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Comparison of Genes Controlling Race 1 Anthracnose Resistance in Arc and Saranac AR Alfalfa

USDA-ARS, Beltsville, MD 20705

^* Corresponding author.

Resistance in ‘Arc’ alfalfa, Medicago sativa L., to anthracnose caused by race 1 of Colletotrichum trifolii Bain is conditioned by a single dominant gene, An₁. A single gene with gene action like An₁ conditions resistance to race 1 anthracnose in ‘Saranac AR’. This tudy was conducted to determine whether or not the gene in Saranac AR coding for resistance to race 1 of C. trifolii is the same An₁ gene operating in Arc. Crosses between three simplex race 1 C. trifolii-resistant clones each of Arc and Saranac AR were made, progeny were evaluated for resistance to race 1 of C. trifolii, and susceptible progeny discarded. Resistant progeny were testcrossed to a susceptible male sterile clone. Those progeny whose testcross lines segregated 1:1 were discarded and those that appeared to segregate as a 3:1 or 5:1 were evaluated further. Ultimately the actual segregation ratio was determined to be 5:1, thus indicating the gene in Saranac AR coding for resistance to race 1 of C. trifolii to be the same An₁ gene occurring in Arc.

Key Words: Colletotrichum trifolii Bain • Medicago sativa L. • Disease resistance • Inheritance • Dominant gene • Segregation ratio

Received for publication November 5, 1987.