hydrangeas -- leaves turning black
 

hello,
zone 5
hydrangeas (greenish flowers and normally leaves)
the leaves become discolored (between the veins) this turns brown and
then the whole leaf turns black -- it has not affected the flowers.
this summer has been EXTRA HOT and DRY -- but i have been watering...
(they aren't wilted)
i cannot see any obvious pests...
i also cannot find info about these plants (all i find is that they are
usually pretty resistant)
any advice?
these have been here for years (also i see no other hydrangeas around
the area that have a similar problem)
thank you in advance...