CAN DUTCH ELM DISEASE BE KEPT AT BAY?
By Peter Dmytrasz, City of Toronto

Winter is a time when trees are in their seasonal dormancy. Roots, which continue to grow after leaf fall, are inactive in the frost filled ground. Destructive insect and pathogen activities have stalled with the extremely low temperatures. Fortunately, the activities of a tireless researcher have not similarly been affected. For the 23 rd year now, Dr. Martin Hubbes, a Forest Pathology professor at the University of Toronto, has been searching for a solution to the fatal Dutch Elm Disease (DED). Recent findings are encouraging, providing a ray of hope that the disease may soon be effectively managed.

The stately Native American elm, with its majestic form and tolerance to both cold and salt, has been a favourite urban shade tree throughout North America. Many of these trees have been decimated by DED, an usually fatal vascular disease. It was first discovered in Holland in 1918, just after World War I.

Introduced to the United States on the Atlantic coast in the 1930's with a second introduction in 1945 to eastern Canada, it has since spread to the continental Midwest. The spores of DED are transmitted from one elm to another by the activities of vectors, either the native or European Elm Bark Beetles.

The pathogen is able to sporulate within the vascular tissues of the host, accelerating the spread of the disease. Presently, in affected areas, only a small fraction of the original American elm population remains. The Ontario Shade Tree Council was initially created as a lobby to encourage government assistance in the intervention of this ravaging disease.

Control of DED has generally been a failure. Physical and cultural controls have been relatively ineffective. Infected branches, usually evidenced by a chlorotic wilting or 'flagging', may only be effectively pruned, if performed aggressively and in a timely manner. The speed and extent of destruction by this pathogen, has given witness to the inability of both the private and public tree maintenance sectors to control its spread. Chemical treatments, involving toxic fungicides and expensive delivery systems, have only been able to aid a few individual specimen trees. Even the hopeful search for resistant elm cultivars, like the once resistant Pioneer Elm, has succumbed to the disease. The only remaining IPM approach, biological control, may soon prove to be the only truly effective control method. Earlier work by other researchers resulted in antagonistic organisms such as bacteria, but the mechanisms of pathogen attack and host defenses, were not understood.

Dr. Hubbes's discovery could not have come at a more timely point in the spread of the disease. In eastern North America many professionals have acquiesced to substituting other tree species for the diminishing elms, especially in Ontario. In the Canadian Prairies, where environmental conditions can not support the same variety of tree species, the importance of the cold tolerant elm was far more critical. The American elm has therefore become a cultural landmark in the west. Here the authorities were willing to be more aggressive in their attempt to preserve local elm populations.

In Winnipeg, Dr Hubbes and his research team originally proposed the development of a strain of DED, which could be released in the field. Their earlier research, employing methods of molecular biology, showed that elms could develop a resistance to DED. This occurred when a non-virulent strain of the disease was first inoculated into the host, then challenged by a virulent strain. The progeny virulence never became higher than that of either parent. This did not always work, due to the genetic diversity of both the DED pathogen and the host elms. Winnipeg foresters declined the offer proceeding instead with fungicide injections, which proved to be both costly and not always effective.

Dr. Hubbes then began examining the natural host defense mechanisms or resistance to DED. He knew that Siberian elms get the disease but do not develop symptoms. He inoculated some of these elms with a non-virulent strain of DED and was able to extract 6 compounds that proved to have toxic properties to the disease. He used the presence of these compounds as markers to host resistance to DED. Timing was critical as elms are only sensitive for about 6 to 8 weeks in the spring, between May and July. He developed tissue culture tests in the lab, with results in a matter of days, rather than resume fieldwork the following year. The results indicated that either dead or living organisms react similarly with the tissue cultures. They also showed the presence of the marker compounds.

The elicitor, in liquid form, was then tested on 5-year-old elm seedlings in Mississauga. Forty of the fifty seedlings received treatment, while ten were left as controls. Each seedling was treated with three million DED spores, when only 40 spores were required for mortality. All fifty trees later wilted, with the controls dying, and the treated trees partially resprouting. Similar results were obtained on larger trees, with elicitor in pellet form in Kingston.

Similar testing for other diseases, by researchers on agricultural crops, produced better results, since individual agricultural field crop genotypes had little genetic variability. Native elms, with a greater genetic variability therefore, produced a far greater variability in their resultant defense reactions. Additional testing has been started in Manitoba, Saskatchewan and Alberta.

Dr. Hubbes is in the process of patenting this vaccine both in Canada and internationally. A local biotechnical company will be producing the product. The cost will be roughly $10.00 per pellet, with the total number required per tree, similar to those of existing implant methods.

The costs are expected to fall to levels of average homeowner affordability, once mass production quantities are reached. Dr. Hubbes also hopes to isolate the genes responsible for DED resistance and develop seedlings, which may retain resistance for approximately 70 years.

The OSTC directors honoured Dr. Hubbes for his efforts and are actively searching for volunteers with mature elm trees to allow his research to continue here in Ontario. 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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