Black rot on apple trees — symptoms and treatment

Severity: high

What is Black rot

Black rot is caused by the fungus Botryosphaeria obtusa. It affects fruit, leaves and bark of apple trees — an unusually wide attack for a single pathogen, and the reason its severity is rated high. Most fungal problems of apple cost you fruit quality; black rot can also girdle limbs through bark cankers and kill wood outright. The cankers are the part growers most often miss, because attention goes to the rotting fruit while the real reservoir sits in the branches.

Symptoms

On fruit, black rot begins as a small brown spot that expands into a firm, brown rot marked with distinct concentric rings, giving a target-like appearance. Affected apples eventually shrivel and dry into hard mummies that hang on the tree rather than dropping. On leaves the fungus produces frogeye leaf spots — rounded spots with a pale centre and a darker margin, resembling an eye. On wood, it forms sunken bark cankers that expand slowly along limbs; the bark above a canker cracks and the wood beyond it dies back.

• Early signs: small brown fruit spots; scattered frogeye leaf spots; slight bark discolouration
• Advanced signs: large ringed fruit rot, mummified fruit clinging to branches, expanding cankers, dead limbs
• Confusable with: other fruit rots — the concentric rings plus frogeye leaf spots on the same tree point to black rot

Causes and conditions

The fungus persists in cankered wood, dead branches and mummified fruit left on or under the tree, and these are the sources that start each new season. Spores are released in wet weather and spread by rain splash and wind onto fruit, leaves and pruning wounds. Warm, humid conditions favour infection, and damaged tissue is the easiest way in — hail injury, insect punctures, sunburn, broken twigs and unhealed pruning cuts all give the pathogen an opening. Trees already stressed or carrying dead wood are colonised far more readily than vigorous, well-maintained ones.

Treatment

Remove mummified fruit — cultural

Timing: dormant season. Remove mummified fruits and prune dead or cankered wood during dormancy. Do this first — spraying a tree that is still carrying mummies and cankers means re-infecting it from its own branches. Cut well below any canker and take the removed wood and fruit away from the orchard; do not leave the prunings on the ground.

Captan — chemical

Timing: green tip to second cover. Apply captan or myclobutanil from green tip through second cover spray. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Prune out dead, cankered and broken wood every dormant season and remove it from the site
• Strip every mummified apple from the tree and clear fallen fruit from underneath
• Prune in dry weather so cuts seal quickly, and keep the canopy open for airflow
• Avoid bark injuries from mowers, ties and careless pruning — wounds are entry points
• Keep trees vigorous with sound watering and feeding; stressed and declining wood is colonised first

Frequently asked questions

Is the fruit safe to eat? No — discard rotted apples. Unlike surface blemishes, this is a rot that runs through the flesh, and affected fruit should be removed from the tree rather than salvaged.

Why does it keep coming back every year? Almost always because mummified fruit or cankered wood is still on the tree. The fungus overwinters there, so no spray programme will hold if the reservoir is left in place.

Can it kill my tree? It can kill individual limbs by girdling them with cankers, and a young or already weakened tree can be lost. This is why the disease is rated high severity rather than merely cosmetic.