Hypoxylon Canker – Identification
Q: Could you please look at this problem with our white oak?
In the last two months the bark has come off, leaving a brownish-gray powdery substance.
I can scrap it off to the wood below. The gray stuff repels water and is continuing to spread.
A: My first guess is hypoxylon canker. This disease is common on white oak. The first sign is sunken patches that are lighter in color than surrounding bark.
This canker occurs primarily on trees which have been stressed by drought or root damage. I see it quite often on oaks whose roots have been covered with soil during construction of a house.
As a class, oaks are very intolerant of low oxygen in the soil so it is important to protect the entire root zone, all the way out to the drip line, when construction occurs nearby.
No spray can help once a tree is infected. The best you can do is keep the tree watered during drought.
Although some trees seem to survive for years with hypoxylon canker patches on their trunk, yours seems pretty badly affected. I don’t hold much hope for it.
You can get a better opinion from a certified arborist.
For further details, read more here.