Black knot tree fungus is getting worse, according to the city’s forestry expert.

"It's all through the city and it's getting into the native areas which is what's really scary," said Jeannette Wheeler, the principal of forestry for Edmonton.

Wheeler said the disease could potentially affect about four per cent of the city’s trees.

City crews prune infected trees as they find them but don’t have the resources to move into natural areas of the city.

The fungus appears as a hard, black swelling on tree branches which can eventually kill the tree if it spreads far enough. New trees are infected by airborne spores.

While the fungus can be brought to manageable levels, Wheeler believes the problem will never go away.