There are several plum diseases such as crown gall, cankers and brown rot but if you first noticed the dropping leaves in mid summer my guess is the tree has verticillium wilt. Even if you spray with a fungicide such as sulfur the odds of stopping this fungus are low. Your best bet is to improve the health of your other plum trees by mulching with compost. Organic matter has been shown to cut back on this fungus in the soil. Healthy trees can defend themselves from the disease so keep your other trees healthy and well nourished. I suggest a slow release plant food such as Osmote around the base of each tree in late spring. Water the root zone during times of drought. Good luck and Keep Growing, Marianne Binetti