Flowering plum, Prunus x blireiana, is a lovely tree, but it's discouragingly prone to diseases. Some infections enter when boring insects expose the sap. Others attack via the soil. Identifying the infection will aid you in controlling it, saving your tree if possible and preventing spread to other trees. To confirm the diagnosis, send a sample branch to the plant diagnostic laboratory at your state's land-grant university.
The fungus Dibotryon morbosum produces characteristic "knots" on the smaller branches of the tree. These are hard black swellings of up to a foot or more in length. On heavily infected trees, the knots may eventually girdle a branch and kill it. Knots sometimes appear and persist on the main branches or trunk of the tree, but these typically don't develop to the girdling stage. Most new infections do not become visible until the following spring. New knots are at first soft with an olive-green velvety covering. They will harden and become black over time.
The knots are D. morbosum's fruiting bodies, and they begin releasing their spores into the wind when leaf buds reach the green tip stage. To prevent spread, remove all developing knots before spring budbreak. Prune 4 to 5 inches below the knot. Bury or burn pruned-off material. Fungicide use is also recommended, but is only effective in conjunction with pruning.
The effects of Monilinia fructicola can be hard to spot in an ornamental plum. The most visible characteristics of the infection are a rapid brown rot developing on the ripening fruit, and, according to the Ashland Tree Commission's Recommended Street Tree Guide, P. x blireiana seldom fruits. You may notice infected blossoms turning brown, then cankers appearing at the fruiting spur.
To control brown rot, remove all cankered twigs and fungus "mummies," including those on the ground. Begin this process in late summer or fall so as to leave nothing behind by blossom time the following spring.
Valsa cincta and Valsa leucostma are the culprits behind perennial canker or valsa canker. Oval-shaped cankers gradually enlarge over the years until they girdle and kill the affected branches or even the tree trunk. In conjunction with the cankers, the tree will exhibit gummosis, a gummy secretion usually intended to protect open wounds.
Control perennial canker by protecting your flowering plum tree from sustaining wounds by which the fungus can invade. Time your pruning for the spring when wounds heal quickly. Control boring insects to avoid further injury and vulnerability. Remove cankers and cankered branches. Burn or bury pruned-off material. Avoid planting new trees next to older, cankered trees to prevent spore spread.