Anthracnose, caused by Colletotrichum trifolii Bain, decreases forage yield and stand persistence of alfalfa (Medicago sativa L.) throughout much of the USA. Little is known about the effects of anthracnose or host-plant resistance on forage quality, including phenolic monomer concentrations. In a controlled environment, we compared forage quality, including phenolic monomers, of alfalfa cultivars Saranac (susceptible) and Saranac AR (resistant) with and without anthracnose (Race 1). In the absence of inoculation, leaves of the two cultivars did not differ significantly for any parameter measured. Leaves had more total phenolics than did stems. Forage quality of inoculated leaves of Saranac often was lower than that of inoculated Saranac AR. Inoculated leaves had higher concentrations of the substituted benzoic acids (p-hydroxybenzoic, vanillic, and syringic) but concentrations of the substituted cinnamic acids (p-coumaric, ferulic, and sinapic) were not altered by the disease. Forage quality of uninoculated stems of Saranac and Saranac AR differed only for neutral detergent fiber. Conversely, infection of Saranac slowed phenological development of shoots, with a concomitant increase in stem protein and digestibility. Inoculated stems of Saranac, however, had significantly more p-hydroxybenzoic, and syringic acids, p-hydroxybenzaldehyde, vanillin, and total phenolics than did stems of inoculated Saranac AR or controls. The presence of resistance to anthracnose in Saranac AR did not affect forage quality, including monophenolic profiles, of uninoculated leaves and stems. Mechanisms of resistance of Saranac AR maintained forage quality of leaves inoculated with C. trifolii.
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