Fungal Diseases

Any time a tree is damaged, fungi will colonize the exposed wood and it will decay. Trees can heal small wounds before dangerous rot sets in, but large wounds almost always result in large pockets of rotten wood inside a tree. There are also many fungi that attack the wood, bark, and leaves of undamaged trees.

Wood-Rotting Fungi

These fungi can result in serious property damage or injury. Everyone knows that dead trees become more hazardous the longer they’re left standing - this is because fungi weaken the wood by rotting it. However, some fungi can colonize living trees, causing the roots or trunk to rot while the rest tree still appears healthy. This is one reason why healthy-looking trees fall over.

These Armillaria mushrooms are a clear sign that the roots of this oak tree have begun to rot

Any mushrooms growing from a tree are a definite indicator of rot, and mushrooms growing from the ground near a tree are often a sign of rotten roots. If you notice mushrooms in or around any of your trees, it’s a good idea to have an arborist come take a look - especially if the tree could cause damage if it were to fail.

Although the canopy of this tree looked healthy, extensive fungal decay caused it to fail.

Bark Cankers

Cankers are wounds in a tree that have become infected. These wounds can be caused by insects or mechanical damage, or they can be the result of a fungus that has entered the tree in another spot. Bark cankers range from small, unsightly blemishes to diseases that can kill large, mature trees.

Most canker fungi need a wound to enter the tree, so it is important to avoid damaging your trees with mowers and other equipment. Stresses such as drought, flooding, soil compaction from machinery, and over-pruning can make trees more susceptible to insect infestations that can lead to cankers. 

The fuzzy coating on this beech tree is an insect infestation. These scale insects cause many small wounds.
The fungi that infects beech trees after scale infestation causes beech bark disease. This will typically kill the tree.

Leaf Spot and Rust

Darkened spots or a powdery coating on leaves are usually caused by fungi. These diseases are usually not a serious threat to a tree’s health, but they can affect fruit production and may warrant treatment.

Leaf spots are more common on trees, while rusts and mildews are more common on smaller herbaceous plants. Many of these fungi alternate between hosts - for example white pine blister rust jumps back and forth between currents or gooseberries and pine trees. If you have a leaf problem on a tree and also on a nearby smaller plant, the two problems may be connected.

Apple scab is a very common problem. The dark spots on the leaves and the hard scabs on the fruit are caused by the same fungus. 
Powdery mildew on a maple tree. Although it looks bad, it isn't a serious problem for the tree. Mildew infections are very dependent on humidity levels so the problem may only persist for a single season.

Leaf spot and rust likely won’t be totally eliminated without chemical treatments, but here are a few things that can help:

  • remove dead or diseased branches that can be a source of infecting spores

  • remove fallen leaves that can reinfect the tree

  • thin the branches to allow more light and air flow

  • avoid spray watering systems that get the leaves or trunk wet

You can probably do some of these things yourself, but if you’re concerned it may be a good idea to contact an arborist to come take a look.

Feel free to email me with any questions and/or pictures of decay or unusual growths you see on your trees