the past week or two, Dahlias in a hanging basket have developed
a white coating on their leaves. It is easy to rub-off, but
I'm wondering what it is, and if it is harmful to our Dahlia plants,
especially if something isn't done about it?
The powdery white substance you are seeing is a fungal disease
mildew. Powdery mildew affects a wide range of plants, from
herbaceous plants like your dahlias to trees and shrubs. There are
different strains of powdery mildew that attack different plants.
Plants that are frequently infected with powdery mildew include
apple and crabapple, azalea, black-eyed Susan, calendula, cucurbits
(melons, cucumbers, gourds, pumpkins and squash), dahlia, flowering
dogwood, common lilac, monarda, phlox, rose, sycamore, verbena, and