Why are my peaches rotting on the tree?

Peaches that brown, collapse, and develop rings of buff-coloured spore pustules while still on the tree are suffering from brown rot, caused by the fungi Monilinia laxa or Monilinia fructigena. It is one of the most destructive stone fruit diseases and can destroy an entire crop within days in warm, humid weather.

How brown rot spreads

The fungus overwinters in mummified fruit left on the tree or on the ground and in cankers on bark. In spring it produces spores that infect blossom, causing "blossom wilt" — flowers and young shoots collapse and die back. Later in summer, spores spread to maturing fruit through cracks in the skin, pest damage, or direct contact between fruit. Warm, humid conditions from July onwards create ideal conditions for rapid spread. A single mummified fruit left from the previous season can infect a significant proportion of the new crop.

Removing infected fruit immediately

Pick off and dispose of every infected fruit as soon as you see browning, before the concentric rings of spore pustules develop and release millions of new spores. Do not compost infected material — bag and bin it or burn it. Check the tree every two days during the main ripening period. Infected fruit that falls to the ground should also be collected immediately.

Removing mummified fruit in winter

Any fruit that mummifies on the tree over winter must be removed during the dormant season. These wrinkled, shrunken fruits contain viable spores that will release and infect blossom the following spring. This single hygiene measure, done every winter, significantly reduces the disease pressure for the following season.

Improving airflow through pruning

Dense canopies with crowded branches trap humidity and prevent fruit from drying after rain — ideal conditions for brown rot. Summer pruning to open the canopy, remove crossing shoots, and increase airflow is a valuable preventive measure. For fan-trained trees, tie in new laterals with adequate spacing so fruit does not touch. Where two fruits touch on the same spur, remove the weaker one when thinning.

Fungicide treatments

Where brown rot is a persistent annual problem, apply a myclobutanil or tebuconazole fungicide at petal fall, then repeat at ten-to-fourteen day intervals through fruit development, stopping at the pre-harvest interval stated on the product label. Copper sprays at blossom time help reduce the blossom wilt phase of the disease, reducing the inoculum available to infect fruit later.