Blossom Brown Rot

Bare, dormant stems begin to swell in spring, transforming from green to red tips, from which tight clusters of pink blossom buds emerge. Those buds will bloom, drop their fruit, assuming they have been pollinated. That is, of course, unless blossom brown rot has taken hold.

Blossom brown rot (Monilinia laxa), also known as brown rot blossom blight, is a fungal disease of almonds, apricots, cherries, and other stone fruits..

Cherry blossoms infected with blossom brown rot (Elizabeth Bush, Virginia Polytechnic Institute and State University, Bugwood.org) CC BY 3.0 US

Blossom brown rot looks similar to brown rot (Monilinia fructicola), affecting flowers from the pink bud stage through petal fall. All parts of the flower are susceptible.

Symptoms of blossom brown rot

The first sign of blossom brown rot is the death of young flower blossoms. What should be a colorful, flower-laden tree buzzing with pollinators looks more like sticks with clusters of brown, dried-up tissue paper. [That would be an extreme case.] More often, infected flowers are intermittent (at first). Gum may ooze from the base of infected flowers, and cankers may form on twigs. Those cankers will have tan centers and dark edges. Blossom spurs and their leaves may collapse. Under humid conditions, you may see tan to gray spore masses.

Blossom brown rot life cycle

Fungal spores overwinter in twig cankers, on mummified fruit, and on any diseased flowers that remain attached to the tree. As temperatures rise in spring, fungal spores begin populating nearby twigs and other blossoms, causing twig and branch dieback and blossom losses. Spores are airborne and spread by irrigation, splashing raindrops, and insects.

​How to control blossom brown rot

​This fungus thrives in rainy weather with temperatures in the 70s(F). High humidity can also encourage spore development. Growth is almost directly related to local humidity and temperature levels, which can be tough to control in the home garden.

Proponents of compost tea recommend foliar sprays as a treatment for blossom brown rot, but research has shown that compost tea either has no effect or worsens the condition.

Unless you want to apply chemical fungicides, you are best off selecting varieties that are resistant to this disease in the first place. The following species of almond tree are most susceptible to blossom brown rot: Butte, Carmel, Drake, Ne Plus Ultra, Winters, and Wood Colony.

You can also reduce the likelihood of blossom brown rot by removing any mummies as soon as they are seen and disposing of them in the garbage. Pruning and training for better airflow can also reduce the time needed for blossoms to dry.