Bronze leaf disease is caused by the fungus Apioplagiostoma populi and affects mainly Swedish columnar aspen and tower poplar trees in Grande Prairie. Symptoms occur in late summer and early fall, when leaves rapidly turn bronze or chocolate brown. Veins remain bright green and eventually turn to black. Infected leaves tend to remain on the tree over winter and in spring the fungus releases its spores from these leaves to be carried on the wind to infect other trees.
As the disease spreads, it slowly girdles the branch, eventually killing it. Bronze leaf disease almost always starts at the bottom of the tree and moves upward.
There are no chemical controls available for bronze leaf disease at this time. The only way to control the disease is through pruning and sanitation.
- Avoid planting Swedish columnar aspen or tower poplar trees, as they are short-lived and susceptible to a variety of insects and diseases. Large shrubs, such as lilac, saskatoon, upright juniper or Amur maple, make excellent hedges or living screens that are longer-lived, easier to maintain and have few insect or disease issues.
- Prune out all infected branches at least eight inches behind the bottom-most point of infection. Due to the columnar growth habit of Swedish aspen and tower poplar, this often results in taking the branch right back to the trunk.
- Sanitize your pruning tools in between each cut to prevent spreading the disease around your trees. Soak your tools in gas-line antifreeze or a 10% bleach solution for at least one minute between cuts.
- Do not keep infected wood or leaves for burning or composting. Immediately double bag all infected material in garbage bags (not clear composting bags) and dispose of it in your municipal garbage container or take it directly to the landfill. The goal is to bury the infected leaves so that when the fungus releases spores in the spring, there's nowhere for them to go.
- Keep the ground around the trees clean of all leaves and twigs.
- Watch your trees each year in late summer and prune out any new BLD infected branches.
- Avoid planting Swedish columnar aspen or tower poplar trees that come from nurseries in Manitoba, where bronze leaf disease originated in Canada. Purchase only certified disease-free trees from nurseries in British Columbia and Alberta.
- These columnar trees are often planted in tight groups of ten to thirty to provide a living screen. Unfortunately, planting too many of the same species too close together makes it very easy for disease to spread rapidly across all trees in your yard. Plant trees further apart to allow for good airflow in between and try to plant multiple species in your yard to prevent one disease or pest affecting all trees at once.