Management Tips for Black Rot Canker in Apples

Black rot is probably wide spread and more common in Ontario apple orchards than growers think. The fungal pathogen Botryosphaera obtusa can infect limbs, trunks, leaves and fruit. Apple trees that were hit with hail last year should be carefully monitored for cankers this spring. Wounds caused by hail, winter injury, mechanical injury, insects or other diseases provide an entrance for the black rot fungus to infect and colonize. Depending upon when the hail or other injury occurred last season and how severe it was, growers may not have recognized the early symptoms of black rot cankers that may have entered wounded limbs or trunks.

Early symptoms of black rot cankers on limbs and trunks are subtle. They first appear as reddish or purplish, brown slightly sunken areas under the bark (Figure 1). Although the sunken areas often remain small and superficial, some can enlarge up to half a meter in length under the bark. Regardless, as the cankers age by the second year, the infected bark dies and peels away from the sunken area exposing black diseased wood underneath (Figure 2).

Figure 1. Subtle Black rot canker forming under bark of infected apple tree.
The Black rot fungus infects through wounds caused by insect, mechanical, hail, and cold injury.

Figure 2. Bark peeling away from sunken area on older Black rot canker.

The severity of damage to the trees due to black rot cankers varies depending on the location of the canker in the tree. Cankers on limbs can cause the entire limb to die back or they can weaken limbs to the point of breaking under heavy fruit loads or during a wind storm. Infections on the main trunk (particularly on young trees), can eventually girdle, the tree resulting in premature death (Figure 3).

Figure 3. Older Black rot canker on main trunk will eventually result in premature death of the tree.

The black rot fungus can overwinter in cankers on both live and dead trees and in mummified fruit left in apple trees from the previous year. Mummified fruit left in trees should be removed whenever found in a tree but particularly during dormant season pruning when they can be seen more easily. Pruning out diseased and cankered limbs and dead wood during the dormant season is an important practice to reduce the inoculum sources. Cankers may also be surgically removed from trees if feasible and or practical. It is important to remove the infected prunings or wood from the orchard since the black rot fungus can survive on dead wood. Burning or chopping up the prunings on the orchard floor with a flail mower should reduce inoculum levels. It is not a good idea to stack wood in piles near orchards either since they can become colonized and act as a source of inoculum.

Since many hardwood tree species are potential hosts of the black rot fungus, attempts should be made to scout surrounding woodlots and identify hardwood trees infected with the disease. If possible and feasible, the infected trees should be removed to decrease potential disease pressure in the future.

There are no apple cultivars that are completely resistant to black rot; however some cultivars are less susceptible to limb cankers than others. Rootstocks can also impart a certain amount of tolerance to this disease, so selecting cultivars that are less susceptible to black rot cankers on tolerant rootstocks will help to reduce yield losses. If a new orchard is being planted near a woodlot, special care should be made to plant least susceptible cultivars on tolerant root stocks closest to the woodlot to reduce the impact of disease moving from the woodlot into the orchard. An application with a fungicide such as Captan or Maestro for summer disease control immediately after a traumatic event occurs in the orchard, such as hail, will help protect the wounds on limbs from being colonized by the black rot fungus and reduce cankers from developing.