Witches' broom of serviceberry

Pathogen and disease type. The disease is caused by the ascomycete fungus Apiosporina collinsii. It acts as a systemic pathogen that induces localized hypertrophy, leading to the formation of dense, abnormal growths on branches commonly referred to as "witches' brooms."

Host plants and symptoms. The primary host is the serviceberry (Amelanchier). The most distinctive symptom is the development of clusters of thin, stunted, and closely packed twigs. Infected leaves often appear smaller, exhibit chlorosis, and tend to drop prematurely during the growing season.

Development and spread. The fungus overwinters as mycelium within the infected woody tissues. As spring moisture levels rise, the fungus produces conidia on the underside of the leaves. These spores are disseminated primarily by wind and splashing rain, allowing the infection to colonize new parts of the host plant.

Economic and biological impact. The pathogen severely impacts the vigor and growth rate of the serviceberry. By disrupting the nutrient flow to the affected areas, the fungus prevents the formation of healthy buds and flowers, resulting in a significant decline in plant health and overall fruit production.

Control and prevention measures. Effective management requires an integrated approach to maintain plant vitality:

• Regular pruning of infected "brooms" during the dormant season, ensuring cuts are made well into healthy wood.
• Strict sanitation protocols, including the removal and destruction of all fallen leaves and pruned materials.
• Application of preventative copper-based fungicides before the onset of the bud-break stage in spring.
• Promoting good tree health through proper watering and spacing to minimize humidity-related stress.