Asked July 29, 2021, 2:46 PM EDT
Mahoning County Ohio
I can see what a special tree this is to you and know that you would be very sorry to lose it. Perhaps with some remedial action you might save it for a while. I recommend you call an arborist to investigate the possible reasons for decline as it is hard to tell from your photos exactly why your tree might be dying.
Often times, decline of the nature you have pictured is brought about by improper planting, the evidence of which does not show up until the tree begins to mature. This can take many years. If there are girdling roots, water and nutrients are chocked off from the canopy. There is a chance this can be partially remedied, though the fungal growth indicates an internal problem which most likely will continue to progress. In your picture of the canopy, the flare at the base of the tree looks healthy; in the picture with the fungus, it looks like one side of the tree goes straight into the soil instead of flaring as a well planted, healthy tree would. The oyster mushrooms are growing because of heartwood or sapwood decay, perhaps this is due to an injury from long ago. Certainly the prognosis is not favorable.
A couple of FactSheets with information:
Because so many factors can cause decline and dieback, the primary causes are listed below in the approximate order of general frequency:
1. Poor soil structure and drainage (important when the soil is predominantly clay)
2. Herbicide injury to foliage, roots, or other parts (Figures 3 and 7).
3. Poor transplanting procedure and lack of proper maintenance after transplanting (Figure 6)
Acacia, alder, ash, beech, birch, chestnut, elm, eucalyptus, fir, hackberry, holly, horse chestnut, linden, magnolia, maple, oak, olive, pecan, persimmon, poplar, spruce, tulip tree, walnut, and willow.
This fungus decays heartwood and sapwood, causing a white, flaky rot. Infections occur through open wounds, and decay is most extreme when wounds are large. A cluster of shelf-like mushrooms, each 2–8 inches wide, is produced annually and can indicate localized decay or heart rot that extends 10 feet in either direction. The mushrooms are smooth on the upper surface with gills that characteristically extend down along the stalk on the lower surface.