I have noticed that these areas tend to be off-color, not like the rest of the leaf. You may be seeing the early stages of powdery mildew infection. By the time you read this, it will probably have progressed to the stage where the white, powdery spores are apparent.
Powdery mildew can cause leaf deformation, leaf drop, and flower deformation.
It is a common disease favored by high humidity and temperatures in the sixties at night. We have now entered the season with the proper night temperatures, but many people wonder how we have enough humidity in a drought year. It doesn't take extremely high humidity. Evening or night watering of flower beds and lawns is enough to provide the necessary humidity. The plants themselves create localized high humidity as they transpire water from the leaves and trap that moisture among the leaves.