Certified GKH Gardening Expert
This is often a sign of a soil born infection. The best and easiest remedy from here will be to apply dolomitic lime and wettable sulfur together over the area. This will kill off the infection, and recondition the soil pH to a proper condition.
This article will give you more information on the care of these shrubs in the meantime: https://www.gardeningknowhow.com/search?q=rose%20of%20sharon%20care