Poplar, Aspen and Willow
Distribution and Disease Cycle
Most poplar cankers in the prairies are caused by fungi. There are several different types of canker resulting in all poplar species being susceptible to at least one or more canker causing agents. Cankers cause a shrinking and dying of tissue at a generally localized area of the bark, on branches and trunks of the host tree. Cankers may appear as circular to elongated sunken areas with a raised greyish to black margin. The sunken areas may range from 5 to 50 cm in length on main branches and stems throughout the lower two-thirds of the tree. This diseased bark eventually cracks open and exposes the wood underneath.
The amount of damage caused depends on the type of canker and the poplar species. A serious type of canker can cause mortality of the host and thus should be removed to prevent further spread of the disease. Other types of cankers are weaker and cause a reduction in growth and reduce the aesthetic value of the tree. These types of cankers still wound the tree, and make it more susceptible to other insects and diseases that may lead to the death of the tree. All cankers weaken the tree resulting in limbs breaking during high winds.
Canker fungi are spread by spores that generally enter the tree through branch stubs or wounds. The spores are usually dispersed in wind currents or by water droplets during a rain, but can also be transported by insects, animals or pruning tools. Trees that are under stress from drought or insects seem to be infected by cankers much easier than healthy trees.
An important measure in control is to maintain high vigor of trees by fertilization and watering. Additional control (depending on type of canker) can be achieved by pruning and destroying all infected branches during dry weather, but do not pollard your trees. Also, to prevent the spread of the disease during pruning, tools must be sterilized after each cut. The only other solution is to plant canker resistant varieties or other tree species.
The main poplar species that were distributed by the Shelterbelt Centre, ("Walker", "Assiniboine", "CanAm", and "Manitou"), seem to be moderately resistant to canker diseases. One should remember, however, that "CanAm" and "Walker" are female clones and will produce seed fluff that some people object to. The Agroforestry Development Centre is currently selecting male poplar clones that are resistant to cankers. Although testing is not complete, several more promising clones have been identified.