It could be something other than rot or fungus.
Ethylene gas can trigger premature leaf and flower drop. Ripening fruit is a source of ethylene, as is incomplete combustion: examples include a fireplace, cigarette smoke, gas or oil burners.
I worked at an apple stand the fall after I graduated, and the owner started selling mums outside the stand on the weekends. I was locking up one Saturday and we were expecting some cold weather, so I moved all the mums into the stand before I shut things up for the night. In the morning not a single plant had a flower left on it.
Your leaf drop sounds more like ethylene.