Cankers in apple orchards: A hiccup from the cold winter of 2015

Apple growers may have noticed more cankers on limbs and trunks of trees in their orchards last year and again this spring. Cankers often show up in orchards 1 or 2 years after a severely cold winter. Although this past winter of 2017 was not severe, the winters of 2014 and 2015 were very cold in some apple growing regions of Ontario. Many canker causing organisms are opportunistic, infecting through wounds caused by machinery, insects, hail or cold injury. Some canker causing pathogens get into wounds and become established without causing noticeable symptoms until the following season, or even 2 seasons after the wounding and infection event occurred. With the cold winters experienced in Ontario during 2014 and again in 2015, it is anticipated that canker causing pathogens that infected through the cold injury wounds have or will manifest themselves this coming season.

Black rot (Botryosphaeria obtusa) is probably the most common canker found on apple trees in Ontario (Figure 1). Wounds on limbs and trunks caused by winter injury provide an entrance for the black rot fungus to infect and colonize. The pathogen does not infect healthy wood directly. Initially the fungus causes a subtle reddish sometimes slightly sunken discolouration under the bark. This discolouration may be a few centimeters in diameter or it can grow up to several meters under the bark (Figure 2). Growers may not recognize the subtle early symptoms of black rot cankers that develop on wounded limbs or trunks during the first season after winter injury. At the end of the season or beginning of the following season, the infected bark dies, cracks and peels away from the sunken area exposing black diseased wood beneath (Figure 1). Some cankers will remain small, but many will continue to grow and over time girdle the limb, branch or trunk. Often it takes 2 years before obvious symptoms of black rot cankers to show up. Many of the larger cankers observed last season (2016) were probably a result of infection of wood injured during the winter of 2014. It is likely that more black rot cankers will show up this year (2017) due to colonized winter injury wounds caused during 2015.

Figure 1. Mature black rot canker with peeling bark exposing black discoloured wood.

Figure 1. Mature black rot canker with peeling bark exposing black discoloured wood.

Figure 2. Black rot cankers initially appear as subtle reddish sometimes slightly sunken discoloured areas under the bark.

Figure 2. Black rot cankers initially appear as subtle reddish sometimes slightly sunken discoloured areas under the bark.

Anthracnose canker (Neofabraea malicorticis and N. alba), sometimes called fiddle string canker, and perennial or false anthracnose canker (N. perennans) look very similar and are difficult to distinguish one from the other. The pathogen infects wounds under very humid conditions, and in Ontario, is sporadically observed in orchards in close proximity to the lakes. The perennial canker causing fungus requires a wound to infect, and is often associated with events that cause injury to the trees such as cold winter injury. Initially the cankers appear as small oval red to purple spots on limbs or trunks of trees. These spots enlarge and eventually become orange to brown sunken spots. As the bark begins to shrink within the enlarging canker, the edges of the oval canker cracks and separates from the healthy wood. The bark eventually sloughs off, exposing the wood and 'blast fibers' that the fungus cannot decompose, giving the canker a 'fiddle string' appearance (Figure 3). Although the fungus can produce spores all season long, spore production tends to peak in the autumn. Fortunately apple trees quickly defend against these types of cankers by producing callus growth around the disease tissue that compartmentalize the infection and stop it from advancing or growing larger within the first year of infection.

Figure 3. Anthracnose canker with 'blast fibres' is sometimes called 'fiddle string' canker.

Figure 3. Anthracnose canker with 'blast fibres' is sometimes called 'fiddle string' canker.

Nectria twig blight and canker (Nectria cinnabarina) occurs sporadically in orchards across Ontario and seldom causes significant damage (Figure 4). The pathogen often infects through wounds on twigs and branches caused by injuries including winter injury. The fungus eventually girdles the twig or branch which wilts rapidly, resembling symptoms of fire blight (Figure 5). Orange to bright pink spore cushions develop in the canker at the base of the wilted branch and twigs. Spores are produced in the autumn and spring. The canker is usually confined to small weakened twigs and branches, and is rarely observed on larger scaffold branches or trunks.

Figure 4. Nectria canker with orange spore cushions on a large branch.

Figure 4. Nectria canker with orange spore cushions on a large branch.

Figure 5. Nectria twig blight can resemble fire blight shoot blight.

Figure 5. Nectria twig blight can resemble fire blight shoot blight.

Many woody plants such as maple, birch, hickory, poplar, beech, and hawthorn are alternative hosts to canker causing pathogens. Orchards planted near wood lots may be more vulnerable to these diseases. Pruning out diseased limbs and dead wood is an important practice to reduce the inoculum sources within the orchard. Cankers may be surgically removed from trees if feasible and practical. It is a good practice to paint over pruning stubs or exposed wood with tree paint to help prevent re-infection into the wound. In addition, it is important to remove the infected prunings or wood from the orchard since these pathogens can survive and produce spores on living and dead tissue. Burning or chopping the prunings on the orchard floor with a flail mower will also reduce inoculum levels. It is not a good idea to stack wood in piles in or near orchards since it may be colonized by canker causing fungi and can act as a source of inoculum.


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