A: Your plant is showing signs of Cercospora leaf spot which is a disease caused by the fungus Cercospora hydrangeae. The spots will start out as small circular purple dots which later will turn tan or grey. Small spores will be in the center of the spot and can be easily spread by water or wind. If the infestation is heavy, the whole leaf will turn yellow and drop off. Any of the common landscape hydrangeas, including bigleaf, oakleaf, panicle and smooth-types are susceptible to this disease. Although plants are not killed by leaf spot, it can cause premature leaf drop which may reduce flowering and plant vigor even for next year’s flowers.
Cercospora leaf spot is favored by warm weather with frequent rain showers – well we have certainly been receiving plenty of rain to produce these problems. Clean up any leaves dropping to the ground. Be sure your plants have plenty of room so air can easily circulate around them and keep them dry. Apply a fungicide for ornamental plants on any new growth, but be sure to rotate and use a different mode of action each time. Remember, the fungicide will protect and help reduce the future leaves from developing spots but it will not cure the disease currently found on the leaves. Avoid over-head irrigation on any shrub or tree – none of them like it. Right now, be sure your irrigation system is off and place it on manual mode. http://edis.ifas.ufl.edu/st298