We have been experiencing some very heavy rain, which although was needed as the gardens were very dry, it has all come at once. Unfortunately, plants will be experiencing a number of issues due to this rain, followed, hopefully, by high summer temperatures. The humidity around the plants will rise and if your planting is quite dense, some plants are susceptible to powdery mildew. Powdery Mildew is a fungal disease which spreads on patches of the foilage and stem and occasionally on flowers and fruit, where a superficial fungal growth covers the surface of the plant.
Symptoms of Powery Mildew are white powdery patches and leaves can be distorted, there can also be a change of colour in infected parts which turn dark brown.
My suggestions are, to increase the airflow around them and remove any dead leaf matter that may be decaying. Prune back and thin out as much growth as possible, picking up dead petals, to prevent any bacteria from rotting around the plants. A chemical control can be used but I prefer not because of wildlife, if you decide to go down that route you can use a Fungal Fighter spray which can be purchased from any garden centre.
If you are watering your garden through a dry spell, try to water around the base of a plant, rather than above as this will help prevent splashback from the soil which also carries the spores of the fungal disease.