Mancana ash fighting frost and fungus attack
ďEvery now and again, an amazing plant is discovered. One that seems to have it all - fine landscape attributes, highly ornamental characteristics, easy culture and no negative traits. And when itís hardy enough for northern yards and gardens, there is cause to celebrate.Ē So says the respected experts on the Northscaping website, in reference to the Mancana Manchurian ash, while deploring the fact the Morden-developed cultivar isnít in wide use. But today, as tree authority Michael Allen tells us, this amazing plant is being attacked by a deadly fungal disease.
Property owners have been flooding me with calls and e-mails about the clearly deteriorating condition of their Mancana ash trees. As can be seen in the attached images the situation is quite devastating.
The situation has come to public attention over the last two years, as the Mancana ash in southern Manitoba has begun showing significant signs of environmental stress, due to frost damage and subsequent fungal diseases. Of these, 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 the damage occurring recently on Mancana ash is quite different.
Initially a deep vertical crack forms 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 expands as the tree grows in the spring. This further opens up the bark, killing the underlying living cells. Eventually, the bark comes off the trunk and stems revealing the interior wood. Several areas of the trunk or trunks (in multiple stem trees) may be exposed, with openings up to 60 cm (two feet) long and seven to 10 cm wide. Prominent wound wood or wood callous forms a protective barrier around the opening.
But the problem doesnít stop there. In many of these injured trees, a massive outbreak of anthracnose leaf disease (Discula fraxinea) is to be found on the leaflets. Ashtrees have compound leaves, meaning each leaf consists of a single leaf stalk supporting separate leaflets. On Mancana ash there are normally nine to 11 leaflets on a stalk. The disease starts out as patterned leaf spots on the leaflets that eventually merge, causing total leaflet death. The main leaf stalk stays green, however.
Manitoba Agricultureís Crop Diagnostic Centre in Winnipeg has looked at fresh samples from these affected trees and determined that the frost crack had been infected with a disease in a broad category of diseases called Botryosphaeria. The specific sub-disease identified is known as Fusiccocum aesuli. To date, the Fusiccocum disease has been associated largely but not exclusively with frost injury in ash trees, especially in the United States.
Small fissures known as cankers have appeared on the twigs of green and Mancana ash that have had heavy leaflet anthracnose fungal infections. These cankers were usually no more than two to three cm in length and were identified as cankers of Botryosphaeria dothidea. Severe fungal infections can cause the death of twigs and ultimately branches, but no additional. twig or branch damage was observed on these trees.
There is no specific treatment to control the fungi that causes the formation of cankers. However, dormant anthracnose can be controlled by spraying lime sulfur fungicide before the buds open in the spring. On the prairies, that typically means mid-April. When spraying, itís important to 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. Two spray applications, usually 10 days apart, will suffice in the case of the copper-based fungicide. The spring fungicide treatments will help control the spread of new infections by spring rains and wind.
The exposed inner wood will split and crack as it dies, due to its location on the sunny south side of the treeís trunk. Thus, it is important to restrict the entry of wood decaying 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 ash tree healthy through early spring and fall fertilization in the first two years of infection, as a bare minimum, will limit the damage done by the anthracnose and the Botryosphaeria diseases. A sanitation program that involves collecting early fallen leaves during summer and again in fall plus fall pruning of dead and dying twigs and branches will help reduce the presence of the fungal spores that could re-infect the tree. All diseased leaves and branch material must be properly disposed of as soon as possible so the tree isnít re-infected.
Because the Mancana ash problem is a recent one I simply do not know what the long term outlook for this stress will be. I plan to monitor selected trees of my clients over the next few years to see how they respond to the problem and the treatments, at least in the short term. I would appreciate hearing from any reader whose Mancana ash trees have been hit by the disease.
Michael Allen, owner of Viburnum Tree Experts in Winnipeg, is a consulting urban forester and certified arborist. He can be reached at firstname.lastname@example.org. His website is www.treeexperts.mb.ca.