West Sweden is home to many Aspen (Populus tremuli). They are fast-growing and the finest tree to listen to on a breezy day.
Unfortunately, Aspen are host to a number of pathogens, some of which can have a significant affect on the tree’s structure and safety. Due to the large number of Aspen in school playgrounds, I’ve recently been paying more attention to these pathogens.
Cankers (”kräftor” in Swedish, which is also the word for prawns) occur when a pathogen kills bark and/or cambium in a localised area. The tree responds by growing new cambium and bark which the pathogen attempts to kill. Warfare. Distorted growth.
Hypoxylon mammatum is an ascomycete (a fungus) that causes cankers on Aspen. Hypoxylon is reported to be an endophyte, existing quietly in the tree and then actively colonising areas that have been freshly wounded.
Xanthomonas populi is a proteobacteria, and appears to be common on Aspen here. The bacteria enters the tree through stipule scars, leaf scars and other injuries, kills bark during winter, eventually killing twigs, branches and can also cause stem cankers.
The cankers themselves are of curiosity interest to myself. However, decay associated with the cankers is a concern. The wood of Aspen is soft, the species’ response to dysfunction is focused on creating new wood and leaving the dysfunctional wood to pathogens. Decay associated with these pathogens would appear to be caused by a secondary agent. Recently, I have witnessed several failed Aspen where the external indications of a problem were a small canker. I have heard of similar problems in the Stockholm area. Most of the Aspen I inspect are in school playgrounds, so I have been paying closer attention to these cankers.
With the specimen in the photos above, decay hadn’t advanced too far yet, and as one would expect with Aspen, the tree had responded to the dysfunction:
Here is another Aspen with stem cankers, the associated tomograph and stem section:
Again, the decay here wasn’t too advanced and the tree appeared to be responding with sufficient new growth. There were other reasons for felling this particular tree.
In the future I intend to follow a couple of specimens with cankers. The first step being to identify the pathogen via laboratory analysis (which is tricky in the case of Hypoxylon), the second step to monitor the progression of decay and the tree’s response with tomography.
Anderson, R.L. & Anderson, G.W. (1979) ”Hypoxylon Canker of Aspen” Forest Insect and Disease Leaflet 6, United States Department of Agriculture. (https://www.na.fs.fed.us/spfo/pubs/fidls/hypoxylon/hypoxylon.htm, page accessed 6 april 2017).
Anon. Hypoxylon mammatum. (Hypoxylon mammatum data sheet, page accessed 6 april 2017).
de Lange, A. & Kerling, L.C.P. ”Aplanobacterium populi, the cause of bacterial canker of poplar.” Tijdschrift Over Plantenziekten (1962) 68: 289. doi:10.1007/BF02044437.
Strouts, R.G.; Winter, T.G. (1994). Diagnosis of ill-health in trees. Forestry Commission Research for Amenity Trees No. 2. Norwich, Storbritannien: The Stationery Office. Pages 72-74.