Reaction of Certain Diploid and Tetraploid Alfalfas to Some Phytopathogens Including the Blackstem Disease

Abstract

Alfalfa in South Dakota as in other parts of the United States and the world is adversely affected by a disease known as Blackstem. This disease is induced by one or more fungi or bacteria whose abundance depends as much on the high susceptibility of the current alfalfa varieties as on the weather. Differences in plant susceptibility to one of these microorganisms, Ascochyta imperfect, has already been shown to occur within and between alfalfa varieties or strains. Reitz working in Kansas obtained F1’s between highly resistant and highly susceptible plants that were intermediate in reaction to this fungus, and in a few crosses they noted a tendency of the resistant reactions of plants to be dominant over the susceptible. By inbreeding and hybridizing selected plants, they obtained a plant population with a higher resistance to this pathogen than the current varieties. Similar possibilities undoubtedly could exist for the other microorganisms including Blackstem, but no attempt has been made to date to determine this. Even for Ascochyta imperfecta, however, the preliminary work of Reitz, has never been extended or applied to the development of a commercial variety resistant to the disease. The purpose of the present study was to find out to what extent alfalfa plants differed in their reaction to the Blackstem disease induced by the various pathogens and to determine to what extent the reactions to these pathogens were heritable. If such plant differences could be shown to be heritable, an opportunity would thus be afforded to develop an alfalfa variety that resists the disease to a greater extent than any of those currently used. Field and greenhouse screening test accordingly, were conducted to measure plant reactions to three of the pathogens and the results obtained are presented herein