Renovation & Design

Variety of galls affect flora in Winnipeg

Major damage usually preventable

CP/Ted S. Warren / The Associated Press Archives Spraying dormant oil on maple leaves before they bud smothers wintering gall eggs.

I've written about specific tree and shrub galls, especially black-knot galls on Schubert choke cherry trees, but here's a broader perspective on the variety of galls out there in the Winnipeg area.

Birch Crown Galls

I recently discovered these rare galls this year. They are bacterial galls that kill twigs and leaves, but eventually will kill the whole tree. There is no treatment for these.

Rose Cynipid Wasp Galls

These destructive wasp galls breed in the rose hips and kill the branches where they occur. They create an unsightly mess but rarely kill the whole rose shrub. Shrubs must be removed when infestations become severe.

Hawthorn-Juniper Rust Galls

This is a complex disease where the fungal cycle occurs on both the hawthorn and the juniper. Both the hawthorn and the juniper weaken within a short time and die. Early removal of the prominent juniper galls and the leaves red-orange spots significantly slows down the effects of the disease on these plants. Early spraying with copper fungicide in spring can kill early disease development on hawthorn.

Maple Leaf Mite Galls

I receive dozens of inquiries each year about the 'bumps", "pimples", and "unsightly growth" on Amur and silver maple leaves. Although the galls can be very unsightly I have yet to see the mites that inhabit these galls kill maple trees. Spraying dormant oil in April before the leaves bud out is a key method of getting rid of them. The oil smothers the over-wintering eggs.

Oak Leaf Woolly Mite Galls

Like most oak leaf galls these are harmless and there rarely occur each year. Oak leaves seem to have a change of leaf mite galls each year. The woolly galls can appear like the old fashioned barber poles -- alternating pink and cream in colour. Often they can either be entirely pink or cream.

Cranberry Erineum Mite Galls

These mites can be very destructive as their feeding strongly distorts the leaf. The distorted leaves can turn shades of pink and red by the mites. Heavy infestations will kill the leaves and the twigs on which they occur. Use dormant oil in April to control this serious pest.

Western Pine Crown Galls

These fungal disease galls are severely destructive on Scots pines and Mugho pines in urban areas as well on native pines in the boreal forest. As the galls grow they block the pines internal vital cell tissues that supply nutrients and water as well as manufactured starches and sugars from the needles. This process will kill the branch. Early removal of the galls limits the spread of this disease in the pines.


Michael Allen M.Sc.F., RPF is a consulting urban forester 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 can be reached at 204-831-6503 or His web site is


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