64. There are other diseases of poplars in North America, some 
of which are of, unknown causation, that are not reported from 
Europe. Little is known about poplar diseases in Asia, but there 
undoubtedly are many diseases there in the widespread native 
poplars that have not been introduced into Europe. 
65. To what extent poplar diseases are spread by cuttings is not 
known but as it is difficult or impossible to detect small incipient 
infections such spread undoubtedly has taken place. Surface treat- 
ments with fungicides followed by cutting off and destroying both 
tips of the cuttings will decrease the risk of importing diseases. 
Little is known about the effects of fungicides on the growth of 
cuttings, but a surface washing with water after treatment will de- 
crease the danger of any adverse influence. Particles of leaves 
that may carry diseases are sometimes found around the buds. As 
the number of cuttings increases, the danger of disease introduction 
also increases. 
CONCLUSION 
66. My conclusion is that European countries should discontinue 
the widespread introduction of poplar cuttings from other continents 
before they bring in new diseases that may greatly limit the future 
development of European poplar culture. In the United States, we 
now permit the introduction of only a few poplar cuttings of some 
clone not available in this country, and these few cuttings are 
grown in posgentry quarantine before being released. Sometimes the 
introduction of seed will suffice. 
67. The present widespread exchange and distribution of cut- 
tings from one European country to another should also receive 
more attention from pathologists, quarantine officials, and persons 
interested in the future of poplar culture. Is the risk of bringing in 
new pests worth the gain? How can the risk be decreased without 
interfering too seriously with poplar culture ? 
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