Marssonina rosae Variants in Mississippi and Their Virulence on Selected Rose Cultivars. Ida Wenefrida, Graduate Student, Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762. James A. Spencer, Plant Pathologist, Department of Plant Pathology and Weed Science, Mississippi State University, Mississippi State 39762. Plant Dis. 77:246-248. Accepted for publication 3 November 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0246.

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 µm in length and from 5.3 to 7.1 µm in width. Isolates were divided into four color groups: clove brown, light pinkish cinnamon, light mouse gray, and clove brown with apricot and salmon 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.