Why Are My Plums Rotting on the Tree?

Plums that begin to rot while still attached to the tree — turning brown, softening, and often developing rings of white or buff-coloured pustules on the skin surface — are affected by brown rot (Monilinia laxa or M. fructigena). It is the most common and damaging fruit disease in plum growing, and in a warm, humid summer it can devastate an entire crop within days of the first symptoms appearing. The good news is that the biology of brown rot makes it very manageable once you understand it.

How brown rot develops and spreads

Brown rot fungi overwinter primarily in mummified fruits left hanging on the tree or lying on the ground from the previous season. In spring, spores are released and infect blossom — causing a disease phase called blossom wilt, where flower clusters collapse and brown before petals fall. From infected blossoms, the fungus may spread into spurs and young shoots. The main fruit infection phase occurs in summer when spores land on damaged or cracked fruit skin and germinate rapidly in warm, humid conditions. Infected fruits are highly infectious and spread spores to adjacent fruits through direct contact.

Blossom wilt — the spring phase

If flower trusses turn brown and fail to develop in spring, brown rot blossom wilt is the likely cause. Bacterial canker produces similar symptoms but tends to kill shoots further back rather than just the flower cluster. Remove affected flower trusses and any associated shoot tissue promptly — leaving them on the tree creates the spore source for summer fruit infection.

Removing all sources of infection

The most important management action is meticulous sanitation. Remove and destroy all rotting fruits as soon as they appear, whether on the tree or on the ground. At leaf fall, look carefully for any mummified plums still clinging to the tree — these black, shrivelled fruits contain millions of viable spores for the following season. Remove every one. This hygiene step alone can dramatically reduce the following year's infection pressure.

Reducing wound entry points

Brown rot enters fruits primarily through wounds — splits caused by uneven watering, insect damage, or wasp and bird feeding. Reducing cracking through consistent watering and managing plum moth and wasp damage all reduce the wound sites available for brown rot to exploit. Avoid dense, overcrowded canopies where high humidity and poor air circulation favour fungal germination.

Harvesting promptly

As fruits approach full ripeness their skin resistance to brown rot declines. Harvest as soon as ripe rather than leaving the crop hanging for convenience, particularly in wet weather.