Answer: I suspect you are seeing a mildew infection, this is usually related to weather conditions. It is not too important this late in the season as the foliage has already done its job for the year; this type of mildew is usually more unsightly than harmful and would not need to be treated. This fall, make sure to clean up all of the fallen foliage from these plants to try to limit sources of reinfection. You might also wish to check with your county extension for a more specific diagnosis and based on that, any recommendations they may offer. They might also be able to help with the mite problem, I am a little surprised to hear these plants are suffering from those. Good luck with your plants!
Q&A Library Searching Tips