Chestnut fusicoccum

The pathogen, known scientifically as Fusicoccum aesculi (often synonymous with the sexual stage Botryosphaeria dothidea), belongs to the family Botryosphaeriaceae within the class Dothideomycetes.

It is a fungal agent that primarily thrives in the plant's vascular tissues and bark, functioning as a facultative parasite that exploits weak or stressed host trees.

The fungus produces spores within small, dark-colored fruiting bodies called pycnidia, which typically rupture through the outer bark of the infected branches.

Environmental conditions such as high relative humidity and temperatures ranging from 20°C to 28°C significantly accelerate the growth and sporulation of the fungus.

The pathogen is highly resilient, capable of surviving for long periods in dead bark, woody debris, or as a latent infection within the host's living tissue.

While Aesculus hippocastanum (horse chestnut) is a primary host, this fungus also affects various other deciduous species, including apple, pear, maple, and willow trees.

The infection causes localized necrosis of the bark and cambium, which restricts the flow of water and nutrients, eventually leading to branch dieback and crown thinning.

In ornamental urban settings, infected trees lose their aesthetic appeal due to wilting foliage and the characteristic appearance of dead branches known as "flagging."

If the fungus penetrates the main trunk, it can lead to severe structural damage and irreversible canker formation, ultimately resulting in the tree's death.

Young trees in nurseries are particularly vulnerable, where dense planting facilitates the rapid spread of spores from infected plants to their healthy neighbors.

Early symptoms include small, sunken, or discolored lesions on the bark that expand into necrotic areas, often characterized by a depressed center and raised margins.

The presence of tiny black pimple-like structures on the bark surface, which are the fungal pycnidia, serves as a definitive indicator of the pathogen's presence.

Leaves on affected limbs tend to curl, turn yellow, and wither prematurely, remaining attached to the branch long after the branch itself has died.

In some cases, the tree may exhibit sap flow (gummosis) around the margin of the canker as a defense mechanism to wall off the expanding fungal colony.

Upon removing the dead bark, the underlying wood often appears stained or brownish, indicating that the fungal infection has reached deep into the vascular tissues.

The most effective strategy is intensive sanitation, which includes pruning and disposing of all infected or dead branches well below the point of obvious symptoms.

Pruning tools must be sterilized thoroughly with ethanol or a bleach solution between every single cut to prevent the inadvertent transfer of spores.

Chemical control involving copper-based fungicides or systemic triazole-group fungicides can be used as a preventative measure during cool, wet spring weather.

Promoting overall tree vigor is essential; this includes proper irrigation during drought periods, adequate mulching, and avoiding mechanical damage to the trunk.

All removed infected material should be promptly destroyed by burning or deep burial to minimize the local inoculum pressure in the vicinity of healthy trees.