Seven isolates of Marssonina rosae (imperfect stage of Diplocarpon rosae) elicited different blackspot disease expression among 39 Old Garden Roses in laboratory tests. ‘Felicite Parmentier’ (Alba), ‘Mme. Hardy’ (Damask), ‘The Bishop’ (Centifolia) and ‘Rosa Mundi’ (Gallica) showed no symptoms. ‘Cardinal de Richelieu’ (Gallica), ‘Hermosa’ (China) and ‘Leda’ (Damask) were the most. susceptible showing a cultivar ratings mean of 2.86 or more. The Wayne variant was the least virulent, causing susceptible reactions on only 46.1% of the roses. The Hinds variant was the most virulent and caused symptoms of 89.7% of the rose plants tested. The remaining five variants caused disease on 69.1% to 79.5%of the plants.
Published as Mississippi Agricultural and Forestry Experiment Station Journal Series No. J7983.
2Professor and Laboratory Assistant, respectively.