Without seeing it, my best guess would be powdery mildew. If you are unfamiliar with it, and blackspot is your usual fungal disease, you likely won't see a lot of it -- but immature plants can mildew when stressed.

No spray will eradicate it. Sprays are preventive, only.

If you have sprayed, it may stop spreading. You can hide it some with an oil spray, but be very cautious. At 75 deg. you're getting up to where oil on leaves cooks them, and a temporary bout of mildew may be preferable to that.

Jeri

If you have highly salty, alkaline water, it can leave calcium spotting on the foliage, as can many sprays. You'd do better posting clear photos of the issue for diagnosis. Kim