THE MICROBIOLOGY OF THE ATMOSPHERE 



slide sampling indicated the same quantitative composition and seasonal 

 variation of the pollen cloud over South Wales as did the Hirst trap. 

 As an exception, the Hirst trap revealed that the abundance of nettle 

 {Urtica) pollen had been greatly underestimated by the gravity slide 

 method. 



Continuous records provided by the Hirst trap have proved highly 

 illuminating in mycology, plant pathology, and allergy — even though the 

 results are still limited to those obtainable by visual identification. Outdoor 

 bacteriology awaits the development of convenient, continuous sampling 

 equipment for cultures {see Miquel & Benoist, 1890), and this would lead 

 also to further precision in knowledge of airborne fungi. 



Air sampling, indoors and out, can have either of two aims: (i) to 

 attain the broadest knowledge of the whole range of organisms in the air, 

 which requires the most complete and undistorted sampling methods 

 possible; or (2) to obtain detailed knowledge about a single species or 

 group, which may require highly selective methods. 



Air sampling has been successful in revealing the diversity of organisms 

 forming the air-spora, in defining conditions for the outbreak of epidemics 

 of some plant diseases, and in measuring dispersal gradients of spore 

 concentration. So far it has proved less useful as a routine measure in 

 forecasting outbreaks of crop disease, because existing methods are mostly 

 insensitive to small concentrations of inoculum in the air (Table XVII). 



TABLE XVII 



ESTIMATED DETECTION THRESHOLDS OF CONCENTRATION (SPORES PER CU. 

 METRE OF air) OF HIRST TRAP AND STICKY MICROSCOPE SLIDES INCLINTID 

 AT 45°, ASSUMING EXPOSURE FOR ONE HOUR AND COMPLETE COUNT OF 28 



SQ. MM. OF SPORE DEPOSIT. (Hirst & Stcdman, 1961.) 



Wind-speed (metres/sec.) 



05 i-i 175 3-2 55 95 



Microscope slide inclined at 4^° 



Lycopodium 941 



Erysiphe 2,000 



Ustilago 28,000 



Hirst trap 

 Lycopodium 2 



Ustilago 2 



As pointed out by Hirst (1959): 'No trap is Hkely to detect spores as 

 sensitively as an acre of a susceptible crop in weather favourable to in- 

 fection. Thus epidemics may be started by spore concentrations which 

 traps will not reveal, so we must define the value of "nil catches". With 

 volumetric traps this can be done by calculating the "detection threshold", 

 or concentration at which one spore should appear in the area scanned for 



106 



