Rosette, caused by the fungus Cercosporella rubi, is an important blackberry disease in the southeastern United States. This disease severely reduces fruit production, and its management has been erratic due to a limited understanding of the host-pathogen relationship. In this study, we expand on previous histological investigations of the development of C. rubi on blackberry at tissue and cellular levels from floral bud initiation through senescence of the flower. Symptomatic and asymptomatic floral buds were examined with light microscopy and both scanning and transmission electron microscopy (SEM and TEM, respectively). Fungal development on the surface of floral buds was examined with SEM. Previous light microscope histological studies were unclear about whether C. rubi penetrated host tissue. With TEM, we demonstrated an intimate association between fungal and host cells with no penetration prior to death of the host tissue. C. rubi was present on symptomatic floral buds before development and through senescence. No morphological differences were seen between healthy and diseased floral buds ≤5.0 mm in diameter other than the presence of C. rubi. Necrosis was observed in symptomatic buds at the 6.0-mm-diameter stage and progressed through floral senescence.
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