10 MASS. EXPERIMENT STATION BULLETIN 343 



Grafting. Contact of affected trees with healthy trees offers excellent op- 

 portunity for the spread of the disease within an area thickly planted to elms. 

 It has long been known that elm branches when in close proximity and sub- 

 jected to adequate pressure, will become grafted together. Likewise, elm 

 roots in the soil may become grafted together. No better setup for the trans- 

 mission of a vascular parasite could be devised than the natural condition of 

 grafted roots, which phenomenon is doubtless widespread. In fact, trees 

 affected by the Dutch elm disease have been found with their roots grafted 

 together (Figure 4). Trees planted relatively closely along the banks of 

 streams have been observed to be affected by the disease. Such observations 

 agree with those for trees closely grouped elsewhere. 



Pruning Tools. Pruning tools used in tree trimming operations have been 

 considered a possible means of spreading the fungus. Persons engaged in 

 trimming work, where the disease is prevalent, disinfect pruning tools to 

 guard against the spread of the disease. This practice of disinfecting pruning 

 tools is a good procedure in all tree trimming activities since, even when no 

 serious diseases are known to be present, newly introduced or little understood 

 parasites may be active, especially if dead branches are numerous. 



Birds. It has been surmised that birds such as woodpeckers and sapsuckers 

 may have a part in the spread of the disease, but there is no proof of this 

 hypothesis. 



Insects. From what is known of the distribution of the disease in America, 

 it would appear that there are no agents of dissemination that carry the causal 

 organism for very great distances. American and European elm bark beetles 

 have been proved to be vectors of the fungus and there may be other carrier 

 insects. Believed to be the most important of all carriers are the European 

 elm bark beetles, one species of which is established in the United States. 

 These insects are known to carry the spores of the fungus within and upon 

 their bodies. For the most part, the flight of these beetles is limited to short 

 distances. In some twenty-eight years since their first American discovery in 

 Massachusetts, they have spread somewhat, but not alarmingly, within the 

 State from the originally observed infestation. 



Comparative data from recent authoritative experiments tend to indicate 

 that prevailing winds are associated with the relatively long-distance travel of 

 the beetles along the Atlantic coast area of infestation. It might happen, 

 therefore, that beetles which carried the spores of the causal fungus could be 

 wind-borne for some distance; but it is essential to remember that, for the 

 most part, prevailing winds in the area mentioned are seaward and would not 

 generally carry beetles great distances inland. Assuming that the beetles are 

 the principal cause for concern in the spread of the disease, it is somewhat 

 encouraging to observe that their further rapid spread inland along the Atlantic 

 coast is at least partially precluded by natural conditions. 



Only time can tell whether additional involved problems are connected 

 with the spread of the disease and it is of utmost importance that laboratory 

 and survey studies on the distribution of the disease and the causal fungus 

 and its carriers be continued in an effort to obtain complete data. 



