There's been considerable discussion lately in the media about black knot disease in the Schubert choke cherry trees. Although I have written about this disease a couple of times in the past, from the inquiries I've been receiving, a short review of the disease is in order.
Unquestionably, the disease is getting worse, and The City of Winnipeg has marked hundreds of trees for removal this year.
The short verdict is this: It's virtually impossible to save one of these trees if there are many large (elongated) knots or black disease galls growing throughout the entire tree. The tree should be removed.
A number of people have asked me if it's now too late to remove the black galls this season. No, but it's difficult because the new leaves are now expanding from their buds. There is a tedious way to prune now and still protect the new susceptible growth from getting infected, but you need to follow these instructions carefully.
Here's what you need to do:
1. Before any pruning is done, purchase a can of tree-pruning sealant. Do not use a product with the name 'tree pruning paint' on the container. These products are not truly effective in preventing the fungal spores of the disease from penetrating the wound cut. I use Wilson's Tree Pruning Paste, a tar-based sealant. Other suitable sealants may be available at garden centres.
2. Cut the twig or branch at least 30 cm (one foot) away from the black gall. Try to make the cut at a junction of the diseased twig or branch with another larger branch or stem. Although this might make the cut more than 30 cm away, it will be alright.
3. Make the cut with newly sharpened tree-pruning tools. Add the sealant to the cut immediately. Spray the pruning tool with a disinfectant such as a mixture of one part bleach to eight parts water, or with methyl hydrate, or with rubbing alcohol before the next cut is made. Repeat this procedure for every cut that is made. The disinfectant will kill most disease spores, preventing the possibility that they will be introduced into the next twig or branch you cut.
4. This procedure will be difficult to carry out in a tall tree. An experienced licensed Manitoba Arborist can help you out.
5. Place all pruned twigs and branches in a sealed bag or container for disposal. Do not leave the cut material exposed in your yard as the spores from this material will re-infect your tree or any other nearby Schubert choke cherry.
6. Please get in touch with me if you need more information.
Michael Allen is a consulting urban forester and certified I.S.A. arborist and owner of Viburnum Tree Experts. He makes house and garden visits to assess tree and shrub problems. He can be contacted by calling 831-6503 or by e-mail at viburnumtrees@shaw.ca Questions can be mailed to Michael Allen, c/o Newsroom, Winnipeg Free Press, 1355 Mountain Ave., Winnipeg, MB, R2X 3B6. His web site is www.treeexperts.mb.ca
