I'm not sure if this should be posted here or on the shrub forum, but my nannyberries have been pruned to be small trees so I'll try here.

I've had these trees for many years and they were trouble free, but the last few years I've had recurring issues with mildew or some kind of fungal problem disfiguring the leaves. I've read that overcrowding could cause this, but I give them lots of space. I've faithfully removed waterspouts and suckers. This spring I pruned off several large branches to give them more air movement.
Now I'm watching closely, and I'm noticing some leaves getting slightly mottled looking. Is there something I can do to prevent this decimation of my trees? I was going to use dormant oil and sulphur on them this spring but read that sulphur was harmful to nannyberries. Does anyone have any suggestions for me?