Dutch Elm Disease has been a devastating event in the history of tree diseases. It is caused by the fungi Ophiostoma novo-ulmi and O. ulmi and is vectored by bark beetles. The disease is referred to as “Dutch” Elm Disease because it was first described in Holland in 1921. The pathogen originated in the orient. The disease began its destructive reign in the United States in 1930. Prior to the arrival of this pathogen, many streets and parks were graced by the upright and spreading branch structure of American Elms. The appearance of a mature American Elm can make nature lovers stop and stare in awe. Due to the incredible interest in these trees, plant breeders have been trying to develop resistant American Elms for years. Unfortunately, many of these trees become susceptible to other Elm diseases such as Elm Yellows.
Symptoms develop quickly within a 4-5 week period and usually when the leaves have reached full size. The first visual symptoms are what is referred to as “flagging” within the crown of the tree. Flagging is a branch of a tree that develops symptoms of wilting and/or yellowing of the leaves on a otherwise apparently healthy tree. Prior to this occurring, symptoms have developed internally. They include the death of xylem cells, the loss of water conducting ability and the browning of the infected sapwood in narrow streaks that follow the wood grain (Fig. 1). The fungus is present in the streaked wood, and isolations taken from this symptomatic tissue are needed to confirm infection by this pathogen. Occlusion of xylem vessels is due to the production of gums and tyloses. In the west where summers are dry, water shortage and heat stress often mask symptoms.
Spores of Ophiostoma novo-ulmi are stored in xylem vessels and reproduce through budding. Dispersal of spores is via the bark beetles that burrow under the bark and lay their eggs in wood galleries (Fig. 2). Ophiostoma novo-ulmi is distributed over long distances in elm logs and in firewood. Elm bark beetles distribute it locally and over distances for several miles. There are two species of beetle vectors known in North America, Hylurgopinus rufipes and Scolytus multistriatus. Insects are attracted to healthy elms by volatile chemicals produced by the trees. Beetles bore into the inner bark and while feeding deposit spores Ophiostoma novo-ulmi. S. multistratus feeds in the crotches of twigs, therefore, most infections occur in twigs. H. rufipes bores in the bark of branches and small trunks causing infections in major branches.