36 U.S. DEPT. OF AGRICULTURE MISC. PUB. 939 



are able to apply embargoes, undeniably the most effective phytosani- 

 tary measures. In practice, embargoes can hardly be made complete. 

 Limited importation by licenses subject to specified precautions must 

 be granted for scientific purposes. In any case spores may enter on 

 aircraft, and passengers may carry diseased material knowingly or 

 unknowingly in their baggage (Sherman 1957) . 



It is generally admitted that inspection of imported material on 

 arrival is a very unreliable way of preventing the entry of diseases 

 (Gravatt and Parker 1949; Gram 1955). In the case of large con- 

 signments only a sample can be inspected and, unless the incidence of 

 disease is very high, it may well go undetected. In some cases the 

 disease may be in a stage which is not detectable even by careful 

 visual inspection. For instance, Chrysomyxa rhododendri was im- 

 ported into the United States as mycelium in azalea leaves. By the 

 time fructifications appeared and the disease became detectable, the 

 plants had been widely distributed (Gould, Eglitis, and Doughty 

 1955) . In fact, inspection on entry, except for the occasional detection 

 of very badly diseased consignments or of material subject to embargo, 

 is valuable mainly in encouraging better phytosanitary practices in 

 the exporting country. 



There is obviously a better chance of achieving satisfactory inspec- 

 tion in the country of origin. If plant inspection is started in the 

 nursery, it can be extended over a period of time and carried out with 

 a knowledge of some of the possibilities of infection to which the crop 

 is subject. Even then it is impossible to state honestly that the plant 

 material is entirely free of all disease. There is no doubt, however, 

 that efficient preexport inspection, especially in the nursery, can greatly 

 lessen the risk so that the danger of importing from any particular 

 country depends not only on the pathogens present there, but on the 

 efficiency of its phytopathological inspection service. Every country 

 is therefore in a position to lessen the chances of disease transmission 

 by improvements in the scope and skill of its inspection service and 

 by the encouragement of phytosanitary practices among its exporting 

 nurserymen. 



The principles underlying plant quarantine regulations have been 

 discussed by Moore (1952, 1955) and by Soraci (1957). There is a 

 strong tendency to base regulations on our present knowledge of spe- 

 cific diseases, in particular on their potential danger and on their 

 distribution, and to condemn widespread embargoes on the ground that 

 they are based on unreasonable fears of the unknown. However, we 

 must remember our almost complete lack of knowledge of potential 

 tree pathogens in some of the phytopathologically less advanced coun- 

 tries. It is also necessary to take into account the obvious possibility 

 that a known pathogen may behave quite differently when moved to 

 a new environment. The behavior of Phaeocryptopus gaumannii on 

 Pseudotsuga and Keithia thujina on Thvja, when moved from North 

 America to Europe, are frightening examples; however, the increased 

 importance of Keithia is due mainly to its severe attacks on nursery 

 plants of Thuja in Europe, as compared with its behavior on natural 

 regeneration in North America. 



Even when a fungus is already widespread, there is a danger that it 

 may have regional strains of differing virulence, the transfer of which 

 could prove damaging. This may well be the case with Lophodermium 

 pinastri, the needlecast of pines. Such considerations may support the 



