Pull that bastard from the room as much as it pains you! Don't want the spores traveling to your neighboring plants. If you decide to keep it, chop the affected branch but remember that PM is systemic. The rest of the plant will still have the white "Aids" that all growers fear. For that late in flowering, you want to avoid any harmful foliar sprays. Mix up a bottle of high ph'd water usually 9.5 or higher alkaline wise and spray 15 minutes prior the lights going off every night until you harvest. This won't kill it but prevent it from potentially spreading to the rest of your crop. The high ph on the top of the leaf surface creates an tougher environment for the spores to populate. In college, I assisted a very knowledgable professor with annuals during winter months in botany with great effectiveness using high ph'd water.
Cheers!