Thank you for your question and for including the photos!

My first thought looking at the goldenrod was rust, but the reddish spots in the photos don't appear typical of rust. Rust generally produces raised yellow, orange, reddish-brown, or brown pustules, often on the undersides of leaves. These eventually release powdery, rust-colored spores that can rub off onto a white tissue. https://extension.umn.edu/plant-diseases/rust-flower-garden

Another possibility is a mosaic virus or another viral disease. Plant viruses can cause irregular yellow-and-green mottling, streaking, rings, and other discoloration similar to what I'm seeing in your photos. Cucumber mosaic virus (CMV), for example, has a very broad host range and can infect plants in many different families: https://extension.umn.edu/disease-management/cucurbit-viruses

However, several viruses can cause similar symptoms, and environmental or other plant problems can sometimes look similar, so a virus can't be diagnosed from photos alone.

It would be helpful to know whether the affected plants, other than the whorled milkweed, are primarily members of the Asteraceae family, such as asters, coneflowers, or related plants. Similar mottling across several related species could be useful diagnostic information. The yellowing of the whorled milkweed may be a separate issue, so I wouldn't assume that a single problem is affecting all of the plants.

For now, I wouldn't recommend treating for rust or applying a fungicide without a firmer diagnosis. If the symptoms continue to spread or you would like a definitive diagnosis, your best next step would be to submit symptomatic plant material to the University of Minnesota Plant Disease Clinic for testing: https://pdc.umn.edu/

Hope this helps!

-Victoria

University of Minnesota Extension Master Gardener Volunteer