Oak root fungus is a common root problem of ornamental plants. It is a common cause for the death of trees and shrubs. The scientific name is Armillaria mellea and the visible signs are mushrooms growing around the base of the plants or on the main roots.
The mushrooms appear in the late fall after several rain storms (however they came this year even when we had no rain.
They grow on the trunk and main roots of plants that are infested with the mycelium throughout the year. Once the mycelium (fungus roots) take over the cambium layer and can support the mushrooms the mushrooms appear almost overnight and start to spread spores in the air.
Here you can see the spores are like a white powder that rain down on the top of the mushroom from the mushroom gills above. The wind carries it long distances and all it needs to sprout is moisture and a host.