All three types of plum trees -- European, Japanese and American -- are susceptible to diseases. Plant resistant varieties when available and meet the tree's needs to promote its healthy development. That's a less costly and labour-intensive alternative to raising the plant without care and having to treat it for disease as a result. In some cases, as with brownline disease, there's no cure. Once the virus enters the tree, the plant begins to die. But a few other plum problems come with a solution.
Treat brown rot blossom and twig blight, caused by two fungi of the Monilinia species, by picking all the fruit the fungal spores mummified. Remove them from the tree and the ground and discard them. Spray late-season plum cultivars with a protective fungicide containing propiconazole, iprodione or pyrimenthanil when the flowers are 40 per cent open. Contact your cooperative extension office or a local nursery for help estimating when that date is for your cultivar.
Alternate spraying plum trees with two fungicides if they become infected with powdery mildew. Select formulas that contain propiconazole, sulphur dust or myclobutanil. Take turns treating the tree with one or the other every two weeks. Suspend treatment after the plum pit hardens. Prune and discard all affected tree parts.
Prune all plum tree wood that has black raised areas, as they're infected with Apiosporina mobosa, the fungus that causes black knot. Inspect the trees in winter so you can finish all trimming before the buds open in spring. Make your cuts 4 inches below the ends of the blackened areas.
Disinfect your shears in a solution of 9 parts water and 1 part bleach. Prepare it in a bucket and take it to the garden when you're ready to prune diseased branches. Dip your shears in it after each cut.