How to Control Brown Rot Fungus on Plum Trees

A disease of stone fruits like plums, brown rot is caused by the fungus Monilinia fructicola. The fungus infects plum tree flowers, fruits and twigs, most often emerging during wet, mild weather. Brown rot fungi can even overwinter in the dried infected plums, called "mummies." You'll likely see brownish-gray spores and powdery tufts on the infected flowers, fruit surfaces and bearing twigs. To treat brown rot on your plum trees, you'll need to establish cultural controls and potentially apply a fungicide.

Pruning shears or loppers

Fungicide

Remove and discard or destroy all infected fruits, flowers and other growth on your plum trees to control the growth of the brown rot fungi. Pick off and destroy all plum "mummies," and don't allow them to remain on the trees during winter. Don't store dropped and rotted plums or allow them to remain on the ground around the trees.

Prune your plum trees to remove all crowded or crossing growth. This will improve air circulation, which can help control the spread of brown rot fungi. In late winter, prune out all infectious cankers growing on the plum tree twigs.

Thin out your plum fruits early, preferably prior to pit hardening. If you thin the plums later, after pit hardening, the fruits will be more susceptible to becoming infected with brown rot fungi when they later fall to the ground.

Apply a fungicide labeled for controlling brown rot on plum trees, such as one containing chlorothalonil, myclobutanil, lime-sulfur, captan or thiophanate methyl. Begin your fungicide spraying when your plum tree starts to bloom, and then continue through the growing season. Follow the application instructions on the fungicide label carefully.

Control any insect pests on your plum trees, especially insects that feed on the fruits like the oriental fruit moth, plum curculio and tarnished plant bug. Keep in mind that plum trees with insect problems and other injuries are more susceptible to brown rot disease, because wounds to the fruits and buds allow entry spots for the fungi.

If you have old and neglected or wild-growing plum trees -- or other stone fruit trees -- on your property or around your orchard, you might need to remove them to prevent the trees from becoming infectious sources of brown rot spores.

You can select plum tree species that are resistant to brown rot disease to plant in your orchard. Hybrid and European cultivated varieties of plum trees are typically more resistant to brown rot fungi than Japanese plums.

Don't use overhead sprinklers to water your plum trees if they have or had brown rot disease. Irrigate the trees so that you minimize the amount of water applied to the foliage, because wet leaves, flowers, fruits and stems can promote fungal growth.