the field in early June 

 with a modified tobacco transplanter and are spaced about 

 15 cm apart within the row with 1 m between rows. The field 

 is cultivated and irrigated to maintain vigorous plant growth 

 throughout the summer. Harmful insects may need to be 

 controlled. During August, dead plants in the very suscep- 

 tible cultivars are removed and recorded. 



About 3 months after transplanting (Sept. 10-15), the plants 

 are undercut about 15 cm deep, pulled, and the taproots 

 sectioned and examined for internal discoloration. Ratings 

 are based on a to 5 scale as follows: = sectioned sur- 

 face is clean and white; 1 = very small spots of yellow- 

 brown discoloration are visible in the stele; 2 = discolora- 

 tion is more evident with up to one-third of stele affected; 

 3 = nearly the entire stele is discolored but the cortex is 

 relatively white; 4 = discoloration extends throughout the 

 stele but plant is still alive; and 5 = root is severely rotted 

 with plant dead or dying. Classes and 1 are considered 

 resistant. 



Levels of bacterial wilt resistance among cultivars within a 

 test can be expressed as percentages of resistant plants or 

 as ASI. The ASI score is more precise than percentage of 

 resistant plants and is more useful in comparing relatively 

 small differences in degree of resistance in a single test (2). 

 The percentage of resistant plants, however, is most useful 

 in comparing cultivars evaluated in different test years. 

 Data from each test are expressed as a percentage of a 

 standard resistant check cultivar (Vernal). These data then 

 are adjusted according to a longtime average (table 1). 

 Nearly all cultivars grown in the United States have been 

 evaluated at the University of Minnesota with these 

 procedures. 



Table 1.— Percentage of bacterial wilt resistant plants 

 of alfalfa cultivars compared with Vernal 



Average Resistant 

 of Vernal plants per 

 Cultivar Tests per test cultivar 1 



Number Percent Percent 



Agate 2 154.0 65 



Iroquois 2 145.3 61 



Narragansett 15 1.5 1 



Ramsey 4 89.6 38 



Ranger 16 35.0 15 



Vernal 16 100.0 42 



'Calculated by multiplying percentage of Vernal by 42.0 (average 

 percentage of resistant plants in Vernal from 16 test years). 



20 



