EPIDEMIOLOGY AND CONTROL OF BLACKBERRY ROSETTE CAUSED BY THE FUNGUS CERCOSPORELLA RUBI

Abstract

Rosette disease may be severe on erect blackberries grown in the southeastern U.S. and often limits fruit production if not controlled. Caused by the fungus, Cercosporella rubi, the rosette disease cycle matches the biennial growth pattern of blackberries. Often rosette symptoms are not evident in a planting until the fourth year when many of the plants suddenly display signs of infection as new growth emerges in the spring. Effective fungicidal control of rosette has been erratic. The objectives of this study were to determine fungicide efficacy, optimum timing for fungicide applications, and rosette infection period. Foliar fungicides were applied to two rosette-susceptible, erect, thorny blackberry cultivars, and disease severity was rated just after the plants began to leaf out in early spring when rosettes were easily visible. Benomyl and bordeaux mixture were the most effective fungicides; however, bordeaux mixture was sometimes 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 fungal sporulation ceased about a month after harvest. The most effective schedule for benomyl applications was two applications before harvest, one mid-harvest, and two after harvest. This schedule reduced the number of rosettes from 40 on untreated plants to 3.3 on treated plants. Since benomyl will no longer be manufactured after 2001, other effective fungicides are needed for rosette control.