There are numerous tree fungus diseases, and some trees are more susceptible to such diseases than others. Some types of tree fungus are very obvious whereas others can be a bit harder to recognize.
Some of the most common tree fungi are root and butt rot and verticillium wilt, which will not be seen until the tree is dying, whereas cankers, powdery mildew, sooty mold, leaf scorch, and needle casts (which only affect conifers) will be noticeable immediately.
Each disease does look slightly different. Verticillium wilt starts in the soil and is an unseen disease that you will not notice until it travels up through the tree to the limbs, twigs, and leaves, causing wilting and death of the branches. By contrast, powdery mildew is a white substance that can easily be seen because it will coat the leaves and branches, causing those areas to die.
Root and butt rot will start in the soil and base of the trunk, eventually causing splitting of the trunk and discolored sections of the tree that will look sunken and twisted. Cankers can appear anywhere on the trunk, branches, and twigs and will appear as open sores that may even ooze sap. Eventually, there will be discoloration of the infected areas.
All these diseases can kill trees. Over time there will be stunted or no new growth of twigs and leaves. Sores will become larger, and the base of an infected tree may begin to sink since the foundation will no longer be sound.
The best way to prevent fungus from attacking your trees is to stop it before it happens. Make sure you water when necessary, but do not over-water. Regular fertilization and pruning of your trees is also important.