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Effect of Selecting for Downy Mildew Resistance in Alfalfa on Saponin Content¹

Alfalfa (Medicago sativa L.) saponin is toxic to several plant disease organisms, including some Oomycetes closely related to Personospora trifoliorum d By., the alfalfa downy mildew fungus. Resistance in alfalfa to downy mildew has been changed significantly during selection to alter the forage saponin content. The primary objective of this research was to determine the effects of selecting for downy mildew resistance in alfalfa on forage saponin levels. Alfalfa ‘CUF 101’ and germplasms UC 123 and UC 143 derived from it by one and two cycles, respectively, of recurrent phenotypic selection for resistance to P. trifoliorum were evaluated for forage saponin content. The percent of plants resistant (symptomless) to P. trifoliorum isolates 1–7 and 1–8, respectively, were 28 and 3 for CUF 101; 62 and 3 for UC 123; and 62 and 72 for UC 143. Saponin indices (higher index = lower saponin content) were CUF 101, 101; UC 123, 92; UC 143, 100; ‘Lahotan’ (low-saponin check), 98; and ‘Uinta’ (high-saponin check), 35. Alfalfa germplasms KS80, KS145, KS167, and K78-10, all higly resistant to P. trifoliorum and several other fungi and insects, had saponin indices about midway between those of Lahonton and Uinta. Saponin apparently played no role in the resistance of UC 143 to P. trifoliorum since UC 143 was derived from the very low-saponin CUF 101 cultivar without affecting the saponin index.

Key Words: Medicago sativa L. • Peronospora trifoliorum d By

² Professor and research assistant, respectively, Dep. of Plant Pathology, Kansas State Univ., Manhattan, KS 66506.

Received for publication September 29, 1986.