Alder rust

Pathogen and disease type. The disease known as alder rust is caused by the obligate parasitic fungus Thekopsora fischeri. This pathogen belongs to the rust fungi group and specifically targets the foliage of its host, disrupting carbon assimilation and reducing the tree's overall vigor.

Host plants. The primary hosts are species of the genus Alnus, including grey alder (Alnus incana) and black alder (Alnus glutinosa). While the infection is rarely fatal to mature trees, it significantly impacts the health of nursery stock and young saplings.

Symptoms and signs. Infection is characterized by the appearance of bright orange or yellow pustules on the abaxial (bottom) side of the leaves. As the disease progresses, these pustules release spores, leading to chlorosis, necrosis, and premature defoliation of the tree canopy during the active growing season.

Developmental conditions. The life cycle of Thekopsora fischeri is heavily dependent on moisture and moderate temperatures. High humidity, heavy morning dews, and frequent rain showers create the ideal environment for spore germination and rapid secondary infection cycles throughout the summer months.

Damage and control measures. The main damage is linked to the loss of photosynthetic surface area, which leads to diminished wood production and tree stress. Control strategies include:

• Sanitation: removing and destroying infected fallen leaves to reduce the primary inoculum for the following season.
• Spacing: ensuring adequate tree spacing to promote air circulation and reduce humidity within the canopy.
• Chemical control: applying registered fungicides in nursery settings if the disease threshold is exceeded.
• Monitoring: regular scouting for early signs of yellow pustules to initiate timely intervention.