Help! Is this Dry Rot?

[Crestfallen End]

8773687737

Replacing approx 12 year old weatherboards with hardiplank and found this underneath.
The boards & studs have a horrible yellowish, puffy I think fungal growth. I'm thinking it could be dry rot and the studs will need replacing.
Hubby put the sisalation on back to front (and from inside) when he replaced the plaster would this have contributed to the problem.
:doh:
Any experienced opinions?

[Bloss]

It sure looks like water damage, and maybe just condensation from there being no air-gap with that sisalation. Moisture has been getting in there and not able to get away. But from just those two pictures the timber itself looks reasonably sound? If the studs do have rot then you will be able to push a finger nail into the timber or a screwdriver, quite easily. But . . . they look a lot like termite tracks too so I would be doing a very thorough pest inspection!

[Crestfallen End]

Thanks, Bloss. Having taken off the rest of the planks the other studs look pretty sound, just this area lower down on the wall is affected.
Our builder reckons just water damage as you suggested.