THE THREAT OF INTRODUCED DISEASES 



T. R. Peace 



Formerly Chief Research Officer (Deceased), Forestry Commis- 

 sion. Forest Research station. Wrecclesham, Farnham. Surrey, 

 England 



Relatively few fungi, bacteria, and viruses attacking forest trees 

 have a worldwide distribution, or even cover all the areas where avail- 

 ability of host and suitability of climate would permit their develop- 

 ment. Many instances are already known, however, where pathogens 

 have spread from one continent to another, and there are alarming pos- 

 sibilities of further spread. This paper attempts to discuss the risk, 

 and how its immediate and long-term effects can be lessened. Discus- 

 sion is limited to diseases caused by fungi, bacteria, and viruses, but 

 similar principles apply to insect pests. 



On land masses where the host tree or trees have a continuous dis- 

 tribution, spread of diseases by natural means is almost inevitable and 

 extremely hard to stop. But where there are breaks in host distribu- 

 tion, for instance a mountain range or a desert, and particularly where 

 there is a sea barrier, natural dissemination becomes much less impor- 

 tant or even nonexistent, and man is the chief agent of spread (Orton 

 and Beattie 1923). Thus intercontinental transmission of disease, 

 except between Europe and Asia, is almost entirely due to man's 

 agency. 



This paper is divided into two main sections. In the first and 

 smaller, the risk is considered briefly. In the second, various actions, 

 which have already been taken or can be taken in the future, are con- 

 sidered. The second part may perhaps be regarded as outside the 

 proper purpose of the paper I was asked to submit. If so, I must 

 apologize to Professor Boyce on whose ground I will have trespassed. 

 Nevertheless it may be valuable if we have these actions in mind when 

 we are considering individual diseases. In any case, this is a matter in 

 which I have some experience and rather definite views. This leads 

 me to welcome an opportunity of putting them before the Group. 



The Problem 



A study of the present known distribution — both in the positive 

 sense of areas known to be infected and in the negative one of areas 

 known to be uninfected — makes it clear that many pathogens already 

 known to be dangerous are not yet generally present throughout their 

 host's geographical range. In particular, they may not yet have pene- 

 trated all the areas where the host species has been planted as an ex- 

 otic. Bacterial canker of poplar, whether it is caused by Pseudomonas 

 syringae'or Aplanobacterium popvli. is still apparently confined to 

 Europe: Phloem necrosis, a virus disease of elm, to North America: 

 the fungus Monochaetia unicornis, causing canker on cypresses, to 

 Africa and New Zealand: and Ceratocystis fagacearum, the cause of 

 oak wilt, to North America. The compilation of any list of dangerous 



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