These are a common type of leaf gall caused by the Exobasidium fungus. Despite being unsightly, they do not seriously harm the plant, and can simply be plucked off and discarded in the trash. Use of a fungicide is not needed, nor would one be able to cure any existing infection. In years with wet weather, such fungi can be more abundant, and once the weather dries out more, this issue will subside on its own.

Miri