Witches'-broom rust

Pathogen and disease type. Witches'-broom rust is a fungal disease caused by the basidiomycete Calyptospora columnaris. It is a heteroecious rust fungus, meaning it requires two unrelated host species to complete its life cycle. The fungus spends its dormant stage on fir trees and shifts to its primary host, lingonberries, during the spring and summer months to reproduce.

Affected crops. The disease primarily impacts lingonberries (Vaccinium vitis-idaea). It can also infect other members of the Ericaceae family, such as blueberries and cranberries, particularly when these crops are cultivated in proximity to infected fir forests. The presence of the intermediate host is a critical factor in the disease epidemiology.

Symptoms and signs. The hallmark of this disease is the development of proliferation shoots known as "witches'-brooms," where branches become unnaturally thin, elongated, and densely clustered. The underside of infected leaves displays bright orange-yellow pustules filled with spores. As the infection progresses, affected shoots lose their vitality, lose leaves prematurely, and eventually die off.

Development and impact. The development of Calyptospora columnaris is highly dependent on environmental conditions, particularly high humidity and temperate weather, which facilitate spore dispersal and germination. The economic impact includes significant yield loss, reduced fruit quality, and the overall weakening of the berry patches, making them susceptible to other stressors or secondary infections.

Protection and management.

• Remove and destroy all infected shoots to reduce inoculum levels.
• Maintain spatial isolation between lingonberry plantations and fir trees.
• Apply preventative fungicide treatments during the early growth stages if the site is in a high-risk area.
• Ensure proper field drainage to decrease relative humidity around the base of the plants.