Hey there, Might not be a pest and could just be general rot due to over saturated soil. As stated above, even in southern more sunny regions, northern windows will work for a very limited number of plants to grow and thrive in. If that is all you have to work with, you will be forced to look into supplemental lighting. Repot into smallest container it will fit into and cut your soil with 50/50 perlite/soil for better drainage. Unfortunately, your conditions are about as bad as possible for this type plant. (ie: water retentive soil, not enough light and too large a container). Don't feel bad, its all about trial and error with these guys. I've killed several trying to maximize my growing conditions. Sounds like you'll have to decide if growing this type of plant in your conditions is worth setting up a grow station or not. Good luck!

I don't know about not doing well near a fence. Mine up against my neighbors solid wooden fence do just fine but the angle of the garden is so that the sun hits my side all the way to the ground from morning on. But I have plenty of daylilies in the front yard. All the beds that run along the very front near the sidewalk get no morning sun but plenty of afternoon sun. They do great and I see no diffence in growth rates between them and the backyard daylilies that do get morning sun.

Plantings. Trees. Shade. Surrounding the home with abundant green- as I live in an irrigated desert. Let the beating sun roll onto canopies of trees surrounding me. Misters. Keeping misters running keeps the back porch twenty degrees cooler than the reported temperature! Arrange the garden & patio with an overstory & understory - with fruit trees in season, and evergreens surround furthering both privacy- and all important to me- a beautiful scene close by during hazardous air quality. Then when the sunlight- which you so kindly mentioned to me- bakes the smog & the air quality is unhealthy to all "hazardous" instead of looking at puffy white air hanging like fog on a 110 degree day- and seeing a gray brown yellow overcast 'cloud'- on a "clear" cloud free day!!! One can simply see the inside of a tree branches cascading merrily. Go out and pick a fig. a nectarine. a peach. make life a peach. But when you leave the house and drive about- ouch! the limited eye sight and polution haze can really hit you by surprize!! Until I learned about sunlight & heat increase of pollution limiting visibility I wondered about early cataracts for me.

As long as the shade is not dense shade, macs can do well in bright shade, dappled sun and morning sun. It will not prevent droopiness in the late afternoon when temps get hot. It may minimize it but I always get some wilting now and then. Just make sure that the shrub is mulched and the soil remains as evenly moist as you can. That way, the hydrangeas can correct themselves on their own at night. In other words, let them be wilted as long as the soil has moisture. One of my sister's neighbor's had one in dappled sun while I have two in bright shade. They all produce flowers. The ones that give me trouble are arborescens and paniculatas in bright shade. When they get little sun, the blooms may start green and either stay green for a long time or for the whole of the season. What is the name of this variety that you have? I am trying to determine if it blooms once or twice.