I just noticed that nobody had ever commented on your question. I cannot pretend to any great expertise on this question, but I hate it when the OP is left with the feeling that nobody is listening! I have no idea what is wrong with your milkweed. However, my general principle with sick plants is, "when in doubt, pull it out." (There are exceptions. I took over a sample of a tomato plant to my agricultural extension service a few weeks ago, and they diagnosed it with spider mites. I started weekly spraying with soap and water and got the infection under control well enough to save my tomatoes and eggplant.) However, in general it is hard to find out exactly what disease a plant has, and most diseases are not treatable anyway. Probably the safest course is to yank it and start anew somewhere else. Otherwise, you run the risk of spreading disease to healthy plants. Not worth it, especially with something as easy to grow as milkweed.

HTH,

Liz