Chestnut fusicoccum

Taxonomy and nature of the pathogen: Fusicoccum aesculi is an ascomycete fungus belonging to the Dothideomycetes class. In its life cycle, it forms a conidial stage (anamorph), which acts as the primary infectious agent causing necrotic lesions on the bark and woody tissues of various tree species, classifying it as a dangerous plant pathogen.

Diseases and host plants: The fungus is primarily known for causing serious diseases in horse chestnut (Aesculus hippocastanum). It causes severe branch necrosis, cankers on trunks, and twig dieback. Besides chestnut, the pathogen can also affect other broad-leaved trees, including species of birch, ash, willow, and poplar, significantly compromising the health of the host.

Biology and life cycle: The infection typically enters the host through wounds, frost cracks, sunscald, or pruning cuts. The mycelium colonizes the cambial layer and bark, disrupting the transport of nutrients. Throughout the growing season, the fungus produces pycnidia, which release spores that are disseminated by wind, rainwater, or insects to infect neighboring healthy tissues or trees.

Development and spread conditions: The fungus thrives in humid conditions and is highly aggressive toward trees already stressed by environmental factors, such as drought, soil compaction, or air pollution in urban environments. The optimal conditions for spore germination and colonization occur during the moist, moderate temperatures of spring and autumn, allowing the pathogen to spread rapidly.

Damage and control measures: The disease manifests as thinning crowns, premature leaf shedding, and eventual tree death if left unchecked. To manage and control the spread of the pathogen, the following practices are recommended:

• Sanitary pruning of infected branches and destruction of debris.
• Disinfection of pruning tools to prevent cross-contamination.
• Application of copper-based fungicides to pruning wounds.
• Improving tree vigor through proper irrigation and soil care to enhance natural resistance.