Dutch Elm Disease
Spread
The main cause for spread of DED is the European elm bark beetle. Over winter, these beetles as adults or larvae tunnel into elm trees. The fungus that causes DED produces spores in the bark beetles’ tunnels and the beetles may pick up the spores as they move through the tunnels.
The fungal spores are then spread to healthy trees when the adult beetles emerge in spring and summer, fly to healthy trees and feed on the twigs. Infected beetles can also move to new areas in infested timber (especially where the bark is retained) used for furniture, firewood or wood chips.
Root grafts commonly form between closely planted elm trees, especially elms of the same species. DED fungi can spread from diseased trees to healthy trees by these grafts. The fungus can kill an entire avenue of trees in this way.
The disease may be spread via elm nursery stock. Elm flowers (fresh) and bark on timber, chips and handicrafts can also carry the pathogen. It is possible that DED pathogens can be spread on non-disinfected pruning tools.