34 U.S. DEPT. OF AGRICULTURE MISC. PUB. 93 9 



pathogens of incomplete distribution indicates clearly that the incom- 

 pleteness rests partly on lack of knowledge. 



A recent unpublished list of forest diseases considered potentially 

 dangerous in Britain was based on reasonably certain information 

 that the diseases listed were not present in this country, but that they 

 were present in other areas. Most records were from North America 

 and Europe, where forest pathological investigations have been reason- 

 ably thorough. The tabulation below, based on this list, shows the 

 number of diseases, absent from Britain but considered potentially 

 dangerous to British forest trees, recorded from the different 

 continents : 



Country Number 



North America only 40 



Europe only 8 



Asia only 3 



North America and Europe 5 



North America and Asia 1 



North America, Asia, and Europe 2 



North and South America 2 



North and South America and Europe 1 



North and South America, Europe, and Asia 1 



Europe and Asia 1 



Africa and New Zealand 1 



The preponderance of North American entries in this list is partly 

 due to the importance of North American conifers in British forestry. 

 The relatively small number of dangerous European diseases, not 

 present in Britain, is partly the result of its proximity to the rest of 

 Europe, so that many diseases have already achieved a common distri- 

 bution. The low numbers for other continents are mainly due to 

 the relatively small importance of their trees in Britain and to the 

 fact that in some cases large areas lie in entirely different climatic 

 zones from Britain. Nevertheless, it is almost certain that the low 

 figures for Asia, where nearly all the dangerous diseases listed are 

 Japanese records, and for other continents poorly represented in the 

 list, are also partly due to lack of knowledge of the pathogens occurring 

 there. 



While any list of known dangerous pathogens with incomplete con- 

 tinental distributions is certainly impressive, particularly in the 

 number of species involved, it is certain that more diseases remain 

 to be discovered when pathologically unexplored regions, such as 

 China or most of the temperate area of South America, are properly 

 investigated. 



Thus, we are faced with a large number of diseases known to be 

 damaging in the countries where they are present and therefore 

 presumed to be damaging in other countries to which they might 

 spread. There is the strong possibility that other diseases, not serious 

 in their present range, might prove dangerous if transported to other 

 areas, either more favorable to their activities or less favorable to their 

 host tree. Finally, there are almost certainly parasites, as yet un- 

 recorded, that will attract attention only when the countries where 

 they now exist are pathologically explored, or when they reach other 

 countries. Thus, we must consider what immediate steps can be 

 taken to prevent or slowup disease spread from place to place, and what 



