The fungus Monilinia fructicola causes brown rot that infects sweet cherries and other fruit with a pit or stone. Brown rot infections are more apt to occur when the temperature is 72 to 77 degrees Fahrenheit or when trees are wet for more than 24 hours. The fungus can spend the winter on cankers, producing spores for several years
Symptoms on Blossoms
The fungus can infect the blossoms, twigs and small branches of cherry trees. Infected blossoms that do not drop off may infect twigs, forming gummy cankers and causing the leaves to wither and turn brown. In the summer, insects, rain and wind can spread the fungal spores from infected twigs to developing cherries. The cankers on twigs grow larger each year.
Symptoms on Immature Cherries
Sunken, rotten spots about ¼ inch wide or spots with a red halo from 3/16 to ¾ inch wide appear on immature sweet cherries. Brown rot can infect green cherries injured by birds or insects. Infection also strikes immature cherries that have been and left on the ground after the pit hardens or frost-damaged cherries left on the tree.
Symptoms on Mature Cherries
Cherries are most susceptible to brown rot as they begin to color. Cherries tend to crack in the weeks before harvest. Rain causes cracking and also promotes infection of the brown rot fungus. On mature cherries, soft brown spots covered with powdery masses of tan spores expand rapidly. The cherries rot quickly, shrinking as they dry on the tree. Thin mature cherries so they do not touch. Stunted cherries are more susceptible to brown rot and should be removed. Harvest cherries carefully to avoid injuries and prevent brown rot infection.
Remove cherry mummies when the trees are dormant. A mummy is dried fruit remaining from the previous growing season. Mummies harbor brown rot fungal spores that will infect trees in the spring. Knock mummies off the branches, rake and either bury or burn. Prune weak and dead wood and wood that has brown rot cankers in the spring. Pruning to allow sunlight to penetrate helps the fruit to dry after wet periods and makes it easier to spray fungicides on the tree. Research conducted by Washington State University indicates that excess nitrogen makes cherries more susceptible to brown rot. Growers should follow the recommended rates for applying nitrogen fertilizer.
Fungicides prevent brown rot, not cure it. In order to protect blossoms, spray fungicides on the buds when they begin to show pink. To prevent fruit rot on cherries, spray three weeks before harvesting. Agronomists at Clemson University recommend the commercial application of demethylation inhibitor fungicides, including those containing the active ingredients fenbuconazole, metconazole, propiconazole and tebuconazole. Concentrated applications have defeated resistant. Agronomists at Washington State University say fungicides with EPA registration for home use include Captan and Daconil.