Dutch Elm Disease

By Guy Scott

Dutch Elm Disease has killed off most of the elm trees that are native to our area. Southern Ontario was famous for its huge elm trees which in the “old days” (pre-disease) often reached over 100 feet in height. Elms could live over 300 years. The shape (long trunk with foliage at the top of the tree) made them popular as shade trees.

In the 1920s, a fungus disease called Dutch Elm Disease invaded North America and began to decimate the native elm. The fungus was carried from tree to tree by bark-burrowing beetles. It was named Dutch Elm disease because a Dutch researcher identified the disease in the Netherlands. Local elms had an immunity to the disease and the native elm were slaughtered by the fast spreading disaster. Beginning in the 1960s, local elms, many of them over a hundred years old, died. All the old elms seemed to perish within a few years, and I wondered if the elm would become extinct. After years of losses, a new generation of young elm trees seemed to spring up, unaffected by the disease. But after a decade or so, many of these young trees also began to die off. Dutch Elm disease is back; if it ever left at all!

Elm trees about 10 feet high or taller are the infected size. The disease does not affect young trees; and if the elm makes it to maturity (60+ years), it will likely survive. The symptoms can be seen when the top of the tree withers, and the leaves die in the middle of the growing season. It simply becomes a dead tree for seemingly no visible reason. The disease strikes randomly: some trees are not affected, other groves are wiped out completely.

How can Dutch Elm Disease be prevented? Scientists estimate that 20% of the elm are naturally resistant or lucky enough to survive. Hybrid trees that are resistant to the disease have been developed by biologists and horticulturalists, but are not widely available. Since the disease is spread by beetles, attempts were made to spray and kill the beetles. Widespread spraying was largely ineffective and people became worried about the side effects on the ecosystem. Scientists developed a vaccine for the fungus, called ‘Elm Fungicide,’ but it must be administered before infection and injected every other year. It has proven effective if used correctly, but can be impractical in most cases.

Governments tried to stop the spread by cutting down infected trees, with limited success. It is difficult to create isolated areas. However, Dutch Elm Disease has not affected Alberta and British Columbia and aggressive culling of trees is credited with this success.