The living cells of the wood in the phloem, cambium and sapwood are invaded quickly soon after a fresh wound exposes those cells. Although sap-feeding insects can carry the fungus from tree to tree naturally, the most severe outbreaks are usually traced to pruning and other work on the trees.
Infected sycamore and London planes have sparse foliage, small leaves, and elongated sunken cankers on the trunk and larger branches. The canker may at first just appear to be a darkened, flattened area on the wood. Beneath the cankers, the wood is stained bluish black or reddish brown in a wedge-shape (when viewed in cross section) with the point of the wedge extending toward the center of the trunk or branch. As the disease progresses and cankers enlarge, water sprouts develop below the cankers. These are killed. Large trees may take several years to be killed but pole-sized trees may die within 2 years.
The fungus enters only through fresh wounds. Infection is least likely to occur in December and January. Once disease begins, secondary wood-rotting fungi frequently invade the cankers and cause severe, additional damage. The fungus produces spores within days of the first infection. Black fungal fruiting structures form on the wound surface and produce slimy masses of spores. The spores ready stick to pruning tools, ropes, ladders, paint brushes used for applying wound dressings, and other equipment.
It is very important to disinfest all tools and equipment immediately after use on a sycamore or London plane tree and before proceeding to another such tree. Do not use wound paints on these trees since contaminated brushes efficiently move spores from tree to tree. Prune sycamores and London planes only during December and January when the weather is dry. When a tree is found infected, remove it. No chemicals effectively control this disease.
Prepared by Gary W. Moorman, Professor of Plant Pathology