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Bean yellow mosaic virus (BYMV) is an important pathogen of red clover (Trifolium pratense L.). A hypersensitive reaction to the most prevalent strain of BYMV in Kentucky (designated 204‐1) conveys resistance. The objectives of the present study were to backcross the genes for the hypersensitive reaction into 10 clones of ‘Kenstar’ to widen the BYMV resistance of this cultivar and to further investigate the inheritance of resistance. Populations of Fl’s and five backcrosses to each of the 10 parental clones of Kenstar were produced using the clones as recurrent parents and a hypersensitive reacting clone as the nonrecurrent parent. Four Kenstar parental clones were found to possess the hypersensitive reaction, and two had a different type of resistance. Inheritance of the hypersensitive reaction to BYMV strain 204‐1 was similar to that reported earlier. Incorporation of the hypersensitive reaction to strain 204‐1 into the Kenstar clones did not increase virus resistance under spaced‐plant field conditions as compared to Kenstar polycross progenies. Lack of infection in broadcast plots prevented a complete assessment of the potential effect of the hypersensitive reaction. A new strain of BYMV was discovered and designated the red clover or ‘RC’ strain. This strain is similar to previously described isolates of BYMV from soybean, and it was hypothesized that the RC strain may have occurred as a result of the increase in soybean acreage in Kentucky. Phenotypic recurrent selection under spaced‐plant conditions where opportunity exists for infection by many different strains of BYMV and other viruses is suggested as aneffective breeding procedure.

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The development of Erysiphe polygoni on leaves of red clover was retarded by poor infection with bean yellow mosaic virus. Conidia germinated on leaves infected with the virus but disintegrated following necrosis of the leaf tissue in the immediate area of the germinating conidia.