What does the disease look like?
Fire-blight disease (Erwinia amylovora) has not been too bad in recent years in southern Manitoba, but this year it is affecting susceptible trees with a vengeance. In our area this bacterial disease occurs mostly on mountain ash, crabapple and pear, but other rose family plants can be infected as well. The disease is often transmitted by bees and wasps in the spring. It can also be easily transmitted by blight contaminated pruning tools.
Early signs of this disease may be all or one of the following: infected flowers that have a brownish shrivelled appearance; or shrivelled leaves that turn brown, orange, red, dark purplish-brown or black; or shrivelled twigs with curled ends of twigs that have a dark brown or black (pear especially) colour. In apple trees the leaves turn uniformly cinnamon-brown colour near the flowers or fruits. Often the inside of the bark will be discoloured reddish brown when the bark is peeled back. This is a lethal tree disease and action on this must be taken right away as soon as its signs become apparent if you want to try to get the disease under control.
How is the disease treated?
Trees and woody shrubs having fire-blight bacterial disease symptoms should be treated as soon as possible when disease symptoms are visible usually in late spring or summer. Spray treatments are not guaranteed to work.
Prune off all infected twigs and dried leaf areas at least one foot into the healthy-looking branches. Always prune back to a nearby branch junction. After each pruning cut always sterilize the pruning tool with diluted bleach (one part bleach to nine parts water) or with denatured alcohol or with methyl hydrate if the cuts were made between April and September inclusive. Sterilizing tools is not required for pruning in late fall and winter or in March.
After these cuts have been made, use Wilson's Tree Pruning Paste or Green Earth Tree Sealer (beeswax) or other approved sealing compound for trees.
Three spray treatments of copper-based fungicide are recommended from mid-late May to June about 10 to 14 days apart. Timing may not be suitable to do three. Our wet springs can make this treatment frequency difficult. Do what you can. Copper spray treatments are also recommended after pruning infected trees with that still have their leaves on. If it rains within 24 hours of the spraying, the plant must be re-sprayed as soon as possible.
Collect all infected leaves ASAP and dispose of them in the recycled green trash bags or if permitted, burn them right away.
Fertilize affected trees and woody shrubs in the spring (May or June), or later in fall (September-October) to improve their vigour and chances of fighting the fire-blight disease. Thoroughly water the tree after fertilization. Annual fertilization may be needed to continue holding back the disease.
Michael Allen M.Sc.F., RPF (ret.) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts, a Manitoba company that provides objective assessments of the condition and the care required for trees and shrubs on home and business landscapes. He is available to visit homes and gardens. He can be reached at 204-831-6503 or 204-223-7709 firstname.lastname@example.org His website is www.treeexperts.mb.ca.