Some varieties of Lupins are prone to mildew and will always get it when the weather conditions are right. In my last garden mine had it year after year, despite treatment, so I eventually dug them up.

Don't take my word as gospel though. Wait until there are more replies.

Just out of interest - how did you see the white stuff on the roots if you haven't dug them up yet?

I think that the white substance on the roots of the lupins are nitrogen nodules produced by the plants.

Good thinking Jimmy!