There is a whitish pest growing on peonies, lilac and phlox. I think it must be some kind of mould or fungus. Does anyone know what it is, or how I can get rid of it?
Pat, it's called 'powdery mildew' and it's a type of fungus. It's also very common on lilacs, roses and more. Usually if your plants get a lot of sun and warmth with good air circulation, it will help to prevent it. When conditions are humid and plants get shade, or when the leaves stay wet overnight, etc. it is more likely to appear. That's why it's best not to water in the evening, especially now that it's getting dark earlier and nights are cooler.
It generally doesn't kill the plant, although it looks unsightly. It can weaken plants though, and make them more susceptible to other, more serious diseases.
You can search online for treatment, but generally a mix of neem oil (available at most garden centers) or even canola oil with a little dish detergent is sprayed on the plants. Best to do this on a cloudy day in the morning, not in bright sun.
So anyway now that you know what it is, you can also look online for more information. Hope this helps.
Another thing that may help for next year is really good sanitation this fall. Remove all the old leaves from your perennials after frost and if you have mulch under plants, remove the surface layer of that. If you don't you might consider adding a thin layer next spring since many funguses get onto leaves by splashing up from dirt when it rains.
As Bill said, good watering and air circulation is important. Some plants are more susceptible to PM than others, so in the future you can check on whether a particular variety is likely to be resistant before you buy.
I agree that it's powdery mildew, but have 1 thing to add.
I've read (and observed, more to the point) that it's most common in drought-stressed plants, so during the growing season thorough, deep watering is essential to keep it at bay.
UMass has a fact sheet about it - it seems a little thin to me, but I'll attach the link. There may be a better one at one of the arboretum sites.
Here is a link that might be useful: UMass extension fact sheet