It's hard to tell what is causing the symptoms, but there are some minor leaf spot infections that can occur on milkweed, which do not cause the plant significant damage, even if they are an eyesore. The wilting and/or disfigured leaves that appear to be part of the reddened area in the second photo might be due to a different cause, but we don't see any distinguishing traits that point to a specific source. There are some native insects, like the Red Milkweed Beetle, that relies on milkweeds as host plants and whose grubs bore into stems and roots (usually at the base of the plant) to feed, killing some top growth in the process. Most milkweed species are so vigorous, though, that they outgrow the damage by regenerating stems from the roots that remain healthy. No intervention in that case is needed, nor would it be recommended or even very effective once an insect larva was inside the stem.

Some pathogens can cause stem wilting as they disrupt or kill the tissues carrying water and sap into the growth above. They can enter though injury (if the stem was wounded by a mower or weed-whacker, for instance) or might infect the plant directly, though generally this happens when the plant is already under stress from some environmental cause (drought, being too wet, etc.). No fungicide would be useful to cure or treat such a condition, and here too, collateral harm to insects or other organisms might occur, even though such products target fungi.

For now, just monitor the plant and cut back any wilted stems if needed. If the roots are vigorous enough and still healthy, the plant should regrow new stems in due time. Leaf reddening in many plants can be a stress response (even the purplish-red color around pinpoints of infected leaf tissue is a typical response to leaf spot pathogens), but it can be hard to determine exactly what stress factor is responsible since symptoms can overlap.

Miri