Submitted to: Lettuce Workshop and Leafy Vegetable International Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: April 20, 2004
Publication Date: August 28, 2004
Citation: Mccreight, J.D. Evaluation of lettuce germplasm for resistance to lettuce aphid. 17th International Lettuce and Leafy Vegetable Conference, Quebec, Canada 2004. p. 19. Technical Abstract: Lettuce aphid (Nasonovia ribisnigri Mosley), a problem since the 1970s on lettuce (Lactuca sativa L.) in Europe, first occurred in the U.S. on lettuce in the Salinas Valley, California in 1998, and is now present in most of the lettuce production districts of Arizona and California. High-level resistance to the lettuce aphid discovered in several Lactuca virosa accessions and conditioned by a single dominant gene, Nr, is commercially available. Genetically unique sources of resistance are being sought as a protection in the event that the lettuce aphid overcomes the resistance conditioned by Nr. Approximately 1,200 lettuce PI lines were evaluated using two greenhouse protocols in 2002 and 2003. A western U.S. strain of lettuce aphid was obtained from a commercial lettuce field in Salinas Valley and reared on 'Parris Island' cos lettuce in insect-proof cages in a greenhouse. In 2002, plants were individually infested with five 24-hr nymphs per plant (controlled protocol), and the numbers of aphids per plant were counted 10 to 14 days post-infestation. Beginning in 2003, plants were mass-infested (mass protocol) with nymphs and alates of various ages and numbers, and the number of aphids 10 to 14 days post-infestation were estimated and categorized using a 1 to 5 scale where 1 = 0 aphids per plant, 2 = 1-10 aphids per plant, 3 = 11-20 aphids per plant, 4 = 21-30 aphids per plants, and 5 >30 aphids per plant. Susceptible, e.g., 'Salinas', and resistant, e.g., 'Barcelona', checks were included in all tests for comparison. Most (1188) of the introductions evaluated were susceptible. Although a few accessions had a few plants with very low numbers of aphids after repeated infestation, their progeny proved to be susceptible in subsequent tests. PI 491093, identified as Lactuca serriola from Turkey, was highly resistant in a mass protocol evaluation. In a subsequent controlled protocol test, PI 491093 (4.5 aphids per plant) did not differ significantly from 'Barcelona' (3.8 aphids per plant) and both differed significantly from 'Salinas' (21.4 aphids per plant), and 'Ultra Green' (19.8 aphids per plant). Further research is needed to determine the inheritance of resistance in PI 491093 and allelism with the Nr gene.