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Tree Diseases

Maureen Sexsmith-West ISA Certified Arborist, PR4600A

“Orange Slime” at this time of year

Be alert to changes in your trees this month. Many of the symptoms of local tree diseases will begin to present. Some are serious and some are more of an aesthetic issue. Most solutions are cultural including proper pruning cuts, good air circulation within the tree (don’t rely on 70 km/h winds), sanitation of tools, appropriate watering and avoiding problem plant pairings. There are a whole host of indicators: blotches on leaves, discoloured bark, fruiting bodies, distortions – too many to list or illustrate here. If you notice these or other changes, we would be happy to provide an educated diagnosis and provide strategies for management.

A sampling:

A common issue we find are RUSTS that result from a pathogen that travels between Hawthorn and Junipers. I am still trying to figure out why this plant combination is so popular with homeowners and landscapers. Here’s hoping.

Underside of Hawthorn

This problem is primarily an aesthetic one. The pictures illustrate typical problems. The Hawthorn has a rusty ‘spot’ on the top side and this curious ‘upside down spider’ on the bottom. On Junipers the infections are slimy in June and harden into a nut-like gall on the twig. The ability for the pathogen to travel a large distance means even if you remove your juniper, your neighbours is still likely to be the allelopathic partner.


Cytospora Canker is a disease that affects primarily spruce trees but can show up on pines. Branches will ‘flag’.

Fire blight: affects apples, pear, mountain ash, hawthorn, cotoneaster. Fire blight is a systemic disease. The term “fire blight” reflect the appearance of the disease (blackened, shrunken and cracked, as though scorched by fire). Primary infections are established during flowering and tender new shoots.


Black knot: The characteristic feature of this disease is the presence of thick, black, irregular swellings on twigs and branches. I think it looks like “burnt marshmallow on a stick”. The galls are easily seen in the winter. The disease is difficult to notice during the early stages of infection. It takes two seasons to complete the cycle. Old knots enlarge every year and may range from ½” to 1 ft. in length. Fungus in old knots may invade other tissues to form new knots. Occurs on trees in the Prunus family: Mayday, Schubert Chokecherry, Chokecherry. Possible on Plum.

Many communities have a ‘BLD’ Hotline. Early detection and treatment is important. There are currently no known chemical controls for BLD, so taking preventative actions and understanding treatment options can save trees and mitigate the risk of widespread infection. Contact us if you notice this on your columnar poplar or aspen mid-season. We have had several confirmed cases in the past two years in Lethbridge and the surrounding towns. Tracking is helpful in determining the movement of Bronze Leaf Disease in our region. Samples may be collected to assist in the development of a control agent currently underway at the Lethbridge Research Centre. I have included a Public Service Announcement produced by the City of Calgary.


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