Grow our native climbing aster, Ampelaster carolinianus, which is smothered in blooms from late October into December for me. One of the best bee plants I have. All it needs is sun and a generous support to grow on, as it is a vigorous grower that can get 10 feet high.
My guess is that the normal Aster does not like the humidity in Florida. Doug has a good solution it sounds like, with the climbing Aster native to Florida. It must be a different zone breed. Good luck!
Mildew on asters is a problem all over. I'm in Kansas City and it's a problem in the Midwest, too. I just use a systemic preventative fungicide/insecticide spray (I use good old Orthonex) on mine and they're great. You have to be diligent, start early in the year when they're only up about 3" or so and continue every 5-6 weeks or so (maybe once a month in FL!) and it works fine. The systemic insecticide also keeps the lacewings that plague asters at bay, too! I have a calendar in the garage to follow so I don't have to try to remember when to do stuff!! Just copy all the info for that week or month to a new calendar every year!
I grow them in WA.State, and have never had that problem. I space them so the air circulates .
The natural movement of the sun throughout the growing season can affect time a plant spends in the shade & restricted airflow due to dence foliage or surronding plants will bring about the formation of mold & mildew. Which is unsightly but I personally haven't seen it deter this vigorous plant or it blossoms. I have bee balms, Black eye susans & Yellow daisies that will do the same from time to time. We can control how & when we water - but no so with the changing nature of the weather. I do make sure I completely clear away any affected leaves & debri throught out the season - when ever possible.
Beautiful flowers - my favorite
I grow them in Minnesota and don't remember having a mildew problem on them but, then again, I grow native asters.