Different fungal diseases produce cankers, or open wounds, on oak trees -- characterized by the development of rotting circles on the tree, with oozing black spots on the bark. These diseases are typically fatal to the oak. In severe cases, the complete removal of the tree is necessary.
Hypoxylon canker centers its attention on several species of oak, including the white and red oak. Specimens that are already stressed through root injury, drought or other environmental or cultural circumstances are most susceptible to contracting this disease. However, Hypoxylon canker also infests healthy oak trees but lays dormant, only displaying itself once the tree becomes otherwise stressed. The first symptoms are the yellowing and wilting of leaves, followed by the defoliation of the oak.
Oozing black spots on the bark in the form of the dead and dying wood of cankers may be accompanied by a surrounding layer of callused tissue. Eventually, the bark sloughs off in segments that may be several yards in length, exposing a tan mass of fungus called stromata. Spores are released into the air when this fungus is disturbed, and the stromata takes on a dark, black color as the fruiting bodies of the Hypoxylon canker mature.
Nectria canker is caused by a fungus of the Nectria genus. Among the species of oak trees, it infests the red oak (Quercus Alba) with the most regularity, and is not especially active in other oak species. Nectria canker is also widespread in a variety of other trees and plants. Like all cankers, it is capable of girdling -- strangling -- the tree as water and nutrition is unable to move past the oozing black spots.
Avoiding tree stress and keeping it healthy is the most crucial and effective canker management strategy. The pruning away of infected twigs and the cutting out of oozing black spots on the bark can help slow and control the spread of the fungi, but oak trees typically develop further cankers in later growing seasons. This is due largely to the tendency of the fungi to overwinter amid leaf litter on the ground and their easy spread through wind and water. Fungicides are typically ineffective against these oak-tree diseases.