Cause
Dutch elm disease is caused by the fungus Ophiostroma ulmi and is spread from tree to tree primarily by insects. In the United States, the smaller European bark beetle (Scolytus multistriatus) and the native elm bark beetle (Hylurgopinus rufipes) are the most common vectors. Adults of these insects construct galleries in or under the bark of dead or dying trees affected by the disease, and lay eggs along the sides of the galleries. After hatching, the larvae feed away from the galleries, and pupation occurs at the end of the larval tunnels (figure 2). In the meantime, fruiting bodies of the fungus are produced from the walls of the galleries. These fruiting bodies produce masses of sticky spores that adhere to the surface of the adult beetles as they exit the bark (figure 2). Immediately after exiting the bark, the adult beetles fly to healthy elms where they feed in the crotches of small twigs (smaller European bark beetle) or in the inner bark and wood of branches 2-10 inches in diameter (native elm bark beetle). It is during this feeding process that the fungus is introduced into the tree. After feeding in healthy trees, the beetles then move to weakened, dying or dead elm trees, or to logs, for breeding, and the cycle is repeated. Usually there are two broods of beetles each year. The overwintering brood emerges in early summer and is responsible for the largest number of new infections. The beetles usually do not travel more than a few hundred feet from the place of emergence to feed, but they may travel for more than 2 miles in search of proper breeding places. In some areas rate of spread is rarely more than 5 or 6 miles per year.
In addition to insects, the fungus may be transmitted from elm to elm by root grafts (figure 2). This is an important means of spread where elms are growing in close proximity, such as rows of trees along streets.