By our resident (amateur) mycologist, Mark Bower. 
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While hiking in the Bull Creek area I noticed a bright yellow log in the distance. I first thought it was a reflection of sunlight, but as I got closer, it was indeed yellow. I then thought that it may have been painted, but as I got closer, it was clear that this was a massive spore deposit.
My suspicion was confirmed when I poked it with my hiking stick and a strip of bark fell off. Underneath the bark were the telltale structures of Xanthoporia andersonii, the canker rot of oak.
This fungus is a polypore which is a pathogen of oaks and sometimes hickories. Its spores enter a living tree through an injury of a branch stub or sometimes the trunk. The spores germinate and form the fungal hyphae, which grow into and infect the heartwood, causing white rot (i.e., preferentially digests lignin). As the infection progresses, it extends outward and eventually reaches the cambium and kills the tree, sort of a natural girdling. It then forms its tubular fruiting bodies and the yellow spores are shed.
Interestingly, peg-like fungal outgrowths are formed, which push out against the bark, causing it to detach from the tree, allowing the spores to escape. The display is brief as this fungus is short-lived, quickly turning dark brown, then black, leaving a log which appears to have been burned.
This is just one more example of the tiny creatures that are all around us in nature when native species are allowed to pursue their own life cycles.
