Unless rosette is controlled, blackberry plantings become heavily infected by the rosette fungus, and berry production is severely reduced.
Blackberry rosette is recognized by the bunchy growth which appears on infected canes in the spring.
Flowers which form on rosettes also have more pink coloration and generally are sterile. As a result, few if any berries are formed on rosette growth. Since the disease becomes more severe in succeeding years, rosette control is necessary if home orchardists want to produce acceptable berry yields. All popular varieties of blackberries are susceptible to rosette, and disease control requires spring pruning to remove rosettes and application of fungicides at intervals through the season to protect new cane growth from rosette fungus infection. Applications number 5 and 6 are especially critical if rosette blooms are still present.