Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/21/2003
Publication Date: 2/1/2004
Citation: Lyman, M.R., Curry, K.J., Smith, B.J., Diehl, S.V. 2004. Effect of Cercosporella rubi on blackberry floral bud development. Plant Disease. 88:195-204 Interpretive Summary: Rosette disease of dewberries and blackberries is a serious problem for growers because it often severely reduces fruit production in the southeastern U.S. While Cercosporella rubi is always found in symptomatic flowers that develop on rosetted shoots in early spring, little is known about the how the fungus infects the blackberry host. Contrary to most fungal pathogens, C. rubi proliferates on the surfaces of developing vegetative and floral appendages without penetrating host tissues. The primary reason for conducting these investigations was to search for evidence that would indicate whether or not fungal penetration of host cells occurred during a particular stage of floral development. Cercosporella rubi was observed in floral buds from 0.08 mm up to 6.0 mm in diameter. Our microscopic investigation showed that C. rubi is present at the time floral differentiation is initiated and persists in opened flowers. Although C. rubi appears to induce rosette formation, its presence does not appear to alter the early stages of floral development morphologically. The fungus seems to induce floral abortion by accelerating senescence. Infected flowers have shriveled, brown pistils and stamens, while healthy flowers have plump, green pistils and stamens. In 6.0 mm diameter floral buds, hyphae growing through the style appeared to interfere with pollen tube development and cause the ovule to abort. This is the first study of fungal and host cell interactions at the ultrastructural level and the first to examine the fungus in floral buds less than 6.0 mm in diameter. These findings directly benefit blackberry growers in the southeastern U.S. because it indicates the necessity of continuing fungicide applications until mid-summer. Other research scientists can use this information as a basis for in-depth studies of the early infection events.
Technical Abstract: Rosette disease limits production of blackberries in the southeastern U.S. The first symptoms of this fungal disease is the emergence of rosettes on blackberry floircanes in early spring on which develop elongated floral buds with reddish sepals and opened flowers with pink, wrinkled petals. Cercosporella rubi is found in these abnormal flowers, and its hyphae proliferate within floral buds throughout their development without penetrating host tissues. Infected flowers do not produce fruit. The availability of information on the relationship between C. rubi and blackberry is limited. The primary reason for conducting these investigations was to search for evidence that would indicate whether or not fungal penetration of host cells occurred during a particular stage of floral development. Cercosporella rubi was isolated consistently from all floral buds with detectable rosette symptoms and never from healthy buds. Microscopic examinations of buds from blackberry plants grown in south Mississippi collected from late April until late July revealed that C. rubi continued to sporulate on some cultivars until late July. Vegetative and floral buds of blackberry were examined with light and electron microscopy. Cercosporella rubi was present from the time of floral differentiation until diseased flowers became senescent. No morphological differences were seen between healthy and diseased floral buds with a diameter of 5.0 mm or less other than the presence of C. rubi. Signs of necrosis appeared as floral buds reached the 6.0 mm diameter stage. Hyphae also proliferated on stamens and ultimately clogged the forming stomia. We did not observe fungal penetration of host cells at any time during the developmental process.