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Effect of Cercosporella rubi on Blackberry Floral Bud Development

February 2004 , Volume 88 , Number  2
Pages  195 - 204

Melinda R. Lyman and Kenneth J. Curry , Department of Biological Sciences, University of Southern Mississippi, Hattiesburg 39406 ; Barbara J. Smith , United States Department of Agriculture-Agricultural Research Service, Small Fruit Research Station, Poplarville, MS 39470 ; and Susan V. Diehl , Department of Forest Products, Mississippi State University, Mississippi State 39762

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Accepted for publication 21 October 2003.

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.

Additional keywords: double blossom, Rubus spp., witches'-brooms

The American Phytopathological Society, 2004