Great topic! I have a thread looking for Gulf Series plum scion wood. We grew all of them except Gulf Rose here in Florida 9b at one point in the past 30 years. Gulf Gold and Gulf Ruby were the best flavored, but the trees got diseases and were not long lived. Gulf Beauty was my least favorite, but Gulf Blaze was good. Scarlet Beauty is the only one that is self fertile and is the best adapted to my area. It was developed by a man in Wachula not by the University of Florida so it may bloom too early in your zone. I have not tasted them, but Robusto and Byron Gold might be good for you to check out as well. Anything with native genetics seems better adapted and less prone to disease. Bruce, Guthrie, Odoham, Excelsior, Hypauxlo, etc. are native plum selections/hybrids that have a reputation for being very disease resistant. I am growing Guthrie at the moment--if it blooms well in the next 4-5 years, I will give some of the others a try.