Is this Dry-Rot? How serious is it?

[McAhuna]

I was just up in our loft when I noticed a few of the joists have these patches of white on them.

I hadn't noticed them before, but we haven't lived here long. The joists feel otherwise solid to touch and the whiteness seems fairly superficial, a little felty and scrapes with fingernails.

Is this a problem and how serious? Whats the best course of action?

Cheers in advance for any advice.

 

[Doggit]

Looks more like staining from the building work when installed. If it was dryrot, it will collapse when you put a screwdriver into it.