Mycorrhizal Fungi are a common infection in most tree roots. But don't worry, it's an infection with benefits.
The fungi and the tree have a symbiotic relationship in which both benefit and neither are harmed. The tree gains increased water and nutrient absorption. In turn, the fungi gain food (carbohydrates) from the tree.
In fact, there are many different species of Mycorrhizal Fungi that are specifically associated with different species of trees.