Brown rot on plum — symptoms and treatment

Severity: high

What is Brown rot

Brown rot is caused by Monilinia species. It causes blossom blight and fruit rot, especially in humid conditions, and its high severity comes from a trait that separates it from most fruit diseases: it attacks twice in one season. The first strike is at bloom, destroying the flowers that would have set the crop. The second is on the ripening fruit, and moves fast enough that a tree heavy with sound plums can be largely rotted by harvest.

Symptoms

The first phase is blossom blight: flowers turn brown, wilt and collapse but hang on the spur instead of dropping cleanly, and the infection runs from the dead blossom into the shoot. The second phase appears on ripening fruit as a brown rot spreading from a single point, soft under the thumb, until the whole plum is gone. The diagnostic sign develops on the rotted surface: grey spore tufts — dusty, cushiony pustules in rough rings. Rotted plums dry and shrink into hard mummies that persist on the tree through winter — not debris, but the disease's overwintering home.

• Early signs: browning, collapsing blossoms that cling to the spur; a soft brown spot on a ripening plum
• Advanced signs: brown rot with grey spore tufts, rot passing between touching plums, mummies on the tree
• Confusable with: other fruit rots — grey spore tufts plus blighted blossoms confirm brown rot

Causes and conditions

The fungus overwinters in mummified fruit and in infected twigs and cankers. In spring, spores from those sources are carried by wind, rain splash and insects onto the open blossoms — which is why a wet bloom produces a severe year and a dry one may pass unnoticed. Blighted blossoms and twig lesions then supply the spores that infect ripening fruit, and humid conditions near harvest accelerate that phase sharply. Any break in the skin gives entry — insect punctures, bird pecks, hail, rain splits on ripe plums. Fruit touching fruit lets the rot pass plum to plum.

Treatment

Sanitation and pruning — cultural

Timing: winter dormancy. Remove and destroy mummified fruit and infected twigs. Prune for good air circulation. This is the foundation of control because it removes the source: every mummy left hanging is next spring's spore supply. Take mummies and prunings off the site.

Thiophanate-methyl — chemical

Timing: bloom and pre-harvest. Apply at bloom and again 2-3 weeks before harvest. Remove mummified fruit. The bloom spray protects the flowers during the infection window that seeds the whole season; the pre-harvest spray protects the ripening crop. Both belong on the tree ahead of humid weather. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Strip every mummified plum from the tree and clear fallen fruit in winter dormancy
• Prune out infected twigs and open the canopy so blossoms and fruit dry quickly
• Thin fruit so plums do not touch — contact carries the rot through a whole cluster
• Control insect damage and handle fruit gently; unbroken skin resists infection
• Pick promptly and keep fruit cool — brown rot spreads after harvest

Frequently asked questions

Can I eat plums from an affected tree? Discard any fruit showing rot. Sound plums from the same tree are fine, but check each one — infection often starts as a small soft spot that is easy to miss, and one rotting plum spreads to its neighbours.

When should I spray? At bloom above all, particularly in a wet spring, because blossom infection seeds the fruit rot later. A second application before harvest protects the ripening crop.

Why does it come back every year? Because mummified fruit and infected twigs carry the fungus through winter on the tree itself. If they are not removed during dormancy, it restarts each spring however well you spray.