DUTCH ELM DISEASE 9 



definitely whether the fungus which causes the Dutch elm disease is associated 

 with any particular wood discoloration. 



Causal Organism 



The Dutch elm disease is caused by a fungus which lives in the sapwood. 

 The fungus, first known in only its imperfect stage, was called Graphium ulmi 

 Schwarz. Later, on discovery of the perfect or ascigerous stage, the fungus 

 was assigned to the genus Ceratostomella, and the name became Ceratostomella 

 ulmi (Schwarz) Buisman. The fungus can be distinguished from other fungi 

 which cause similar wilting in elm by the type of growth and reproductive 

 structures it produces in artificial culture media (Figure 3), and may be isolated 

 in culture from infested trees at any time of year. 



Figure 4. Roots of TwoIElm Trees Grafted Together. 



Bark removed to show streaks caused by Ceratostomella ulmi passing from one root into 

 the other through the graft. 



(Reproduced from Phytopath. 25: 1040. November 1935. Photograph furnished by 

 Division of Forest Pathology, U.S.D.A.) 



How the Fungus is Spread 



The manner in which the fungus is disseminated is not completely under- 

 stood. Numerous possibilities have been considered, including the following: 

 wind, grafting, pruning tools, birds, and insects. At least one class of trans- 

 mission agents has been established experimentally. 



Wind. From the delicate nature of the spores, or reproductive units, of 

 the causal fungus, it is apparent that these structures are not adapted to 

 withstand the desiccation to which they would be subjected if borne by wind 

 for considerable distances. Moreover, the fact that the fungus in its role as 

 a vascular parasite is, for the most part, not accessible to air currents outside 

 of the host definitely limits the possibility of direct spore dissemination by 

 the wind. 



