Purple blotch of raspberry

Purple blotch of raspberry, caused by the fungus Septocyta ruborum (syn. Didymella applanata), is a destructive fungal disease that primarily affects the canes and buds of raspberry plants. This pathogen is responsible for significant yield losses, as it directly damages the vascular system of the stems and prevents the proper development of fruitful laterals.

The primary host for this fungus is the common red raspberry, although it can also affect blackberries. The disease targets all parts of the cane above ground, with the most severe damage occurring during cool, wet growing seasons. Neglected plantations with poor airflow are particularly susceptible to widespread infections.

Symptoms typically appear during the summer as irregular purple or reddish-brown lesions around the leaf axils. As the infection progresses, these blotches enlarge and coalesce, often causing the epidermis of the cane to crack. By the following spring, the buds within the infected areas are usually dead, leading to the withering of the entire fruiting cane before harvest.

The disease cycle is heavily influenced by high humidity and precipitation, which facilitate the release and spread of fungal spores from existing pycnidia or perithecia. The pathogen survives the winter in infected canes and leaf debris on the ground, creating a continuous cycle of infection if sanitation measures are not strictly followed by the grower.

Effective control requires an integrated approach. Essential practices include the prompt removal and burning of old, infected canes immediately after harvest to eliminate the inoculum source. Improving canopy aeration through proper pruning and thinning is vital. Additionally, applying protective fungicides during the early leaf-out and pre-flowering stages is highly recommended to protect new growth from early infection.