Every year, starting in late June I see limb failures that appear to be caused by a fungus developing a pressure pad directly in the inner fibers of the wood. The pressure from this growth causes the wood to shear and the limb fails.
I have never been able to positively ID the offending fungi. My hunch is all that I see is the same species as they all look the same. If I had to guess on the genera, I’d say Phellinus. I’d hardly call that an educated guess; my presumption is based almost entirely on the fact that the are many different species of Phellinus and I have seen some where the conks have some of this striking white color. If any mycologist, or plant pathologists out there read this post, I’d love to hear your thoughts. I have gone so far as to send samples to the Texas A&M plant pathology lab. The response I got back was, “this is heartwood rot.” Yeah, I already knew that. Duh. Apparently the lab coats don’t have any way to figure it out either.
I primarily see this in Arizona ash. However, oaks succumb as well. I’ve witnessed both red oak and live oak damage from this fungal summer limb drop phenomenon. I’ve even seen an entire red oak fall over because the growth occurred just 12″ above ground. Other species may also be susceptible, I just can’t recall seeing any others as I sit here and type.