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

Abstract

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 An 1 gene in Arc codes for resistance only to race 1. The An 2 gene in Saranac AR codes for resistance to both race 1 and race 2. A gene similar to the An 1 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 An 2 gene.