Ash rust

Pathogen and disease type. Ash rust is a fungal disease caused by the pathogen Puccinia sparganioides. This fungus is heteroecious, meaning it requires two unrelated plant hosts to complete its complex life cycle. The ash tree serves as the host for the aecial stage, while the telial stage typically completes its development on various species of cordgrass (Spartina) or sedges.

Affected species. The primary hosts for this pathogen are ash trees, particularly Green ash (Fraxinus pennsylvanica) and White ash (Fraxinus americana). Young leaves, petioles, and current-season succulent stems are the most susceptible parts of the tree to initial infection.

Symptoms and signs. The disease becomes visible in late spring. Infected tissues show bright yellow-orange pustules known as aecia. These pustules typically form on the undersides of leaves and on distorted petioles. Severely infected shoots often become swollen, twisted, and stunted, sometimes resembling a witch's broom growth pattern.

Conditions for development. Spore dispersal occurs during the spring, usually triggered by wind currents moving from the alternate host (sedges/cordgrass) to the ash trees. The infection intensity is highly dependent on humidity and rainfall; frequent, cool, and wet weather during leaf emergence is essential for the germination of spores and subsequent disease spread.

Damage and management. While mature trees often survive, repeated infections cause premature defoliation and significant dieback of branches, weakening the tree over time.

• Prune and destroy infected branches to reduce local inoculum.
• Remove cordgrass or sedges from the immediate vicinity of ash plantings.
• Apply preventative fungicide sprays during early spring budding if disease history is high.
• Select resistant tree species or cultivars when planning new landscapes.