I received this note from a new member at Dave's and don't have any answer for her. Do any of you? Name is Darshobby, from Oswego, NY...
Darshobby wrote: "Would you happen to know why my coreopsis plants (3 of them) have developed a powdery white apperance on the once lovely green fern like bush? We have had so much rain this summer---not sure if that has anything to do with it. I planted them about 4 or 5 weeks ago."
Coreopsis, powdery white?
Sez here "powdery mildew may coat the leaves (of coreopsis) with a white powdery growth".
http://www.msue.msu.edu/imp/modzz/00000412.html
a quick scan of other Googles on "coreopsis mildew" shows a number of cultivars described as 'mildew resistant' indicating to me that PM is a problem with the genus in general...
The MI State site links to fungal and other info are all (currently as I speak) broken, sorry.
Other sites I scanned briefly recommend destroying affected leaves and ALL plant debris from under/around the area... as a fungus, it reproduces by spores and "it only takes one"... also one source suggests first year plants are most at risk because they are weak/stressed, and another that plants are more likely to succumb when in conditions not optimum for them.
From mildew on roses, I suspect that all their rain (can you imagine THAT problem???) is a big contributor -- but I also wonder about you posting this to Shady Forum, as coreopsis (those not in Moab) like full sun to PART shade. If Dar's are in shade, as well as wet and brand new plants, the deck is maybe stacked against them... of course, if they are only in shade because they are not getting much sun through the rain clouds, not much can be done about that!
~'spin!~