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In Vitro Assay of Excised Cotyledons of Alfalfa (Medicago sativa) to Screen for Resistance to Colletotrichum trifolii. J. D. Cucuzza, Plant Genetics, Inc., 1930 Fifth Street, Davis, CA 95616. J. Kao, Plant Genetics, Inc., 1930 Fifth Street, Davis, CA 95616. Plant Dis. 70:111-115. Accepted for publication 21 June 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/PD-70-111.

An in vitro assay using excised cotyledons of alfalfa has been developed that accurately detects resistance to Colletotrichum trifolii. The adaxial surfaces of excised cotyledons are placed on sterile moist filter paper in petri plates. The cotyledons are sprayed with a spore suspension, and resistant and susceptible reactions can be determined after incubation for 14 days at 24 C with a 16-hr light period. Susceptibility is based on the presence of sporulating acervuli. Five isolates of C. trifolii reacted similarly in the assay, and the range of effective inoculum was 2,50010,000 spores per spray. Excised cotyledons and the seedlings from which the cotyledons were obtained were simultaneously tested in vitro and in a greenhouse. Saranac AR and Saranac had 86.5 and 96.2% agreement of observations, respectively, between the two screening techniques. A blind test of six cultivars was done, and the percent resistance obtained with the in vitro screen agreed with published values. Results indicate the usefulness of this technique as an alternative to greenhouse screening methods.