Short answer
Curling in the middle of summer is usually the plant shrinking the surface it exposes to sun and wind, and where that has been studied closely the rolling does no lasting harm. Check the soil at depth before anything else. If it is damp down there, look at the newest leaves for insects, then step back and check whether several unrelated plants are affected at once, which points away from a pest entirely.
Listed in the order they are worth ruling out. It is normal for more than one of these to be happening at once, so work down the list rather than stopping at the first match.
Curling in the middle of summer is most often the plant reducing the leaf surface it exposes to sun and wind, not a disease. It is worth ruling out first because it is the cheapest thing to check and because the published guidance is that this kind of rolling does no lasting harm.
What to check first
Push a finger two inches into the soil in the middle of the afternoon. Dry at that depth points here. Still damp at that depth points at one of the causes below instead.
These insects work on the softest tissue, so the damage shows up as curled and puckered new leaves at the tips while older leaves lower down stay flat. That split, new growth distorted and old growth normal, is the useful signal.
What to check first
Turn over the newest leaves at two or three shoot tips and look closely. Clusters of small soft-bodied insects, or a sticky shine on the leaves below them, settles it.
Leafhoppers curl the margins of a leaf rather than the whole blade, and they leave a fine pale speckling behind that a heat response does not. They are easy to miss because they leave the moment you get close.
What to check first
Tap a branch sharply over your open hand and watch. Quick-flying, tiny, pale green insects scattering off the plant is the confirmation, and it takes about ten seconds.
This one is worth knowing about because nothing you do to the plant will help, and because it is the cause most often mistaken for a pest. The tell is the pattern: several unrelated plants affected at once, usually along the side of the garden facing a lawn or a road.
What to check first
Step back and look at which plants are affected and where they sit. Damage crossing several unrelated species at once, strongest on one side of the garden, points here rather than at anything living.
Still not sure
This is exactly what your county Extension office answers, for free. They are the local horticulture experts for your area, they know what is going around this season where you live, and they will look at a photo.
Find your local Extension officePollinator Patch gives you watering and seasonal care for the plants you already own, written for your region rather than for gardening in general.