Pathogens - Dutch Elm Disease

Dutch elm disease (Ophiostoma ulmi (Buisman) Nannf. or Ceratocystis ulmi)

DESCRIPTION:  One of the most familiar tree diseases imported to this country, Dutch elm disease is a fungus spread by bark beetles or by root grafting to nearby trees.  It is a wilt disease that spreads rapidly and clogs the tree’s vascular tissues, stopping water from moving inside the tree.  The tree forms gums within these water-conducting vessels in response to the presence of the fungus, causing the tree to wilt and eventually die. (31)  The fungus must enter the host through a wound in the bark, and almost invariably the wound is caused by two bark-boring insects, the European and American Elm Bark Beetles.  As the beetles chew their way into the sap that flows in healthy conductive cells, they carry on their legs the spores of the Dutch Elm disease picked up from infected trees. (32)  

INTRODUCTION HISTORY AND DISTRIBUTION:  Sometime in the 1920's, the fungus entered the United States, inadvertently introduced by shipments of imported European elm logs on at least three separate occasions. Its presence was first confirmed in Ohio, in 1930. (27)  Since 1930, the pathogen rapidly spread in native and urban elm populations throughout North America.  It swept through midwestern elm populations in the 1950s and has since been reported in all states except the desert Southwest. (32)   

IMPACTS ON ECOSYSTEM: Millions of American elm trees have been lost across the United States.  The elms would be extinct now but for their prolific seed production.  However, even so, there are now relatively few elm trees compared to the large forests that once covered North America, England and Europe. (27)  Trees infected by beetles first show wilting, curling and yellowing of leaves on one or more branches in the upper portion of the tree. (32)  Large trees may survive and show progressively more symptoms for one or more years.  Trees infected through root grafts wilt and die rapidly; this frequently occurs in the spring soon after the trees have leafed out and progresses from the base of the tree upward.  Trees infected with Dutch elm disease usually develop brown streaks in the sapwood of wilting branches, reducing salvage timber value (31)   The elm tree was important not only for its quality of wood and constituency in the forest ecosystem, but also as an economically important ornamental that lined many city streets as a shade tree and enhanced property values. (27) 

CONTROL EFFORTS:  There is no way to eliminate Dutch elm disease once it begins; control programs have as their object the management of the disease so that losses are spread out over a long period, minimizing the impact of the disease. (31)  Pruning of affected branches and severing root grafts can help slow infection rates.  Injection of fungicides has been tried, but these are very expensive. (32)  Microorganisms have been tested to determine if some sort of biocontrol can rid the disease from the elms.  So far, results have proved negative or are not yet approved by Environmental Protection Agency.  It would not be impossible to imagine this tree becoming extinct despite the efforts to control the disease. (27)