Orange Hobnail Canker is having some taxonomic issues. It’s been known as Endothia gyrosa but Melogramma gyrosum seems to be the current name. The genus Cryphonectria seems to be quite similar.*
Some fungi are benign, but this particular one is a pathogen. It’s associated with pin oak blight. See comments below for reason for deleting this.
I’ve seen it oaks and beeches, usually on fallen limbs but also, as in this case, on the exposed root of this mature red oak.
Here it is on the exposed roots of a beech (F. sylvatica).
*”Cryphonectria has one‐septate ascospores and valsoid stromata with ectostromatic and entostromatic areas in predominantly prosenchymatous tissue. This is in contrast to the ascospores of Endothia that are nonseptate, and the stromata diatrypoid with predominantly pseudoparenchymatous, entostromatic tissue.” Ok, back to your regularly scheduled Saturday morning activities, or not….
Backyard and Beyond 
Hi Matthew,
Thank you for sharing your experiences and insights with the community. I admire your dedication and consistency and always enjoy reading your posts and looking at your wonderful photos.
However, I take issue with one item in your post today. I think it is unfair to characterize the Orange Hobnail Canker as a pathogen without proof.
Since about the mid nineteenth century, Robert Koch’s postulates have been the gold standard used by scientists to prove a particular organism causes a particular disease. Koch’s four criteria are:
(1) The organism must be found in diseased but not healthy individuals; (2) The organism must be cultured from the diseased individual; (3) Inoculation of a healthy individual with the cultured organism must recapitulate the disease; and finally (4) The organism must be re-isolated from the inoculated, diseased individual and matched to the original organism. I would add a fifth postulate: a control.
With or without proof of causation satisfying all four of Koch’s postulates (with control), the scientific method requires consideration of the null hypothesis.
It therefore must be considered that the Orange Hobnail Canker is present at the site of injury, poisoning or nutrient deficiency as an opportunistic symbiont. We have to remember correlation is not the same as causation. What if the Orange Hobnail Canker was actually feeding on necrotic tissue (caused by poison, starvation or injury) and might even serve to isolate the necrosis and clear the debris?
With best wishes,
Daniel Atha
Thanks, Daniel. Your comments are always welcome. My knowledge of fungi is… advancing beginner. Found the pathogen bit from the aether of the web, so I appreciate your in-depth discussion here.