Fellenberg, I believe, is considered similar to Italian, but as I said, I never have grown it. It tends to be rated a moderate cropper. I consider Italian here less than a moderate bearer so I'm guessing Fellenberg would likely be same. There's a lot of tricks I use to accelerate bearing so I'm not sure if Fellenberg actually can't be speeded up significantly. I consider Castleton a very flavorful prune plum but what makes it stand out is its reliability- it tends to set fruit in clusters almost like grapes. Some years other plums set little or no fruit but Castleton always seems to have a crop at all sites. It is also a natural semi-dwarf with a wonderful spreading branch habit. My favorite prune plum is Valor because it is very big and has that rich super-sweet quality and it can ripen from the last week of August here until the first hard frost. That is an extremely long picking time even for a prune plum. The reason I recommend Castleton over Valor is because of its consistency at many sites I manage. The funny thing is that at some sites plums are generally much more reliable than other sites for reasons I've never been able to decipher. That means that to really know what is best at your site you'd have to try several varieties and find out yourself but if you're just going to try one I think Castleton is your best bet. ACN carries it but you'd better order ASAP because the commercial growers are sucking it up.

Lorraine, I'm in the same boat. I live in San Diego and have a dwarf Santa Rosa plum tree that I've had at least 4 years. This is the first year that I've had a significant amount of blossoms set fruit (although last year I moved the tree from a pot to in the ground a bit late in the year, losing almost all of my blossoms). Some of the lower ones are yellowing and will probably fall off. I've got my fingers crossed for this year.

I grew them here in Arizona. Low chill, early bloom. For the first few years, I had two plums on a huge tree grafted on Mariana -- so it needs a pollenizer if you want more than a few plums. It definitely has an acidic/tart aftertaste that I didn't care for, which is probably why it was never a commercial success.

had to google caliper lol I'm pretty sure you mean Prunus angustifolia because that's the only one I see get called Chickasaw plum but there's also Flatwoods/Hog plum, Prunus umbellata. I have P. umbellata and it was over 5' 6" feet when i bought it (this is a species that can be sour tho) from this place in Jax http://nativeanduncommonplants.com/ I fertilized it in mid to late summer again with citrus fertilizer and it responded with a second burst of growth about equal to initial spring one so you might be able to experiment with that to get it to grow faster. Not sure what its diameter size is, sorry. It's certainly thicker than my thumb, though. Edit: I've never ordered from these people but i have considered it. https://www.wilcoxnursery.com/plants-and-supplies/trees/chickasaw-plum-15-gallon/