PHOTO BG
May 18, 2012
Every year at this time I bring attention to a tree disease
that rewrote the book on urban forestry. Dutch Elm Disease is a fungal disease
that, after decimating the elm forests of Europe early in the last century, was
discovered in New York in the 1930s and has since crossed the continent.
Described as 99% fatal to Ulmus americana, the American elm tree,.
Rapid growth and the ability to survive tough urban
conditions are two of the characteristics that made American elms the go to tree
for urban settings. Graceful vase like shapes lent themselves well to over street
canopies and spectacular alles.
The only major drawback was its inability to survive the
introduced fungal pathogen Ophistoma ulmi. This disease combined with the
presence of a previously harmless bark beetle, created the perfect conditions
for a country wide epidemic.
D.E.D. moved at a rapid pace, its sticky spores clinging to
the bodies of both native and introduced elm bark beetles. This combined with
people moving beetles and diseased firewood spread D.E.D throughout most of the
native range of this stately, street tree
With such a high mortality rate there were many who
predicted the eventual extinction of the American elm. However, there was right
from the beginning tiny numbers of trees that, in spite of being in the middle
of epidemic numbers of dead and dying trees, and plagued by hordes of bark beetles,
survived, continuing to grow and
prosper.
There have been a number of programs funded and promoted by
governments of Canada and the United States that have brought forward many
potentially resistant examples of American elm. Through rigorous testing some
are proving to be very resistant to D.E.D.
American efforts have resulted in American elm varieties
like “ Princeton”, “Valley Forge” and “ Deleware” all of which have been shown
to have some resistance to D.E.D. Canadian efforts to date have resulted in
several trees of note, “ Brandon” an American elm and Jacan a Japanese elm
pioneered by Dr. Wilbur Ronald formerly from Agriculture Canada’s Morden,
Manitoba facility. This later selection is in fact growing in Brandon,
Manitoba, on 10th St. and on Pacific Ave. at 15th St.
Jacan does not have the characteristic vase like shape but has proven to be an
excellent tree in other respects.
As we move forward in an age of cloning and micro
propagation, efforts in Guelph, Ontario spear headed by Dr. Praveen
Saxena, have resulted in the ability to
mass produce resistant trees. This greatly increases the efforts to grow and
test resistant varieties as well as producing them for introduction. With so
many skilled and dedicated people working to find resistant American elms, one
can’t help but have hope that from the thin numbers of survivors a new
generation of elms will emerge and take their place on our city streets
Your I.S.A. Certified Arborist is up to date on the latest
research and can help preserve your elm trees.
