Your mostly-empty fuel tank is mostly to blame, because that head space provides the perfect environment for condensation to form. Going forward, I'd suggest buying 5 gallons of ethanol free gas (or 93 octane), treating it with star tron + sta-bil + a splash of Heet. Fill your fuel tank at the beginning of the season and run the blower for a few minutes. Keep the tank topped off after every use, and keep your gas can closed tightly.
I've been doing this for years with no issues. I also start and run the blower at least once a week to keep fresh fuel in the line and carb.