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Historical Significance

The first North American Dutch elm disease epidemic began when Ophiostoma ulmi was introduced in the 1920s by furniture makers who used imported European elm logs to make veneer for cabinets and tables. Some of the beetle vectors of the Dutch elm disease pathogens also were brought here from Europe, years before the fungi were introduced. When the more aggressive pathogen, O. novo-ulmi, was later introduced in North America, it killed many elms that had survived the original epidemic. Dutch elm disease epidemics that resulted from movement of Ophiostoma species between and across continents vividly illustrate the dangers inherent in our movement of plant material around the world.

A Dutch scientist, Marie Beatrice Schwarz, is credited with first identifying the causal agent of what was to become known as Dutch elm disease. Another Dutch scientist, Christine Johanna Buisman, who had seen the disease in her homeland, first identified Dutch elm disease in Ohio in 1930. The disease spread up and down the U.S. East Coast and west across the continent, reaching the West Coast in 1973. Over 40 million American elm trees have been killed by this disease, and today it is still a very destructive disease of shade trees in the U.S.

Many of the elm trees in North America and Europe were planted in rows along streets and walkways, or in hedgerows, or on dikes. The elm trees made effective windbreaks (Figure 22), and the large, overarching branches created beautiful shady canopies (Figure 23). These dense plantings of elm trees are examples of monocultures.


Figure 22	Figure 23

Monocultures are created when plants of the same species are grown in close proximity, with few other types of plants present. People have planted monocultures for hundreds of years and there are many reasons why monocultures are desirable. Monocultures provide uniformity, which is desirable both for aesthetic reasons and for production practices. Planting, management, and harvest are all simpler when one kind of plant is grown in an area.

Dangers, however, are inherent in monocultures. Because all of the plants in a monoculture are very much alike, they are all subject to the same catastrophic problems. A disease, insect or weather condition that harms one plant is likely to harm them all. Monoculture is the main reason why Dutch elm disease has been so devastating in our towns and cities. The pathogens can move between closely spaced trees via insect vectors or root grafts, leaving devastation in their wake (Figure 24). The Dutch elm disease epidemics illustrate the value of diversity in plant populations.

Figure 24