It could be anthracnose but it is difficult to diagnose when you are not there to see it. Be aware that fungus in hydrangeas is a very common thing. But, only in the fall season.
This being so early, I would address other cultural aspects that could be causing the problem. For example, make sure that there is good air circulation around the
plant; do not water or mist the leaves; instead, water the soil under the
plant with a water wand and do this only in the mornings; discard
plant debris or leaves that may accumulate under the
plant in the Fall (do not put the any affected leaves in the compost pile but rather, throw it in the trash); keep the soil moisture level constant with the use of 2-3" of mulch; water when the soil feels dry; replace the mulch in order to remove any spores that may be found there.
For a list of possible diseases/fungicides, see this link:
http://www.aces.edu/pubs/docs/A/ANR-1212 Your local extension service or a good local
plant nursery could help with the diagnosis. Take a sample in a sealed plastic bag for analysis to them.
Good luck,
Luis
