Why Are My Apples Rotting on the Tree?

Brown rot (Monilinia fructigena) is the most common and destructive apple disease in temperate gardens, and it causes exactly this symptom: developing fruit develops brown patches that rapidly expand across the entire apple, the flesh turns soft and rotten, and the surface produces concentric rings of pale buff or white spore pustules. Affected fruit may remain hanging on the tree as shrivelled, mummified husks long after harvest — these mummies are the primary source of infection for the following year.

How brown rot enters fruit

The brown rot fungus cannot penetrate unblemished apple skin directly — it needs an entry point. The most common entry routes are: wounds made by codling moth or other insects boring into the fruit; cracks in the skin caused by irregular watering or calcium deficiency (bitter pit); hail damage; or skin-to-skin contact between adjacent fruit during wet weather. A spell of warm, humid weather in late summer, combined with a crop load that brings fruits into contact with each other, creates ideal conditions for rapid spread.

Managing an active outbreak

Remove and destroy (in household waste, not compost) every affected fruit as soon as you see it, including those already on the ground. Pick off and dispose of any hanging mummified fruits from previous years — these contain vast numbers of viable spores. Improve air circulation by removing any fruit touching others. In a severe outbreak close to harvest, harvest unaffected fruit early and store carefully rather than leaving it on the tree to risk further infection.

Prevention for future seasons

The most important preventive measures are: removing all mummified fruit before bud break in late winter; thinning the crop to prevent fruit touching; controlling codling moth to reduce wound entry points; and ensuring the tree has good air circulation through appropriate pruning. Copper-based fungicide sprays applied from petal fall and repeated through the growing season provide additional protection in orchards with a history of severe brown rot.

Brown rot in storage

Brown rot also attacks apples in storage, spreading from infected fruit to neighbouring healthy ones. Check stored fruit every two weeks and immediately remove any showing signs of rot. Never store damaged or insect-affected fruit with undamaged ones — they will infect the whole store.