I am not familiar w/fungus attacking milkweed (A. syriaca) but that's not to say it doesn't....there could be other reasons for your plants not blooming, though, besides fungus. I have some beautiful common milkweeds....their foliage is lush, glossy and they, themselves, have grown to over 5 ft. These plants have not bloomed either and haven't even set buds! I think the reason might be that they are growing in extremely fertile soil with LOTS of mulch! I didn't mulch them but they seeded themselves in the mulch....A. syriaca typically grows on roadsides under impoverished soil conditions, often, in gravel and very often in full sun. Where are your plants growing? Do they have full sun? Too much rain and too little sun could be causing the blooms to fail.