What is this?

[Noidios]

Opened up an exterior wall today and fond this white stuff on the plywood. There were traces of the same material higher up the wall - almost like it had dripped down.
The structure is kind of an A-frame so asphalt shingles on the other side of this plywood. White stuff in question wiped right off the plywood completely with no effort.

 

[FredC]

white wood rot/fungi

my guess anyway

 

[Fat Mike]

That cavity definitely looks saturated