I suspect that you have seen this before on your annuals, particularly petunias and geraniums. The flowers begin to mold and develop a mass of gray fuzzy fungal spores.
When the weather conditions warm up and we get sunshine, the disease development ceases.
However, on the peonies, the infection has already occurred and you will likely see large, brownish irregularly-shaped spots on the leaves. The flower buds may never have opened, and if they did, they might have blackish spots or moldy petals.
You may see the fuzzy fungal spores after a rain on the petals or leaves.
The problem is the weather. Fungicides sometimes may control the disease but the timing of application is extremely challenging.
If your peonies look like this now, good sanitation is helpful to decrease the amount of spores hanging around to affect next year’s growth.
Remove any diseased flower buds as well as foliage. If plants are in the shade, try to thin trees to expose them to better sun that encourages drying of the foliage. If they are crowded in the garden, try to open up the area around them as well.
In the fall, cut the plants back and destroy the foliage. Don’t compost and reuse in the perennial garden.
If this disease occurs every year, you should consider replanting them to an area where they have lots of space and full sun.
Botrytis blight also affects Asiatic lilies and can prevent blooms from looking spectacular. My Asiatic lilies are just beginning to bloom and the ones in full sun that have good air circulation are perfect.
However, those in the side garden that are surrounded by thick daffodil foliage that remains green are affected by botrytis blight.
This is a perfect example of the need for better air circulation needed to dry the foliage. I keep saying I need to replant the daffodils but haven’t gotten around to doing this yet. Maybe next year.
The bottom line for some of these weather-induced diseases on annuals and perennials may be to wait for nice warm weather. Clean off any diseased foliage, even if it means cutting the plant back to the ground (perennials) and trimming annuals.