The Dutch Elm Disease fungus can infect a tree in two ways. The most common method of transmission is on the bodies of elm bark beetles. The fungus also spreads from diseased to healthy trees through grafted roots.
An elm bark beetle's life revolves around elm trees. It breeds in dead and dying elms, and depends exclusively on elm tissue for food creating a devastating cycle for elms when the Dutch Elm disease fungus Ophiostoma novi-ulmi is thrown into the mix. The adult females in one piece of wood the size of a fireplace log can lay thousands of eggs.
If the tree died from Dutch Elm Disease, every beetle that hatches and emerges as an adult from that wood may be carrying the fungus. They will be looking for a healthy elm on which to feed and will inadvertently inoculate the tree with Dutch Elm Disease via the spores carried on their bodies. The majority of new infections, and the bulk of the losses to Dutch Elm Disease are due to this method of transmission. That is why prompt detection, removal, and destruction of dying elms are so critically important.
The other method of disease transmission between elms is through grafted roots. When elms are growing near each other, their roots come in contact in the soil and graft together. In the absence of a vascular wilt like Dutch Elm Disease, this is an advantage for the trees. The Dutch Elm Disease fungus, however, can pass from diseased to healthy trees through these grafted roots and continue to spread indefinitely through a stand of elms.
