38 U.S. DEPT. OF AGRICULTURE MISC. PUB. 9 39 



Research Action 



Any advance in our knowledge of tree diseases is naturally of value 

 when we come to consider the possibilities and results of their trans- 

 mission from one area to another. The testing of the trees of one 

 country or continent for their resistance or susceptibility to the dis- 

 ease of another, whether by chance exposure, deliberate exposure, or 

 inoculation, is particularly valuable. In many countries there is so 

 much information already available, but not necessarily collected, 

 from existing plantations of exotics that it is doubtful if plantings de- 

 signed merely for disease recording purposes can be justified. These 

 test plantings are really only useful when placed in regions where the 

 pathogen population is unexplored. The possibility of doing this is, 

 of course, usually limited by the lack of pathologists in those regions. 

 It may therefore prove easier to observe the development of diseases 

 in carefully sited plantations, rather than over the unexplored area 

 as a whole. 



In addition, forest plantings of exotics are probably lacking in a rel- 

 atively undeveloped country, so that stands specially formed for path- 

 ological study may provide the only chance of getting data on the sus- 

 ceptibility of normative trees to the local pathogens. Provided the 

 limitations imposed by siting in relation to virtually unknown sources 

 of disease and by the limited number of plantations are taken into con- 

 sideration, there is no doubt that such special plantations could yield 

 results of very definite international value. 



In pathologically explored regions, plantations designed to test 

 under natural conditions the reactions to specific diseases and there- 

 fore deliberately placed in regions of high infectivity would be more 

 valuable, and there is certainly room for their extension. It is also 

 desirable to extend inoculation tests in one country on tree species 

 of importance to another (Gravatt and Parker 1949; Riker 1957). 



With any of these methods, care must be used in the interpreta- 

 tion of results. The presence of a disease i,i a plantation is always 

 more significant than its absence ; very good evidence is required before 

 the absence of a disease from a tree can be taken by itself as proof of 

 immunity or resistance. Even when disease is present, care must be 

 exercised, for unequal distribution of sources of infection may cause 

 unequal distribution of damage, which thus may have no relation to 

 the inherent susceptibility and resistance of the trees affected. For 

 this reason, information from a large number of exotic plantations, 

 preferably growing under a wide range of infective and climatic 

 conditions, is likely, if intelligently interpreted, to be of more value 

 than that collected from a limited number of specially designed test 

 plantations. 



None of the methods mentioned above will eliminate the effect on 

 diseases of differences in climate and other environmental factors 

 between one country and another. For this reason, the disease reac- 

 tions of a tree species in one place cannot be taken as a certain indica- 

 tion of its behavior towards the same pathogen elsewhere. They do, 

 however, give the best information which can be procured without 

 actually moving the pathogen, a method which naturally is completely 

 inadmissible. 



There is danger even in moving cultures of pathogens from one 

 country to another (Wheeler 1957) . Transfer of cultures can be justi- 

 fied only if it serves some useful purpose and provided the cultures 



