Hydrangeas typically will show stress in the form of yellow leaves. I would think you are probably right about too much rain. I wouldn't think it is a fungus - just stress. Typically they recover just fine - although it sounds like yours have lost quite a few

Mine do the same thing - after our huge rainstorm last week (quite a few inches of rain within one day) there were many yellow leaves. Now it seems to be fine again, and the blooms keep on coming! Oh well, I guess it's just a first year plant and it's a bit more delicate than established one...

ihave a hydrnagea that was potted and i transplanted it into a bigger pot and the leaves are yellowing and falling off what am i doing wrong should i plant it outside