Struggling rhododendron

Debbie Bowman

Hi Doug. I was happy earlier this year to see the many buds on my rhododendron and fed it Hollytone in mid April. But I only had a few flowers and for some reason, the majority of the buds did not open up (per the photo). They also seem to have a powdery mildew on them. What could have caused this and how do I prevent it next year? I already deadheaded them.