should both be used to describe cultivar reaction to dis- 

 eases. Cultivar evaluations should be based on individual 

 plant scores rather than on solid-seeded plots. A 1 to 5 rat- 

 ing when feasible (1 = no disease, 5 = severely diseased) 

 is favored. Ratings for each pathogen should be based on 

 specific lesion types or degree of infection (some diseases 

 may require fewer or more than five classes). Rating values 

 should be consistent from test to test. Generally, at least 

 100 plants should be used for evaluating a cultivar, and 

 resistant and susceptible check cultivars should be included 

 in every test. The susceptible check cultivar can be used to 

 estimate percentage of escapes in a test. The cultivar being 

 evaluated should be compared to the resistant check. 

 Standard seed lots of the check cultivars (when available) 

 should be used. Although no agreement was reached on the 

 number of tests required for evaluation, individual scientists 

 should base their decision on the number of escapes and 

 relative precision of their data. 



In describing the relative disease resistance of alfalfa culti- 

 vars, the word "tolerant" should be used to denote a type of 

 host pathogen interaction but should not be used to denote 

 low levels of disease resistance. The term "immune" prob- 



ably will have no place in alfalfa cultivar descriptions be- 

 cause of the difficulty with reaching homozygosity for any 

 trait. 



Summary of Resistance Information by Disease 



Distribution and severity maps for 19 important diseases in 

 the United States appear on pages 14 to 19. These maps are 

 based on observations and opinions of research and exten- 

 sion specialists from nearly every State. Scientists with 

 expertise on specific diseases or pathogens are identified 

 as resource people who can supply additional information 

 about developing and evaluating specific types of disease 

 resistance in alfalfa. Check cultivars are identified for dis- 

 ease evaluations where sufficient information was avail- 

 able. 2 In nearly all instances, public cultivars are designated 

 as checks instead of proprietory cultivars. This designation 

 ensures the availability of seed after the cultivars are no 

 longer commercially available. We attempted to keep the 

 number of check cultivars to a minimum. For these reasons, 

 some cultivars with higher levels of resistance and more ex- 

 tensive distribution may not have been selected as checks. 

 The availability and type of screening and evaluation pro- 

 cedures (L = laboratory, F = field tests) also are indicated. 



Tabulation summarizing available resistance information by disease 



Bacterial Leaf Spot 



Xanthomonas alfalfae 

 (Riker, Jones & Davis) Dows. 



Bacterial Wilt 



Corynebacterium insidiosum 

 (McCull) H. L. Jens. 



Distribution and severity map: Figure 1 

 Scientists and locations with expertise: 3 31 and 32 



Check cultivars: 



Dormant and semidormant types: 

 Nondormant types: 



Resistant 



Susceptible 



KS76 



Screening procedures: L 

 Evaluation procedures: None 



Distribution and severity map: Figure 2 



Scientists and locations with expertise: 1, 8, 16, 19, and 35 



Check cultivars: 



Dormant and semidormant types: 

 Nondormant types: 



Resistant 



Susceptible 



Vernal 



Narragansett 

 Sonora 



Screening procedures: L and F 

 Evaluation procedures: F (p. 20) 



2 See appendix, p. 37, for approximate levels of pest resistance 

 expected for check cultivars. 

 3 See listing of resource scientists on p. 12. 



