Q: I have lost three good-sized oak leaf hydrangeas that were in dappled sun but a fairly wet spot. The leaves wilted from top down over a two to four week period. They were well-mulched and fairly well-watered (on a drip system) over the summer drought. I have three more nearby and would like to preserve them but am not adverse to trying another hydrangea if necessary.
A: It was difficult to kill plants with too much water last summer but I think you accomplished it. Oak leaf hydrangea is a tough plant in most gardens but my experience is that it suffers in a soggy spot. Your description of symptoms is consistent with phytophthora root rot.
To fix the situation, thoroughly mix a bag of soil conditioner into each spot where you plant another hydrangea. This will increase drainage and lift the plant above any waterlogged soil. Water next summer only when you see the leaves drooping.
Once the plants are established, remove fallen leaves out of the base of the plant each January. This entire process should improve growing conditions enough to allow the shrub to fight its own battles against disease.