Hardy in zones 5 to 7, the Blue Damson or Damson plum is a European type of plum. It bears small oval-shaped fruit with dark-purple skin and yellow flesh. Blue Damson plums work well in jams and pies due to their sweet-tart flavor. The trees range in height from 10 to 20 feet and have been cultivated since ancient times, notes Arbor Day.
Damson plum trees can contract brown rot during the spring months, primarily when spring weather is wet and warm (70 to 75 degrees Fahrenheit). Crusty black 1-inch-thick growths appear on the branches, which spread vertically rather than horizontally. If no action is taken, the tree will continue to produce growths. As they grow larger, growths can choke and kill a branch by encircling it. Clip off affected branches to control black knot, since the infection spreads from current growths, or use preventative fungicides, including lime sulfur.
Caused by warm, wet spring weather, leaf spot is a fungal disease. Affected leaves develop purple or brown spots. With the shothole leaf spot disease, the spots drop off the leaves, creating holes. This disease is fairly harmless and will not affect the fruit, but it does mar the cosmetic appearance of your plum tree. Rake up and dispose of affected leaves that drop off the tree, since the fungus can remain in the ground. You can also used a copper-based fungicide to treat this disease.
Brown rot (Monilinia fructicola) affects many types of stone fruit, including the Damson plum. This fungal disease can infect twigs, branches, fruit or flowers of trees. The first symptoms appear in the spring, when the flowers open then wilt and turn brown. The wilted flowers don't fall off the tree. As the fruits mature they become more susceptible to infection; infected fruits develop brown spots. As the infection expands, the fruit wrinkles and shrivels, resembling a dried-out prune. Disease spores remain in the mummified fruits and in affected twigs, so a tree can be reinfected once it contracts brown rot. Brown rot can be controlled with an approved fungicide. Call your local county extension office to check on current approved fungicides for brown rot, advises the University of Rhode Island.