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Inheritance of Resistance to Race 1 and Race 2 Anthracnose in Arc and Saranac AR Alfalfa¹

Previous inheritance studies of resistance to anthracnose in alfalfa (Medicago sativa L.) caused by Colletotrichum trifolii Bain included only race 1. The objectives of this study were to simultaneously study the inheritance of race 1 and race 2 and to determine the association between resistances. Twenty plants were randomly selected from each of ‘Arc’ and ‘Saranac AR’ alfalfa, cloned, and ramets needle inoculated with race 1 and race 2 of C. trifolii. Seven of the Arc clones were susceptible to both races, 12 clones were resistant to race 1, and one clone was resistant to race 1 and race 2. Five of the Saranac AR clones were susceptible to both races, three clones were resistant to race 1, and 12 clones were resistant to race 1 and race 2. Each of the 40 clones were selfed and testcrossed to a race 1 and race 2 susceptible, male-sterile clone. The progeny were evaluated for anthracnose resistance, and the segregations tested for fit to tetrasomic segregation ratios. Three reaction types were observed in the progeny: a.) susceptible to both races (SS); b.) resistant to race1—susceptible to race 2 (RS); and c.) resistant to both races (RR). No plants susceptible to race — resistant to race 2 (SR) were observed. Segregation ratios supported the hypothesis that two independent dominant genes with tetrasomic inheritance control anthracnose resistance in Arc and Saranac AR alfalfa. The An₁ gene in Arc codes for resistance only to race 1. The An₂ gene in Saranac AR codes for resistance to both race 1 and race 2. A gene similar to the An₁ gene in Arc that codes for resistance only to race 1 also occurs at low frequency in Saranac AR and is frequently masked by the An₂ gene.

Key Words: Breeding • Pathology • Genetics • Colletotrichum trifolii • Medicago sativa

² Research agronomist and research plant pathologist (now with W-L Research, Inc., Highland, MD 20777), respectively, Field Crops Laboratory, Plant Genetics and Germplasm Inst., Beltsville Agric. Res. Ctr., Beltsville, MD 20705.

Received for publication September 4, 1984.

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