Jane wrote:
Hi Jack,
I'm sorry to be the harbinger of bad news but I've never
successfully rid my Rosemary of powdery mildew which I imagine you
have, because the cure is something that poisons the food. Fungicides
are available to treat it on roses or ornamentals. Oil sprays to help
stop Black spot can be used as a preventative but have no business in
anyone's mouths. Yuck. Mildew is obnoxious. It spreads and destroys
other plants if allowed to stay in your garden. I'd start over with a
new plant in clean soil or a different spot in the garden and treat
the ground with a fungicide in quarantine earth space until the
following season so it's out of the soil. If you have aton of old
rosemary and you're determined to save the plant you can treat it,
without eating it for a few years, then let it grow without fungicide
for a season before eating it to be safe. Jane

Thank you Jane. You have confirmed my fears. I'm going to let the plant do
its thing for a few weeks, spraying it regularly with soapy water. If it
cures it, good. If not, I'll dig it up and start a new plant in a different
place.
Jack