Submitted to: Acta Horticulturae
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/17/2002
Publication Date: 9/1/2002
Citation: Smith, B.J., Killebrew, J.F. 2002. Epidemiology and control of blackberry rosette caused by the fungus(Cercosporella rubi). Acta Horticulturae. 585:319-320.
Interpretive Summary: Rosette disease may reduce yields of erect blackberries in the southeastern U. S. more than 50% by the fourth year of production. Distorted growth or witches' broom symptoms of axillary buds on the floricanes which are first evident in early spring. The leaves on rosettes are pale green and smaller than normal. Infected flower buds are more elongated, larger,coarser, and redder than uninfected buds. Petals of infected flowers are wrinkled and twisted with a pink tint. Infected blossoms usually abort or produce poor quality fruit. Chemical control of rosette has been erratic. In this study we identified fungicides for rosette control, determined the optimum application schedule, and by inference determined the infection period of the pathogen. Benomyl and bordeaux mixture were the most effective fungicides; however, bordeaux mixture was phytotoxic and benomyl will no longer be manufactured after this year. DCNA, myclobutanil, ferbam, metalaxyl, and propiconazole gave limited control. Triadimefon, vinclozolin and iprodione were ineffective. Fungicides were most effective when applications began at bloom and continued until sporulation ceased about a month after harvest. These studies combined with field observations indicate that the infection period for rosette extends throughout the time the fungal pathogen is sporulating on infected flowers. In the southern U.S. the infection periodmay be as long as 16 weeks from mid-March to mid-July. Since infection occurs during harvest, fungicides are needed that can be used during this critical time.
Technical Abstract: Rosette (also known as double blossom) is a severe disease of erect blackberries, and if not controlled, often limits commercial production of blackberries grown in the southeastern U. S. Most erect blackberry cultivars are very susceptible to rosette which is caused by the fungus, Cercosporella rubi. The disease cycle of rosette matches the biennial growth pattern of blackberries. Symptoms of rosette are often not obvious in a planting until the fourth year, and many growers do not recognize the disease until it is wide-spread and yield is drastically reduced. Effective fungicidal control of rosette has been erratic. Our objectives of this study were to determine the infection period for rosette,fungicide efficacy, and optimum timing of fungicide applications. Benomyl and bordeaux mixture were consistently the most effective fungicides; however,bordeaux mixture was phytotoxic. DCNA, myclobutanil, ferbam,metalaxyl, and propiconazole gave limited control, while triadimefon, vinclozolin and iprodione were ineffective. Fungicides were most effective when applications began at bloom and continued until sporulation ceased about a month after harvest. We concluded that rosette infection starts in early spring when the pathogen begins to sporulate on fully open, infected flowers and continues as long as the fungus sporulates on the dead infected flowers which may persist on the rosetted canes until late summer. Since benomyl will no longer be manufactured after 2001, other new, effective fungicides are needed for rosette control.