Bitter rot in apples

Cause

Bitter rot is a fruit-rotting disease of apples and pears caused by the fungus Colletotrichum gloeosporioides and is more common in orchards with minimal fungicide spray programs.

Symptoms

Bitter rot symptoms will usually appear as small, light brown circular spots when the fruit is almost full size. These spots can enlarge rapidly and become sunken. If left untreated, the rot can penetrate deep into the flesh. Occasionally, if disease pressure is high, leaves of infected trees might show small, red flecks on the surface.

Affected fruit might have only one spot about 10 mm or more in diameter, or many smaller ones. The spots become dark brown to black with age. The fungus spores, which form pink masses on the brown spots, are sometimes arranged in concentric circles, giving the spots a target-like appearance. The rot penetrates deeply into the flesh.

The causal fungus survives the winter mainly in rotted and mummified fruit of the previous season. New-season fruit is first infected during warm, wet weather, usually after about mid-November. Spores spread from fruit to fruit and from tree to tree by rain splash.

Infection on fruit makes it unsaleable and prone to decay. Fruit infection is less common in orchards that receive a full protective spray program for apple or pear scab.

Control

Monitoring: bitter rot is usually seen in warmer coastal districts after November, particularly in the Sydney basin. It is favoured by warm, humid and wet conditions during the growing season. Careful monitoring of susceptible areas is recommended from 3 weeks after petal fall until harvest.

Cultural and physical: good orchard hygiene is critical for bitter rot control. The disease can survive over winter on mummified fruit and dead wood; this must be removed and destroyed. When pruning, clear away and destroy all dead wood and mummified fruit and remove infected fruit regularly during the season.

Chemical: a spray program for the control of bitter rot and other late summer diseases of apples is given in the Orchard plant protection guide.