A prune tree is actually a plum tree, as prunes are plums that have been dried after they have been harvested. Getting a plum to the point of harvest requires proper care of the tree and the fruit, including disease prevention. Many diseases can affect fruit bearing trees, with a few that are specifically related to the plum tree.
Black knot is a very common, and serious, disease that affects the plum tree. North Dakota State University Agriculture and Extension explains that black knot appears in the spring, on small branches, and the knots appear soft and green. The knots then become dark and hard, and can get larger over the course of a few years, eventually destroying the entire branch, and ultimately the tree if it is not treated.
If you discover black knot on your plum tree, prune the infected area away immediately and burn the pruned pieces, as this can help stop the disease from spreading. NDSU recommends using a fungicide in addition to pruning.
Plum Pox Virus
According to Oregon State University, plum pox virus, also known as PPV, was not seen in North America until 1999. It is a disease that can take a devastating toll on the industry aspect of plums, and is spread through orchards by aphids. Symptoms of PPV can vary, but usually appear on leaves as bands or rings, and physical leaf deformity. The fruit becomes deformed with brown flesh and drops almost a month before it is due to mature. OSU states that the fruit that remains on the tree has a low sugar content and is lacking in flavor.
According to Penn State University College of Agricultural Sciences, it is very difficult to control an outbreak of PPV, even with spraying for aphids, as it only takes one aphid to infect an entire tree. Control measures include eradication of the tree or trees once the virus has been discovered.
Plum pocket is a rare disease of the plum tree, but when it strikes, it can cause severe damage. Plum pocket starts as little white blisters that are on the plum fruit, and eventually it covers the fruit. PSU explains that the fruit becomes spongy, hollow in the center as it dries up, turns brown on the outside and then drops from the tree.
The disease doesn't leave the tree with the dropped fruit, however. New shoots, leaves and fruit are also usually infected. NDSU recommends an eradicant spray be used on the tree early in spring, when the buds are still dormant, and be careful to spray the entire tree.