I've no idea as to the cause but it does look severe. Bless your heart, I'd be terribly upset too. Has anything around your MW bed changed? It has been terribly hot and dry in my area. Do you normally water this bed? If not, perhaps the plants are heat stressed. I can't tell from your id where you are located so I may be totally off base with weather conditions. I live in central Arkansas and we've been experiencing a very hot, dry summer, causing me to water my milkweed every day. The single plant with the leaf node growth is shocking to see. If heat stressed, I'd think the plant would slow down the growth.

I have seen the thin leaves on some of my Common MW earlier in the season, after the hot dry conditions began. Once I began watering each day, the newer leaf growth above the thinned leaves returned to normal. My baby plants are all healthy. It might not have anything to do with the watering and I might have the first signs of whatever it is you are experiencing. I will follow this thread with great interest.

Mary

I just did some digging and found this article. It sounds like some of the conditions you are experiencing, especially the witches broom condition (could this be your heavy leaf node growth off of one stalk?) I may have the same conditions beginning in my garden and if so, is spread by leaf sucking insects like leaf hoppers.

... Click for Milkweed disease article ...

Mary

Thank you for your comments. I live in Northwestern Ohio near the Michigan border. We have actually had a lot of rain and flooding this year, but the condition started to happen at the end of the season last year as well. The bed is watered by an automatic sprinkler which is controlled by a rain gauge. So, I do not think it has anything to do with watering.

mary_litterocker, I also read that article when I began my search and also came to the conclusion it was suffering from witches broom growth. Thank you for digging around. I am hoping for some more in depth information. There seems to be very little out there because Common Milkweed is considered an aggressive growing weed and most people want to get rid of it rather than heal it. It is sad, and one of the many reasons why Monarch's are suffering.

Right now I have to go and get my milkweed from fields. My daughter and I drive around searching for good clumps of milkweed for our 50 caterpillars. Hopefully I can get my bed back in good order next year. I am in the process of cutting all the sick plants down, but it is slow going because the Monarch's are still laying eggs on it. So, I need to carefully check each plant once I cut it down.

Thank you again for you help and if I ever so find a clear answer I will post. But, I am leaning toward the Milkweed Mosaic Virus spread by the leafhopper right now.

the Milkweed Mosaic Virus or the Milkweed Yellows Phytoplasma I meant to type...

I've never seen that happen before. I'd wonder whether some other plant took over my milkweed patch.

I grew up in Toledo about a mile south of the border. Was always easy finding milkweed in Michigan but that was in the '70s. The fields where I got milkweed in Toledo are long gone.