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Ministry of Agriculture, Food and Rural Affairs

Black Rot

Frog-eye leaf spot symptoms on apple Black rot canker on infected apple tree limb  The wood of two-year-old black rot cankers shrinks, turns black, and layers of bark peel Initial black rot symptoms on mature apple fruit appear as black sunken spots Black rot symptoms on mature fruit  Light or tan brown rot with concentric dark and light rings Late in the season or in storage tiny pimple-like black pycnidia Sepal infections that occur early in the growing season eventually develop into calyx end rotCaptan injury
Click to enlarge.

Beginner

Scientific Name
Botryosphaeria obtusa

Identification
On leaves:

  • The disease first appears as a tiny purple fleck which eventually enlarges into a circular lesion about 4-5 mm in diameter.
  • As the lesion enlarges, the margin remains purple while the centre turns tan or brown with a light centre (“frog-eye” appearance).
  • Severely infected leaves eventually turn yellow and drop.

On limbs:

  • Limb cankers first appear as reddish or pinkish, brown sunken areas in the bark.
  • Cankers often remain small and superficial but sometimes enlarge up to 50 cm in length along the infected limb, killing and cracking the bark. 
  • As the canker ages, the wood shrinks and becomes black as the bark peels away from the infected area.
  • Cankers on limbs cause the entire limb to die back. Severely cankered limbs are weakened to the point that they may break under heavy fruit loads.

On the main trunk:

  • Infections eventually girdle the tree resulting in premature death.

On fruit:

  • Initial symptoms on young fruit appear as red flecks that develop into purplish, slightly raised pimples
  • As the fruit begins to mature and ripen, lesions begin to enlarge and develop into a sunken black spot surrounded by a red halo.
  • The lesion eventually enlarges into a light or tan brown rot with concentric dark and light rings.
  • Black rot often appears late in the season just before harvest as black spots on the fruit associated with mummified fruit
  • Black pycnidia may appear on fruit late in the growing season or in storage.
  • Sepals of fruit become infected early in the season just after bud scales become loose.
  • Red flecks that turn purple surrounded by red rings on sepals are the first symptoms of early infection.
  • As the fruit matures, the fungus penetrates the fruit from the calyx causing calyx end rot.
  • Eventually, concentric dark and light rings form and the fungus grows into the core causing core rot and resulting in premature fruit drop.

Often Confused With
Leaves

  • Spray burn - Spray burn on leaves can appear as brown spots. Frog’s eye leaf spot an be distinguished from spray burn by the characteristic tan center or brown with a light center giving the lesion a frog’s eye appearance.

Limbs and Trunks

  • European canker - European canker on trunks and limbs has bright orange fruiting bodies in the cankers during the winter. The wood of black rot canker shrinks turns black and layers of bark peel. Often difficult to distinguish cankers and lab diagnosis may be necessary. 

Fruit

  • Bull eye’s rot, calyx-end rot and dry end rot - Black rot can be distinguished late in the season or in storage by the appearance of tiny pimple like black pycnidia on the rotted fruit.  Often it is difficult to distinguish fruit rots and lab diagnosis may be necessary.

Period of Activity
Black rot activity begins at the end of bloom and it continues to be a threat through harvest. The disease often first shows up on leaves one to three weeks after petal fall. The optimum temperature for leaf infections is around 26.6°C with 4.5 hours of leaf wetness.

Scouting Notes
Make an attempt to scout surrounding woodlots and identify hardwood trees infected with the disease.

Thresholds
None established.

Advanced

Scientific Name
Botryosphaeria obtusa

Identification           
The black rot pathogen infects limbs, trunks, leaves and fruit resulting in reduced productivity and quality of yield.

On leaves: The disease first appears as a tiny purple fleck, which eventually enlarges into a circular lesion about 4-5 mm in diameter. As the lesion enlarges, the margin remains purple while the centre turns tan or brown with a light centre (“frog-eye” appearance). Severely infected leaves eventually turn yellow and drop. Defoliation year after year stresses trees and greatly reduces vigour.

On limbs: Damage to limbs caused by cankers from other disease – or wounds caused by insects, pruning and hail or winter injury – provide an entrance for the fungus to invade and become established. Fire blight cankers are often colonized by black rot fungus. Limb cankers first appear as reddish or pinkish, brown sunken areas in the bark. Cankers often remain small and superficial but sometimes enlarge up to 50 cm in length along the infected limb, killing and cracking the bark. As the canker ages, the wood shrinks and becomes black as the bark peels away from the infected area. Cankers on limbs cause the entire limb to die back. Severely cankered limbs are weakened to the point that they may break under heavy fruit loads.

On the main trunk: Infections eventually girdle the tree resulting in premature death.

On fruit: Initial symptoms on young fruit appear as red flecks that develop into purplish, slightly raised pimples, which often go, unnoticed at first.  As the fruit begins to mature and ripen, lesions begin to enlarge and develop into a sunken black spot surrounded by a red halo. The lesion eventually enlarges into a light or tan brown rot with concentric dark and light rings. Colonized mummified fruit are often located in close proximity to the black-rot infected fruit. Black rot often appears late in the season just before harvest as black spots on the fruit associated with mummified fruit left from chemical thinners. Black pycnidia may appear on fruit late in the growing season or in storage. Sepals of fruit become infected early in the season just after bud scales become loose. Red flecks that turn purple surrounded by red rings on sepals are the first symptoms of early infection. As the fruit matures, the fungus penetrates the fruit from the calyx causing calyx end rot. Eventually, concentric dark and light rings form and the fungus grows into the core causing core rot and resulting in premature fruit drop.

Often Confused With
Leaves

  • Spray burn - Spray burn on leaves can appear as brown spots. Frog’s eye leaf spot an be distinguished from spray burn by the characteristic tan center or brown with a light center giving the lesion a frog’s eye appearance.

Limbs and Trunks

  • European canker - European canker on trunks and limbs has bright orange fruiting bodies in the cankers during the winter. The wood of black rot canker shrinks turns black and layers of bark peel. Often difficult to distinguish cankers and lab diagnosis may be necessary. 

Fruit

  • Bull eye’s rot, calyx-end rot and dry end rot - Black rot can be distinguished late in the season or in storage by the appearance of tiny pimple like black pycnidia on the rotted fruit.  Often it is difficult to distinguish fruit rots and lab diagnosis may be necessary.

Biology
Black rot fungus overwinters in cankers on twigs, branches and trunks of many hardwood trees, as well as on mummified fruit. Apple cultivars which retain mummified fruit such as Gala, Honeycrisp, Cortland and Northern Spy are often susceptible to black rot. Trees damaged by winter injury or fire blight infections are more susceptible to black rot.  

The fungus produces two types of inoculum – ascospores (sexual spores) and conidia from pycnidia (asexual spores).  Ascospores from cankers or colonized mummified fruit tend to be released early in the growing season as buds begin to swell and open. The heaviest ascospore release occurs for a four to six week period following petal fall. Conidia are released from pycnidia throughout the growing season starting soon after bud break. Ascospores are wind blown, and conidia require a wet period and rain splashing for release. The optimum temperature for leaf infections is 26°C accompanied by four to five hours of leaf wetting, and fruit infections occur at 20-24°C with nine hours of wetting. Risk of infections is low when temperatures are below 10°C (more than 24 hours of leaf wetness required), and none occur below 8°C. Leaf and fruit infections occur simultaneously in the spring.

Period of Activity
Black rot activity begins at the end of bloom and it continues to be a threat through harvest. The disease often first shows up on leaves one to three weeks after petal fall. The optimum temperature for leaf infections is around 26.6°C with 4.5 hours of leaf wetness.

Scouting Notes
Regular scouting is the best way to assess disease levels. Make note of infections on leaves, limbs and trunks and fruit. Make an attempt to scout surrounding woodlots and identify hardwood trees infected with the disease.

Thresholds
None established.

Management Notes

  • Selecting cultivars that are less susceptible to black rot for planting near sources of disease (woodlots) may help reduce yield losses. For listing of cultivars susceptibility to Black rot see OMAFRA Publication 310, Integrated Pest Management for Apples.
  • Pruning out diseased limbs and dead wood is to reduce the inoculum sources within the orchard. Remove prunings from the orchard or burn them since black rot fungus can survive on dead tissue. Alternatively, chopping up the prunings on the orchard floor with a flail mower reduces inoculum levels.
  • Woodpiles must not be stacked in or near orchards since they can be a major source of inoculum.
  • Make an attempt to scout surrounding woodlots and identify hardwood trees infected with the disease. If possible, remove and burn these trees to decrease potential disease pressure. 
  • Mummified fruit left in trees often become infected with the fungus and remain as a source of inoculum within the orchard. Removal of mummified fruit helps reduce the spread of the disease. Some chemical thinners leave small immature fruitlets that become mummified and infected with the black rot fungus, and a source of inoculum later in the season. Research shows applying chemical thinners when fruit are 6-8 mm rather than 12-14 mm reduces the number of mummified fruitlets left in the orchard, resulting in less black rot on mature fruit at harvest. 
  • In orchards with a history of black rot, initiate applications of registered fungicides at silver tip and continue on a 10-14 day schedule to protect leaves and fruit from black rot infection.
  • For a list of products effective in managing black rot, see OMAFRA Publication 360, Guide to Fruit Production - Chapter 3 Apples (PDF) or Apple Calendar only (PDF): .