DANGEROUS INTERNATIONAL FOREST TREE DISEASES 35 



long-term work can be undertaken to mitigate the effects of such 

 diseases should they eventually reach other countries or other con- 

 tinents. 



Partial Solutions 



Phytosanitary Action 



Phytosanitary action can be taken in a number of ways. General 

 reviews of these have been made by Giissow (1936), McCubbin (1946, 

 1954), and Soraci (1957). The means used fall roughly into three 

 categories: (a) measures taken in the exporting country, (b) measures 

 taken in the importing country, and (c) embargoes, whereby carriage 

 of specified plants or parts of plants from one country to another is 

 completely forbidden. 



As far as trees are concerned, there are three main types of material 

 on which fungi, bacteria, and viruses are likely to be transported, 

 namely seed, plants, and timber. Seeds present the smallest risk. Al- 

 though a good case can be made for the routine treatment of many 

 agricultural seeds, there is so little evidence of disease transmission 

 on tree seeds that no general action can be justified. Only in the case 

 of chestnut blight, Endothia parasitica, on Castanea seed is there a 

 strong case for seed disinfection. 



Timber is much more dangerous, especially when it is unbarked. 

 Ceratocystis ulmi, the cause of elm disease, was taken from Europe to 

 North America on unbarked timber, and Endothia parasitica may have 

 reached Europe from North America in the same way. There has been 

 a very general tendency, except in the case of fruit and vegetables, to 

 neglect the risk involved in the movement of final products, partly no 

 doubt because restrictions on them would almost invariably involve 

 serious restrictions on trade. Yet transmission of some pathogens, 

 such as Endothia parasitica on Castanea and on Quercus, or Hypoxylon 

 pruinatum on poplar, is much more probable on timber than on small 

 plants. Morgan and Byrne (1957) have recently stressed the dangers 

 inherent in the uncontrolled movements of timber from country to 

 country or from continent to continent. 



There are of course several well-known examples of the transmis- 

 sion of tree diseases on young plants. The most notable is the importa- 

 tion on nursery plants of Cronartium ribicola, the cause of white pine 

 blister rust, into North America from Europe. Most of the quarantine 

 regulations designed to prevent the entry of tree diseases have referred 

 to living plants, rather than to seed or timber. 



Although embargoes obviously give the best chances of success, 

 there is rather general objection to their use. Where trade exists they 

 are bound to be restrictive, and there is always the possibility that they 

 may be applied for economic rather than phytosanitary purposes. A 

 complete embargo on seed would obviously impose too severe a restric- 

 tion on any country which was largely dependent on exotic trees. 

 A complete embargo on young trees and parts of trees would stop 

 the international distribution of trees such as poplars, which are 

 normally raised as vegetatively propagated clones and of grafting 

 material of selected trees for breeding work. A complete embargo 

 on timber would only be possible for a country that possessed a suf- 

 ficiency of all kinds of timber for its own use. Conditions vary in 

 different countries and, therefore, so does the extent to which they 



