A variety of plum trees thrive in Minnesota, providing gardens with beautiful blossoms in the spring and tasty fruit in the fall. But a few problems prevent some plum trees from reaching their full glory. The best way to protect and grow healthy plum trees requires examining the tree, especially in the springtime, and taking preventative measures when problems appear.
Late Spring Frost
Unexpected late spring frosts may occur in Minnesota, causing the leaves to wilt. While the tree may not look very healthy, there's little you can do to help the tree. The best solution requires waiting until the tree produces more buds, then the tree will leaf out again and look better.
Plum pocket occurs when fruits become enlarged and hollow, resembling large bladders. This problem occurs when spores disperse in early spring and get stuck in crevices and bud scales where they stay dormant until the next year. One way to control the problem requires spraying a delayed dormant spray right before the buds open. Add lime sulfur in the spring after temperatures rise above freezing or after the leaves fall off the tree in autumn.
Black Knot Disease
This disease gets its name from the hard black knots that appear on plum tree branches. Eventually these knots girdle and kill the branches. Around blossom time, new knots develop, sporting a felt-like, olive green appearance. If you see these knots appearing, prune the branches four to five inches below the knot at blossom time. A fungicide may also be applied in combination with pruning. Plan to prune the tree for a few more years to thoroughly control this disease.
Plum Rust Mites
This pest feeds on new foliage, causing the leaves to turn brown and curl up. The female mites spend the winter in the dead buds, then move to the foliage as soon as the buds open in the spring. Gardeners need to monitor new growth in July, watching for leaves turning brown. A chemical spray program should lower populations of the mites, but it may take several applications over a few years to thoroughly rid the tree of the problem.
While most gardeners welcome birds to their backyards, these feathered creatures wreak havoc on plum trees once the fruit matures. One way to prevent birds from pecking holes in the fruit or eating the plums requires securely netting the entire tree. Another option involves hanging aluminum pie pans or mylar tape around the perimeter of the tree. This only works for a short time until the birds figure out what's going on, but it might work long enough to harvest the plums.