Viburnum flower disease

Hi. Hoping someone can help. For the past three years my viburnum bodnantense ‘dawn’ has increasingly suffered from a disease which causes the flower buds to rot. Last year I treated it with Provado Ultimate Bug Killer, thinking it had thrips, but without luck. This year I’m wondering whether to try Tumbleblight, in case it’s a form of mildew. Any ideas as to what the problem is and how I should treat it? Many thanks.