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Monogenic Resistance to Red Clover Leaf Rust Associated with Seedling Lethality¹

Monogenic dominant resistance to leaf rust (caused by Uromyces trifolii (Hedw. f. ex De.) Lév var. fallens (Desm.) Arth.) in a red clover (Trifolium pratense L.) clone, KyCL 75, was studied to determine its value as a source of resistance for cultivar development. KyCL 75, reported to be heterozygous for rust resistance, was crossed with a ,rust susceptible clone, L12, and the F₁'s were intercrossed and backcrossed to the parents. Equal segregation of the F₁ population into resistant and susceptible types indicated L12 was homozygous recessive for rust susceptibility. When resistant F₁'s were intercrossed or backcrossed to the resistant parent, the F₂ and BC populations segregated 2 resistant:l susceptible rather than the expected 3:1. When susceptible F₁'s were intercrossed, all progenies were susceptible. Resistant plants selected from the F₂ and BC populations were backcrossed to the susceptible parent L12, and all populations segregated for leaf rust. The unexpected genetic ratios and failure to obtain homozygous resistant genotypes were attributed to a close linkage between a dominant allele conditioning rust resistance and a recessive allele causing seedling mortality. Hence, the homozygous resistant genotypes die in the seedling stage. Unless this linkage can be broken, KyCL 75's rust resistance cannot be used in developing a rust-resistant cultivar of red clover.

Key Words: Trifolium pratense L. • Uromyces trifolii var. fallens Arth. • Pleiotropism • Linkages • Lethality

² Research geneticist, ARS/USDA, Grassland-Forage Research Center, Temple, Tex. (former research assistant, Dep. of Agronomy, Univ. of Wisconsin); research geneticist, ARS/USDA, and associate professor of agronomy, and associate professor of plant pathology, Univ. of Wisconsin.

Received for publication August 26, 1976.