Whatever you do, don't use the white milky looking stuff called "mildew cure"
Smells like shit and doesn't work.
If it is only on one branch cut the whole branch off??
when you are done with this grow (whether you decide to battle it or chop em down and start again) make sure you spray down and wipe everything with 50/50 bleach/water...hell I'd do it twice. I mean everything the hood, glass, cords, hangers, any duct work you have...EVERYTHING!!