AEROBIOLOGY 



In the unbalanced condition necessary to agriculture, this claim obviously 

 fails; it is comparatively easy to limit the dispersal of Sclerotium cepi- 

 vorum and Synchytrium endobioticum w^hich have poor dispersal mechan- 

 isms, difficult with Phytophthora infestans, and impossible with Pitccinia 

 triticina whose dispersal mechanism is excellent. Willis's claim refers to 

 floras which man has not greatly disturbed, and, it seems, mainly to 

 higher plants. 



The rules of M. W. Beijerinck and L. G. M. Baas Becking, as given by 

 Overeem (1937), state that 'as far as microbes are concerned; ""Everythhig 

 is everywhere^'' and, from this "everything", ''''the milieu selects^' '. This is 

 an exaggeration, a microbiological half-truth that is useful as a corrective 

 to narrow parochialism ; but, if it were universally true, aerobiology' would 

 not be interesting. For example the rubber-tree is attacked by two serious 

 pathogens, Oidium heveae and DothideUa ulei, which fortunately are at 

 present limited in their distribution, the former to Asia and the latter to 

 the Americas. 



Many saprophytic microbes have a world-wide distribution, and the 

 soil bacteria and moulds tend to be similar on all continents. Floras of 

 different areas are remarkably uniform in their coprophilous fungi, and 

 in their green and blue-green algae — a testimony to the effectiveness of 

 various dispersal processes where similar environments, existing over 

 long periods, have given time for equilibrium to be established. With 

 local disturbances from time to time, sites become available in the midst 

 of a population of organisms in apparent equilibrium. Man is the most 

 active disturber of this equilibrium, and human activity therefore fre- 

 quently operates in the border-line region of a microbial concentration 

 gradient. In this region we can attempt to interfere ^^■ith the dispersal 

 process. 



Normally in a field-population, multiplication and elimination are 

 going on simultaneously. Altering the balance between these two pro- 

 cesses may sometimes lead to the disappearance of an organism or a dis- 

 ease. Isolation is one of the methods of doing this. Isolation of allergic 

 patients from allergens out-of-doors may need greater distances, because 

 the threshold may be so low that one pollen grain is enough to provoke an 

 attack. Isolation and quarantine in medicine scarcely falls within the pro- 

 vince of this book; intra-mural aerobiology is mainly the concern of 

 hygiene and public-health workers. But the use of distance to control 

 cross-pollination and plant diseases out-of-doors is obviously relevant. 



Where the geographical distribution of a pathogen is more restricted 

 than that of its host crop, the existence of natural barriers, for practical 

 purposes uncrossable by airborne plant pathogens, makes control of some 

 crop diseases by official action a feasible procedure. 



Where natural wind-transport fails, other methods may dominate 

 dispersal. With freshwater and soil micro-organisms, transport by birds 

 may be important. But of all animals, man is the most dangerous, because 



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