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...
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