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Brown rot in cherries

Immature tart cherry fruit infected with brown rot. Photo: Shane Bugeja

Home orchards should be on the lookout for brown rot on their sweet or tart cherries. This fungal disease can reduce the amount of fruit you can enjoy this summer, as well as cause damage to twigs and leaves. Thankfully, this disease is often not a fatal one, but nevertheless homeowners should keep an eye out not just during bloom, but well into the season.

Brown rot can appear suddenly, and soon become a problem that stubbornly refuses to leave. Spread via greyish white spores that overwinter on infected plant material, this fungus can also arrive from several sources outside of a sick cherry. These can include cherry relatives such as chokecherry, black cherry, or another stone fruit (apricots, plums). Insects such as the invasive spotted wing Drosophila, a type of fruit fly, can also introduce this disease into your orchard.

Brown rot (also called blossom twig blight) is most commonly found on flowers, branches, leaves, or mature cherry fruit. It is caused by the fungi species Monilinia fructicola and Monilinia laxa. This fungus can cause brown water-soaked areas on blossoms, crisp up leaves, and/or create sunken wounds called cankers on twigs. A gummy, weeping sap can also appear in these infected twig areas, a symptom called gummosis.
Canker with gummosis on an infected cherry tree. Photo: Shane Bugeja

If mature fruit is infected with brown rot, over time they will shrivel into “mummies”. These mummies can also be covered with grey spores that can spread the fungus, whether they are on the ground or hanging in the tree. While immature green cherries tend to not get damaged, they are not immune, as some recent photos from Iowa can attest.

Brown Rot or a Cold Snap?

As many gardeners know, early May was quite cold, with several freeze warnings. For cherries, temperatures must stay 28° F for longer than 30 minutes to see at least some frost damage. Brown rot and frost damage on blossoms can look similar, with browning/death of the flower. However, if killed by cold instead of disease, we would expect the blossoms to fall off completely instead of lingering well into the growing season.
Brown rot-infected blossoms staying attached. Photo: Shane Bugeja

With Minnesota’s strange weather in the spring/early summer, it is important to be able to tell whether brown rot has hit your blossoms or if the damage was due to a cold snap. Brown rot enjoys an environment that hovers around 70° F, with ample rainfall. In the state, south central and south east regions are near or above precipitation norms (as of this writing). However, a significant part of the state is abnormally dry, with even some drought in west central Minnesota. With brown rot, there is no specific amount of rainfall needed to start infection, just that the infected plant material is wet for a prolonged period. However, being wet for 2 to 3 hours can be enough to cause infection to susceptible plants—provided spores are present and the temperature is warm enough.
Ripe tart cherries with brown rot. Photo: Annie Klodd

The UMN Extension page for brown rot is a great resource regarding prevention and control of this disease. Focusing on healthy airflow in the canopy by regular pruning is an important step to dry blossoms and fruit faster. This can reduce the length of time water (and maybe the disease) is in contact with the plant. Cleaning tools properly between trees and composing/disposing of brown rot affected plant parts are also effective practices.

Fungicides are an option if brown rot tends to be an issue year in and year out. These products are typically applied twice; once as flowers start to open, and another a few weeks before picking. If you are not sure the disease you are seeing is brown rot, consider submitting a plant sample to the UMN Plant Disease Clinic to avoid applying an ineffective fungicide. Here is a list of chemicals that are often labeled for brown rot.
1. Inorganic
a. Captan
b. Myclobutanil
c. Propiconazole
d. Chlorothalonil - Do not use for fruit rot control. Apply only on blossoms.
2. Organic
a. Sulfur

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law. When treating fruits or vegetables, make sure the plant you wish to treat is listed on the label of the pesticide you intend to use. Observe the number of days between pesticide application and when you can harvest your crop.

Additional Resources

Author: Shane Bugeja, Extension Educator, Ag Production Systems, Blue Earth/Le Sueur County

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