We've consulted our plant pathologist but can't determine a cause for the bleaching. Some research articles suggest high heat can cause symptoms like these on geranium, but since we often have hot weather in Maryland summers and this is not a widespread occurrence, we're not certain how it might be responsible here. Perhaps pots in locations where the roots are also getting too hot or the plants experiencing reflected heat from a nearby wall, decking, or other structure is part of the cause. If it's been awhile since you last fertilized, perhaps a light dose would be of use to spur more chlorophyll production now that our nights are cooling.

Miri