dead, the test is terminated and surviving seedlings in each 

 row are counted. Resistance is expressed as percentage of 

 seedling survival and is compared with that of a resistant 

 check cultivar. In some Southwestern States, large field 

 cages have been used successfully for aphid evaluations. 



Blue Alfalfa Aphid Resistance 



M. W. Nielson 



Forage Insects Research Laboratory, Tucson, Ariz. 



Techniques for evaluating resistance to the blue alfalfa 

 aphid, Acyrthosiphon kondoi Shinji, in the greenhouse are 

 similar to greenhouse tests for the pea aphid and spotted 

 alfalfa aphid (63, 64). Because the blue aphid is difficult to 

 rear in sufficient quantities in the Southwest, even during 

 the cooler months (Dec-April), the best results often are 

 obtained with a field-cage technique. Rows of the suscep- 

 tible entry PA-1 (resistant to pea aphid and spotted alfalfa 

 aphid) are planted in the fall. The length and number of rows 

 are determined by the size of the cage. In the spring, the 

 field plot is caged; then the rows are infested manually to 

 start the culture. When the population begins to build up, 

 the test entries are planted between the PA-1 rows. After the 

 test entries germinate, the PA-1 rows are cut back to force 

 the aphid populations to feed on the young seedlings. The 

 number of test entries that germinated are counted before 

 cutting the PA-1 rows; then, the number of plants that sur- 

 vived after the susceptible check planted with the test en- 

 tries is destroyed are counted. Resistance is expressed as 

 percentage of seedling survival. 



Spotted Alfalfa Aphid Resistance 



M. W. Nielson 



Forage Insects Research Laboratory, Tucson, Ariz. 



Seedling tests used for evaluating spotted alfalfa aphid 

 (Therioaphis maculata (Buckton)) resistance are similar to 

 those used for pea aphid evaluations. Although resistance 

 tends to increase in mature plants, resistance ratings from 

 seedling cultivar evaluations are much like those of mature 

 plants (40, 41). Tests described here are similar to proce- 

 dures suggested by Hackerott and others (36) and Klement 

 and Randolph (52). 



All entries are planted in galvanized flats. Each flat con- 

 tains 13 rows, 50 seeds to a row; 8 rows are test entries, and 

 5 rows are susceptible checks. The rows are arranged so 

 that each test row is next to a susceptible check row. Each 

 test includes four replications of each entry. 



Seedlings are counted in the unifoliolate stage (approxi- 

 mately 7 days old) and infested manually with 4 cm 3 of 

 aphids per flat. Tests are conducted in the greenhouse at 



temperatures between 20° to 30°C. When all plants in the 

 susceptible check rows are dead, surviving seedlings are 

 counted in each test entry. Resistance is expressed as per- 

 centage of seedling survival. Test entries are compared with 

 a resistant check included in each test. 



Nielson and Don (61) and Nielson and others (62) reported 

 the existence of biotypes of spotted alfalfa aphid in the 

 Western United States. Cultivars were evaluated for resist- 

 ance to four biotypes by Nielson and others (66) using these 

 techniques. If evaluations are conducted with aphids of 

 unknown biotypes, a description of when and where the 

 aphids were collected is useful. 



Potato Leafhopper Yellowing Resistance 



W. R. Kehr and G. R. Manglitz 

 University of Nebraska, Lincoln 



Field tests to select for resistance and to evaluate materials 

 for resistance to potato leafhopper (Empoasca fabae (Har- 

 ris)) yellowing have usually been more successful than 

 greenhouse seedling tests (45, 47, 48, 49, 51, 59, 86, 87). Nat- 

 ural field infestations and manual infestations of field 

 cages have been successfully used to evaluate the resist- 

 ance of breeding materials and cultivars. The mechanisms 

 of resistance are not well defined. 



Spring seedings are made from mid-April to early May. 

 Greenhouse-grown seedlings 6 to 8 weeks of age are trans- 

 planted to the field in late April or early May. Single row 

 plots are used with a minimum of four replications. Although 

 transplanted plots consist of 25 plants spaced 30 cm apart, 

 spacings of 30 to 90 cm between rows have been successful. 

 Seeded plots 3 to 5 m in length have also been used suc- 

 cessfully, as have spring-seeded cultivar tests using 

 multiple-row plots. Plantings are kept weed free and insec- 

 ticides are not used. 



The initial growth from spring transplanting or seeding can 

 usually be scored for yellowing (or reddening, depending on 

 plant genotype) by mid-July (48). After the first cutting in the 

 year of establishment and in subsequent years, harvest 

 intervals are extended 10- to 20 days beyond the normal 

 32-to 35-day cutting interval to maximize infestation. Plants 

 are usually scored twice during the growing season, that is, 

 the initial growth and first regrowth (second cutting) in the 

 year of establishment, or the second and third cuttings after 

 the year of establishment. 



Individual plants or seeded rows are visually scored for leaf- 

 hopper yellowing using a 1 to 9 scale (19th Alfalfa Improve- 

 ment Conference, CR-54-56:63, 1964). A score of 1 = up to 

 10 percent of leaves show yellowing, 2 = between 10 and 

 20 percent of leaves yellow, 3 = between 20 and 30 percent 



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