MARSSONINA-ROSAE VARIANTS IN MISSISSIPPI AND THEIR VIRULENCE ON SELECTED ROSE CULTIVARS

Spore size, colony color, and pathogenicity of Marssonina rosae, imperfect stage of Diplocarpon rosae, were determined to ascertain the occurrence of fungus variants. Isolates were obtained from 10 rose cultivars in the rose disease research garden at Mississippi State University and from two unidentified cultivars in Oktibbeha and Wayne counties. Spores ranged from 22.1 to 25.3 mum in length and from 5.3 to 7.1 mum in width. Isolates were divided into four color groups: clove brown, light pinkish cinnamon, light mouse gray, and clove brown with apricot and sa[mon buff spot mixture. Four pathogenic variants were found among the isolates. Isolate ELT from the cultivar Electron was the most virulent and was more severe on Electron than on other cultivars, Rosa roxburghii was immune to all isolates.