A CALL TO REPORT LARGE ELMS
by Henry Kock, Interpretive Horticulturist, The Arboretum, University of Guelph

Many people today do not have an experience of the incredible canopy that the white elm produced over towns and roads. Few trees have the stamina for the difficult conditions that White (American) Elm will grow in. Two years ago, one would look at a remnant, large white elm tree in the landscape and surmise that the Dutch Elm Disease (DED) simply had not reached the tree due to its isolation.

Today we know a little more. Elms, and trees in general, have natural immunity responses to disease invasion. In the case of the elms of eastern North America, the immune response was not aggressive enough to save many of the elms from the disease that swept through in the 1950s and 60s and continues taking young elms, in particular, to this day. However, some did survive.

The survivors are now understood to be potentially tolerant of the disease - the disease is present in the tree but not able to get pass the chemical barriers that are produced in response to the initial invasion. It is now understood that the immune response varies from tree to tree and that the disease has to enter the tree before the immune system and cascade of chemical reactions takes place to halt the advance of the disease. In other words, the first sign of DED (a branch with yellow leaves) in an elm tree does not mean that the tree will die. It is quite possible that many of the naturally resistant elms were inadvertently (for all the right reasons at the time) cut down in efforts to halt the advance of DED across North America.

The isolated, large elms in the landscape represent the hope for future elm planting. They are, however, generally too far apart to breed with each other with any great frequency. The pollen is carried by wind but not for great distances. Seeds that are produced by the large elms are likely the result of pollination by young trees that are the seedlings of trees that died in the 60's. The horticultural industry might be tempted to propagate a surviving elm by grafting but cloning a tree is a dangerous game, as the risk of disease mutation to a stronger form, may result in the loss of the entire clone - just as plantings are beginning to make their mark in the landscape.

In natural systems, genetic diversity is the corner stone of survival. The Arboretum at the University of Guelph initiated the Elm Recovery Project in June of 1998. The Project is designed to work with nature's rules but speed up the rate of natural recovery of the white elm by setting up a breeding orchard of up to 50 representative trees that have been grafted and screened for their ability to tolerate DED. The seeds produced will be genetically diverse from a high frequency of resistant x resistant matings.

Dr. Martin Hubbes from Forest Pathology at the University of Toronto will test candidate specimens for the breeding orchard. Dr. Hubbes is well known for his work on DED and recently developed a "vaccine" for short term DED management on old elms that dovetails nicely with this long term effort to introduce resistant seedlings. The first seeds from the orchard are expected in about 10 to 12 years.

Over two hundred elms have been reported to date, many in eastern Ontario. The Guelph and Waterloo areas seem to be well reported as well as elms growing along major roads. Some elms are as large as 15 feet in circumference. Members of the Ontario Shade Tree Council can help by reporting old elms that they know of that are greater than 7 feet (213 cm) circumference.

The reporting of elms will produce an inventory of large elms in Ontario to make collection and monitoring possible. We need to know your name and address in order to send out a reporting form.

Anyone who is able to contribute a specimen or more is asked to contact Dr. Hubbes by phone at (416) 978-6831.

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