Our parsley was growing very well and suddenly it began to developing a white powder on all over its leaves. I don't believe it to be a watering issue. The weather conditions here has been hot and very humid. Thank you, Mike O' Hare
Extension Educator, Horticulture
University of Illinois Extension firstname.lastname@example.org
Sounds like it might be powdery mildew which is ai fugal disease affecting leaves and is often more of a problem when we have warm days cool nights and not so much free water on the leaves.