It’s well past cherry season here in Seattle and Western Washington, but earlier this summer you may have noticed a brown mold covering fruits just as they were ripening. This is due to a fungus called cherry brown rot or blossom blight which belongs to the genus Monilinia. Unfortunately it can infect all Prunus species (stone fruits), so there’s still an opportunity to see it out there on fallen plums.
This entire cherry twig has died, most likely due to the fungus Monilinia laxa. Gumming has occurred where blossoms emerged from the stem. This entire twig (back to healthy tissue) should be removed and destroyed, as the fungus can produce spores on the twig and re-infect the tree next year. The brown mass at the end of this twig are leaves and blossoms that have died from the infection.
Monilinia laxa and Monilinia fructicola are spread by spores during various (usually wet) times of the year. The fungus infects a number of areas on the host plant. Early in the season it causes blossoms to wilt and die, and sometimes entire branch tips will be killed. The branch ends will then wilt and turn brown but remain on the tree, this is called flagging. Later in the year it can attack ripening fruit, rapidly causing a gray mold to engulf the entire fruit and spread to others that are touching it. It can also cause cankers and twig die-back, sometimes causing the tree to exude sap or gum.
Cherry brown rot can be hard to control, but here are a few suggestions:
Avoid overhead watering. Avoid wounding the fruit during harvesting (the wounds are places for infection to occur. Control insects that cause wounds to fruit, (i.e. fruit flies). Prune out infected twigs in late spring or summer- don’t wait until the leaves fall, as it’s much harder to tell what is dead at that point. Remove mummified fruit and dead blossoms from the tree and underneath it, these can lead to re-infection. Space plantings and prune for good air circulation. This allows for quicker drying after rains, and limits moist conditions which are favorable for disease development.
Sources: Washington State University Hortsense
Cornell Integrated Pest Management
Diseases of Trees and Shrubs 2nd edition. Sinclair and Lyon. page 76. 2005