XIV 



LONG-DISTANCE DISPERSAL 



Dispersal of microbes over long distances is an ever-present, world-wide 

 phenomenon. Its experimental study is almost non-existent; but there is 

 circumstantial and observational evidence of its magnitude, some of which 

 has been reviewed by J. J. Christensen (1942). 



CONTROXTRSY ON THE IMPORTANCE OF THE AIR-SPORA 



The discovery of air dispersal in any group of plants has usually been 

 followed by controversy between opposing specialists, some maximizing 

 and some minimizing the significance of the phenomenon. Exaggeration is 

 dangerous, and the object of this book is to present evidence from which 

 balanced conclusions can be derived. 



Views current on air hygiene affect the design and planning of hospitals. 

 The air dispersal of human pathogens was confidently accepted in the 

 golden age of bacteriology, but was gradually discounted after the experi- 

 ments of Fliigge which focused attention on the limited scatter of drop- 

 lets from the mouth and nose (cf. Chapter XII). The balance has now been 

 restored by Wells (1955) and others who have stressed the floatabilit}' of 

 'droplet nuclei'. 



In plant patholog}' the two schools of thought have been prominent 

 simultaneously. Butler (191 7) held that spores of plant pathogens could 

 be transported for short distances by air or by rain-splash, but claimed 

 that 'the distance to which spores may be carried in the air has often been 

 exaggerated in the past, and is much less than might be expected'. The 

 discontinuous spread of plant pathogens, he considered, was likely to 

 occur on seeds, plants, and horticultural produce; but 'infection by spores 

 carried through the air from remote centres is not a contingency which 

 needs to be taken seriously into account'. Naumov (1934) held that human 

 activity accounted for most long-distance transport of fungus pathogens 

 and that, in the absence of host plants, fungi were dispersed with extreme 

 slowness. Endothia parasitica in North America could not cross, in a 

 period of 1 years, a 45-60 km.-wide tract that was free from chestnut 

 trees; the spread of a disease of palm trees was estimated at only 4-5 km. 

 per annum. 



The literature contains many instances of fungus pathogens failing to 

 infect hosts at distances of a few metres (e.g. H. W. Long, 1914), or to 



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