I have a number of plum trees of different varieties. Some are only a year or two old and others are 5 to 10 years old. I am yet to harvest any plums. The older trees form plums, but once they reach the size of a marble, they fall off. The trees appear very healthy. The fruits, when they fall off, usually appear fairly normal. The varieties - Santa Rosa, Italian Prune, Green Gage, Burbank, Shiro, and another Italian variety. Are plums difficult to raise in Maryland? Have I chosen the wrong varieties? I appreciate any suggestions!
You have hit the nail on the head as far as ease of growing. Tree fruits are not easy to grow well in Maryland without more intense management than most homeowners are willing to give. Insect pests, diseases, and weather/humidy/cold can all cause problems, and often a strict spray schedule of fungicides and pesticides is used.
We spoke to our fruit specialist and he wonders, among other things, about lack of pollination/pollinators (some of whom don't fly unless it's warm and dry), lack of sunlight from over- crowding, including not pruning so light can get through.
Also, we've had hard winters with late frosts and freezes, which can kill pollen.
Here is our page on tree fruits, that discusses cultural management of new trees: