Michael Allen / Winnipeg Free Press
In southern Manitoba, Swedish aspen and tower poplar are highly susceptible to bronze leaf.
As you look around Winnipeg, you may notice tall trees with rust-coloured leaves. Tower poplar and Swedish aspen are known to be highly susceptible to a common fungus disease called bronze leaf (Apioplagiostoma populi), especially in older trees.
You can identify these trees easily, as the leaves are highly symmetrical. The disease progressively kills the leaves, twigs and branches on infected trees. Genetically, the two types of poplar trees have a great deal of similarity. The leaves become prominently discoloured, going through colour phases from yellow to orange to red brown to bronze.
In southern Manitoba, only tower poplars and Swedish aspen leaves, when infected with this disease, turn a dark, very prominent bronze colour. Often, very distinct colour edges of the disease can be seen adjacent to bright green areas of the leaves.
Only one part of the tree is infected for a while, but after two or three years, the entire tree will become affected, killing twigs and branches. Peeling back the bark on these twigs will reveal patterns of brown staining. Making things even worse, stressed trees can be attacked by carpenter worms (formally called poplar borers) and by lethal pathogenic fungal diseases called Hypoxylon and Cryptosphaeria.
Starting in early August, poplar bronze leaf disease shows up on columnar poplar trees such as Swedish aspen and tower poplar. The colour is unmistakable — no other tree will exhibit this bronze fall leaf coloration. However, not all columnar poplars and aspens are affected by this disease. If one were to look closely at the trees, it would become obvious that the tops of the tree are dying first.
With time, the progression of dying branches and leaves moves down the tree. I have noticed while observing trees that columnar poplars have a short life span. That is their nature, and I strongly believe this life span gets shorter over time within the population of columnar poplars. I believe it is caused by overbreeding to produce a perfectly shaped columnar tree. This breeding, in my opinion, weakens the genes’ resistance within the poplar cells to succumb to aggressively lethal fungal diseases such as Hypoxylon and Cryptosphaeria.
What can I do to save my tree — poplars or any tree?
Good sanitation is the best method of controlling diseased trees, especially in the early years of their lives. Prune out affected branches 20 to 30 centimetres into healthy wood, while still attempting to maintain the tree’s shape. Often in these trees, this means you prune back to the trunk.
Pruned wood should be bagged and disposed of in the garbage or burned immediately.
Removal of dead and/or dying trees is important, as they will infect nearby trees. Chipping or composting infected wood will help only to spread the disease and should be avoided in the event you use this material in your landscaping program.
The tools used to prune trees, especially poplars, should be sanitized between cuts to avoid transferring the disease.
A simple solution can be made out of one part household bleach and three parts water. Methyl hydrate, obtained from a hardware store, also works well. Cleaning metal cutting blade tools with methyl hydrate will speed up the sanitizing process.
If there are not any poplars on your property now and you live in a populated urban area, I strongly recommend you do not plant any and also avoid planting poplar tree species in areas where you can see older poplars not too far away. A fungus disease will be present in those older poplar trees. Poplar fungal spores are readily transferrable by wind over short distances.
Further attention should be paid to how close together trees are planted. By planting these trees far apart from each other, increased air flow and reduced humidity will decrease the incidence of disease.
As a general rule of thumb, for all your mature trees of any species, ensure that there is noticeable air movement through the trees’ canopy. Maintaining good and regular pruning practices also will discourage the development of tree diseases. Pruning trees is best done in the fall.
To limit the chance of your tree becoming diseased, the best defence is to keep it healthy through following the suggested practices. Make certain your trees are well pruned, well-watered and fertilized appropriately. In southern Manitoba, tree roots growing in clay loam soils roughly extend about half the height of the tree from the trunk in all directions unless physically blocked. This is the zone where trees should be fertilized.
Dealing with trees that have an untreatable disease is a sad realization for property owners. Do not waste your time on treating them. Have them removed as soon as possible, and look at other tree species for possible replacement.
Michael Allen M.Sc.F., RPF (ret’d) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts. He can be reached at 204-831-6503 or 204-223-7709. His website is treeexperts.mb.ca.
viburnumtrees@shaw.ca
