Sick Ash tree
 

I have a fully grown, 25 year old ash tree. A few years ago some crunchy,
crab-apple type growths appeared on some of the branches. I must say I
didn't pay much attention, but last year a significant amount of the tree
was covered.

When removed, these growths crumble into small pieces. They are dark brown
in colour. The tree seems healthy otherwise.

I'm in Canada, Southern Ontario. I'd appreciate some advice on what is
happening., and what I can do about it.

Thanks,

George