From your photos we don't think that what you are seeing is powdery mildew. It looks crystalized and somewhat granular. Can you wash it off and does it dissolve in water? We can't tell what it is but did other plants on that side of the house have it?

We are not familiar with Banish. As a brand name multiple pesticides come up, including a fungicide containing geranium oil. Fungicides are preventative and not curative so they are usually repeated. In this case we don't think you should spray anything else before knowing what is really going on. This plant isn't known to have disease issues and the remaining leaves that we can see look good.

The insect in the photo is definitely NOT the problem, but it does help give clues. That is the larval stage of the Asian Lady Beetle (aka 'ladybug'), which both in its larval and adult stage are hungry predators that help control pests like aphids. Look carefully under the leaves of the plants for these small, soft-bodied sucking insects, particularly where you see puckered leaves. Smoosh those you find or blast them off with a hose. Do not apply insecticides, to protect beneficial insect populations.

The damage you saw on the leaves could have been caused by late frosts, or even perhaps the  dried 'honeydew' , which is the sweet, sticky excretion that aphids drip after sucking plant saps. It could even come from overhanging trees that have sucking insects.

We think that your groundsel should rebound just fine, but if you continue to see issues, contact us again and we'll continue to try to help.

Christine

You're welcome.