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Mancana ash trees -- and owners -- stressed

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Split trunk in Mancana ash

Every now and then a tree problem suddenly emerges. When that happens many property owners deluge me with calls and e-mails. The tree in question is Mancana ash -- a variety of Manchurian ash.

This is the third year that major problems have been reported in this shade tree. As can be seen in the attached image the effects are quite devastating. What has happened to these trees to have them suddenly appear so stressed? Over the last three years Mancana ash trees in southern Manitoba have shown significant signs of environmental stress due to frost damage and subsequent fungal diseases. Of the stresses seen in Mancana ash, frost crack damage is the most severe.

Native green and black ash trees growing on the Prairies are known to experience frost crack damage caused by extremes in heat and cold within the living tissues under the bark during late winter. Yet recent frost damage occurring on Mancana ash is quite different from what is normally seen on the native ashes.

Initially one or more deep vertical cracks form on the south to southwest facing sides of the main trunk bark, as well on the upper secondary stems arising from the trunk. The crack then expands as the tree grows in the spring. This further opens up the bark and kills the underlying living cells. Eventually, the bark sloughs away from the trunk and stems revealing the interior wood.

Several areas of the trunk or trunks (in multiple stem trees) may expose core wood. Those openings may be up to 60 cm (2 ft) long and 7 to 10 cm (3 to 4 in) wide. Prominent wound wood or wood callous (revealing a curve smooth edge) forms a protective barrier around the opening.

In addition to the bark cracking by frost, there is massive anthracnose leaf disease (Discula fraxinea) occurring on the leaflets of many Mancana ash trees. Ash trees have compound leaves, meaning each leaf consists of a single leaf stalk supporting separate leaflets.

On Mancana ash there are normally 9 to 11 of these leaflets. The disease starts out as patterned dark brown leaf spots on the leaflets that eventually merge, causing total leaflet death. The leaflets appear withered and crushed although the main leaf stalk usually stays green.

Fresh samples from these affected trees were delivered to Manitoba Agriculture's Crop Diagnostic Centre in Winnipeg. Lab examination of the samples revealed that the frost crack had been infected with a disease in a broad category of diseases called Botryosphaeria.

The specific sub disease identified in this broad group is known as Fusiccocum aesuli. To date, the Fusiccocum disease has been largely associated, but not exclusively, in the tree literature with frost injury in ash trees especially in United States.

Small fissures known as cankers appear on the twigs of green and Mancana ash that have had heavy leaflet anthracnose fungal infections. These cankers were usually no more than 2 to 3 cm (1 to 1.5 in) in length and were identified as cankers of Botryosphaeria dothidea. No other twig or branch damage was observed. Severe fungal infections can cause the death of twigs and ultimately branches.

There is no specific treatment to control the fungi that causes the formation of cankers. However, the best means of controlling anthracnose is to spray dormant lime sulfur fungicide before the buds open in the spring. On the Prairies, that typically means mid-April. When spraying, ensure the buds, twigs and branches are thoroughly soaked.

Early June spraying with a copper-based fungicide or any other approved tree fungicide is also very important. In the case of copper fungicide, two spray applications, usually 10 days apart, will suffice. The spring fungicide treatments will help control the spread of new infections by spring rains and wind. Our spring this year has been cool and wet which made spraying on schedule difficult if not impossible.

The exposed inner wood will split and crack as it dries due to its location on the sunny south side. Thus, it is important to restrict the entry of wood decay fungal spores. Apply a wood preservative such as clear shellac to the exposed wood. Repeat twice a year for two or three consecutive years. Keep the shellac off the sides of the newly forming wound wood. Tree sealant tar, paste, or grafting wax can also be used to preserve the exposed wood.

Keeping the tree healthy through early spring and fall fertilization in the first two years (bare minimum) of infection will limit the damage done by the anthracnose and the Botryosphaeria diseases. A sanitation program of collecting and disposing ofearly fallen leaves during summer and again in fall, plus pruning dead and dying twigs and branches during the fall, will help reduce the presence of the fungal spores that could re-infect the tree or nearby ash trees.

It is too early to say what the long term outlook for Mancana ash problem will be. Not all mature trees are affected. I plan to monitor selected client trees over the next few years to see how the trees respond to the problem and the treatments at least in the short term. I would appreciate hearing from any reader who has this problem in their Mancana ash trees.

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 viburnumtrees@shaw.ca His web site is www.treeexperts.mb.ca