Dutch elm disease has spread in a number of waves from the Atlantic coast and the Great Lakes into the Midwest. It now threatens even the westernmost provinces and states in North America. Dutch elm disease has also been reported in the southern United States. Internationally, the UK and continental Europe have been dealing with the disease longer than North America. DED is present in New Zealand and threatens Australia.
Most elm trees can successfully defend themselves against the non-aggressive strain of the disease.
This is because the tree s natural defense response is rapid enough to shut down the slow spread of this strain of disease. When the aggressive strain of DED infects a tree, it very rapidly overtakes the elm tree, before the elm tree can fully detect its presence and mobilize its natural defense response. As a result most elm trees, if left untreated, become susceptible and die when infected within a short period of time. In the case of the aggressive strain of the DED fungus, the elicitor treatment activates the tree s natural defense response early enough for the tree to respond, even to the rapid activity of the aggressive strain of the disease. Through the extensive research conducted by Dr. Hubbes and his University of Toronto team, they have been able to demonstrate that elm trees, just like humans, have their own immune response system, which serves to fight off disease and other infections.