Black Rot Cankers In ApplesHere

Apple trees that were hailed 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. The time and severity of the hail may affect how severe this problem is. It is important to recognize the subtle early symptoms of black rot cankers in wounded limbs.

Early canker symptoms first appear as reddish, purplish, or brown slightly sunken areas under the bark. Although the sunken areas often remain small and superficial, some can enlarge up to half a meter in length under the bark. Regardless, by the second year, the infected bark dies and peels away from the sunken area exposing black diseased wood underneath.

The extent of the damage varies with the canker location on 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 and kill the tree.

Pruning out diseased and dead wood during the dormant season helps reduce the inoculum sources. Summer pruning should be avoided if possible, because pruning cuts can provide an entrance for the fungus. It is important to remove the infected prunings from the orchard or burn them, since the fungus can survive on dead wood. Alternatively, chopping up the prunings on the orchard floor with a flail mower will reduce inoculum levels. Avoid stacking wood piles near orchards since they can act as a major source of inoculum.

The black rot fungus overwinters in cankers on twigs, branches, and trunks of many hardwood trees. Scout surrounding woodlots to 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 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 less susceptible cultivars on tolerant rootstocks will help to reduce the problem. A fungicide application for disease control immediately after a traumatic event, such as hail, will help protect the wounds on limbs from being colonized by the fungus and reduce canker development.

The black rot pathogen can also cause Frog-eye leaf spot and black rot on fruit therefore directly impacting yield. The management options are different for each phase of this disease and should be integrated into a program to reduce yield losses.