Brown Rot Blossom Blight and Fruit Rot

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  Strong resistance to brown rot does not exist in commonly grown varieties, but plums are generally not as susceptible as peaches.  However, European plums are more susceptible than Japanese plums. 
• The brown rot fungus infects flowers when wet weather occurs during bloom and infects fruit when warm, wet weather occurs during the ripening stage. Infected fruit that is not harvested will dry and persist into the next season to continue the spread of infections, and should therefore be removed during dormant pruning.  
• Fungicides applied to protect blossoms and ripening fruit should be timed according to susceptibility and weather that favors disease.

Disease Cycle and Management

Brown rot, a fungal disease caused by Monilia fruiticola, infects blossoms, shoots and fruit, and causes significant losses for growers when not controlled.  Plums are less susceptible to brown rot than peaches and cherries, but can be severely infected when conditions are ideal for the fungus.  The brown rot fungus overwinters on mummified fruit that remain attached to twigs, infected blossoms that cling to twigs and on infected shoots.  Fungal conidia are dispersed by wind and rain.  The disease is more severe with wet weather. 

Conidia produced on infected blossoms and shoots spread to fruit.  Infected blossoms wilt, turn brown and cling to the shoot.  The infection can move into the twig and cause a canker.  Blossom infections are not common on plum.

Fruit are susceptible to infection.  Immature fruit can be infected when insect control is poor.  Thinned fruit can serve as a source of innoculum, but thinning fruit so that they do not contact each other will reduce disease at harvest.  Signs of infection begin as small, tan colored circular lesions on the fruit surface.  In warm, humid weather, lesions develop conidia, a mass of grayish brown spores over the fruit surface.  During warm, humid conditions, grayish brown spores are produced which can be diagnostic of brown rot.  Infected fruit that remain in the orchard will shriven and cling to the branch, and serve as a source of disease the following spring.  These shriveled “mummies” should be removed during pruning.

The brown rot fungus can be spread by insects and can also spread from alternate hosts such as wild plum and cherry. Improve air circulation with annual pruning and sucker removal.  Thin fruit so that they are not in contact with other fruit.  Harvest fruit before they become over ripe.

Fungicide resistance has occurred.

Activity Rating of Fungicides

• Copper – not applicable
• Captan - good, but phytotoxic to some varieties
• Chlorothalanil (Bravo, Echo) – excellent for blossom blight, not labeled for fruit rot
• DMI (Indar, Tebuzol, etc.) – excellent, except for Rally
• Strobilurin (Pristine) - excellent
• Strobilurin (Flint, Gem) - suppression
• Sulfur (Microthiol Disperss, etc.)– fair for blossom blight, slight for fruit rot
• Biofungicides (Double Nickel, Serifel) - suppression
• Thiophanate-methyl (Topsin M) – not recommended because of resistance