Standard Tests To 

 Characterize Pest Resistance 

 in Alfalfa Cultivars 



Need to Standardize Pest-Resistance Evaluation 



Characterizing Disease Resistance 



Alfalfa (Medicago sativa L) ranks fourth in the United States 

 in acreage (26,394,000) and dollar value ($5.6 billion) among 

 all agricultural crops (Agricultural Statistics, 1982). Alfalfa is 

 produced in all 50 States. Alfalfa's extremely wide area of 

 adaptation and its perennial growth habit cause it to be ex- 

 posed to a large number of disease, insect, and nematode 

 pests. In 1970, alfalfa scientists estimated that the com- 

 bined annual losses from all pests amounted to about 40 

 percent of the national production. 



The importance of pest resistance was first demonstrated 

 in 1942 with the release of two bacterial wilt-resistant 

 cultivars— 'Ranger' for the northern Great Plains area and 

 'Buffalo' for the central Plains area. Since then, cultivars 

 have been released that have resistance to anthracnose, 

 alfalfa weevil, blue alfalfa aphid, verticillium wilt, fusarium 

 wilt, pea aphid, phytophthora root rot, spotted alfalfa 

 aphid, and stem nematode. Research techniques are being 

 developed for selecting resistance to other important 

 pests. 



The development of pest-resistant alfalfa cultivars has been 

 an important contribution to agriculture. Most alfalfa acre- 

 age in the United States now is planted with cultivars that 

 were selected for resistance to one or more pests. During 

 the past decade, alfalfa acreage in the United States has 

 remained constant, whereas yields have increased nearly 

 20 percent. Much of this increase can be credited to culti- 

 vars with increased types and levels of pest resistance. 

 Some types of pest resistance also have increased forage 

 quality. 



A primary goal of most alfalfa-breeding programs is to 

 develop new cultivars with multiple pest resistance. There 

 are many pests, however, and cultivar development takes 

 time. Alfalfa breeders, plant pathologists, and entomol- 

 ogists must assign priorities to each type of resistance. 

 They also must use the most effective methods for selecting 

 and evaluating resistant germplasm. Growers, extension 

 personnel, marketing specialists, and research scientists 

 need uniform procedures to describe levels of pest resist- 

 ance of cultivars. The Plant Variety Protection Act of 1970 

 also increased the need for standardizing the characteriza- 

 tion of pest resistance. This publication presents a sum- 

 mary of pest resistance information to help the alfalfa in- 

 dustry develop and describe pest-resistant cultivars. 



Background 



Plant disease may be caused by either abiotic agents or 

 biotic agents. Abiotic agents include nutrient deficiencies, 

 toxicities, physiogenic spotting, and air pollution damage. 

 Differences occur among cultivars in their reaction to some 

 abiotic agents. The selection of strains resistant to air pollu- 

 tion by Howell and others (42f is an example. In general, 

 however, little progress has been made in developing resist- 

 ance to abiotic agents. This publication, therefore, is con- 

 fined to diseases caused by biotic agents. 



Biotic agents that incite diseases in alfalfa are bacteria, 

 fungi, viruses, and mycoplasmas. Graham and others (29) 

 reported that at least 70 different pathogens occur in alfalfa 

 and that about 30 limit the growth and production of the 

 crop. A survey of the United States by the Alfalfa Improve- 

 ment Conference Committee identified 22 diseases that 

 were problems in more than one State. Of these diseases, 

 19 were identified as problems in enough States so that 

 national distribution and severity maps could be developed. 



Some research information is available on nearly every 

 important alfalfa disease in the United States (27,46), but 

 often only one scientist may have worked with a particular 

 disease. Even though information is available and variation 

 in host-plant resistance has been reported for most impor- 

 tant diseases, reliable evaluation procedures are available 

 for only bacterial wilt, anthracnose, common leaf spot, 

 downy mildew, fusarium wilt, lepto leaf spot, phytophthora 

 root rot, rust, and verticillium wilt. 



Sufficient information is available to screen for resistance 

 to bacterial leaf spot, rhizoctonia crown rot, sclerotinia 

 crown and stem rot, spring black stem, stemphylium leaf 

 spot, summer black stem, yellow leaf blotch, and alfalfa 

 mosaic virus. Unfortunately, the procedures used to deter- 

 mine resistance to these diseases usually rely on natural 

 field epiphytotics; therefore, reliable field evaluation proce- 

 dures are available only on occasion. With additional 

 research, it may be possible to develop evaluation pro- 

 cedures for most, if not all, important diseases in alfalfa. 



Reliable evaluation procedures require that uniform infec- 

 tions be repeated in subsequent tests. The percentage of 

 resistant plants and average disease severity index (ASI) 



1 1talic numbers in parentheses refer to Litera- 

 ture Cited, p. 33. 



