you'll have to expect that if youre using real branches this is inevitable. to keep it from happening for a longer period of time use fresh branches off of a live tree. i perfer killing off the surcafe layers with hydrogen peroxide or isopropyl alcohol and if you can peel the bark off that helps.
everything gets old and rots, its just going to happen with organic material. especially in humid cham environments. find replacements or use something inorganic. even though driftwood and grapevine are porous they do last a while as compared to stuff you might pick out of the back yard.
spores are everywhere. fungi will come