Hysterographium fraxini

Hysterographium fraxini is a fungal species within the Dothideomycetes class, primarily known as a pathogen that causes necrosis on the bark and branches of ash trees (genus Fraxinus). While often classified as a saprotroph, it frequently behaves as a weak parasite, colonizing stressed or weakened trees.

The fungus target host is predominantly the common ash (Fraxinus excelsior). It attacks branches, twigs, and trunks, hindering the transport of nutrients and water by destroying the phloem and cambium tissues. This process leads to localized dieback and progressively affects the tree's overall structure.

Symptoms are visually distinct due to the formation of hysterothecia—small, black, elongated, slit-like fruiting bodies. These structures appear on the bark surface as dense clusters of dark cracks or ridges. Infected bark often cracks, becomes brittle, and may eventually slough off, revealing dark, stained underlying wood.

Environmental conditions play a critical role in the prevalence of this fungus. High humidity and persistent dampness facilitate the production and dispersal of spores. The pathogen typically enters through mechanical injuries, pruning cuts, or natural fissures, quickly spreading throughout a weakened tree canopy when conditions are favorable.

Effective management focuses on reducing the inoculum density and strengthening tree resilience through several key strategies:

• Regular sanitation pruning to remove and destroy infected branch wood.
• Proper disposal of infected material to prevent further spore dispersal.
• Application of protective wound dressings or fungicides to significant pruning cuts.
• Ensuring proper site drainage and tree spacing to minimize humidity and improve airflow.