You are growing mildew on your plant. You can not undo what you have, but you can limit the progress into the rest of the plant. Spraying with a fungicide labeled for powdery mildew is recommended, or just spraying with jets of water to wash off the mildew spores will help. Al

Thank. I suspected as much. This isn't normally a plant that's prone to that, but it's been incredibly humid here all summer.

they grow in the ditches around here ...

as far as the plant itself.. you need not do anything .. IMHO ...

as for the aesthetic ... well.. as al said.. you arent going to make it pretty ...

so all i ask is.. why bother with chemicals ...

in fall.. cut it down.. and send it away ... no composting..

and the start wondering if preventative is necessary next season ... and it sounds like you already know that answer.. odds are low ...

ken

I don't know what causes it but my Asclepias tuberosa looks like that too! It hasn't been a good year for this plant because it doesn't usually look this bad. I have trimmed back some of the plants to see if there will be new growth. The pregnant Monarch females seem to prefer the tender fresh growth tips any, and I routine clip back other species of Asclepias.

This year, I have NO A. incarnata (swamp milkweed) at all - I didn't start any from seed in the spring and the slugs or something ate the older plants right down to nubs.

Asclepias isn't the easiest genus to grow but I keep trying because I love to raise Monarch butterflies.

Ken, I wasn't figuring on spraying. Too many bees and butterflies enjoy it for me to endanger them.

terrene, I bought three small starts of Asclepias tuberosa 'Hello Yellow' back in 2005. One died, the other two took two years to get really established. I now have a big stand of them. I love them and I normally get two bloom cycles.