Reddish residue on bottom of Common Milkweed leaves

[ButterflyLdy]

All of my common milkweed plants have a reddish / rust-colored spotty residue on the underside of the leaves that wipes off leaving my fingers red. My tropical and swamp milkweed don't have this. Is this a disease or a pest?
Rebecca

[Keith Petrosky]

I think thats caused by air pollution.

[psi_chemie]

Possibly spores from a fungus?

[Wendy_T]

If you look really close, are the spots moving? Do you notice if the milkweed leaves are curled with some webs? If so, it sounds like spider mites.

[harpo787]

Well, from my rather unfortunate extensive experience with red spider mites, I'd say it sure sounds like them. I've also seen a reddish rust coloration on my milkweed leaves that doesn't rub off and doesn't leave a residue on my finger. I think that's just got to do with the plant's growth etc., it's not an "outside problem" so to say.

But the residue on your fingers? Spider mites. Also, if there are enough on there, you'll start to see very fine webbing that they weave.