AIR SAMPLING TECHNIQUE 



each sample. In our routine scanning of hourly samples from the Hirst 

 trap the detection theshold is less than lo spores per cubic metre of air. 

 This high value explains why spore traps are of little practical use in 

 forecasting epidemics of potato blight which start from minute local 

 sources, but are valuable for apple scab or black rust, with which initial 

 spore concentrations may be high because of the sudden liberation of 

 accumulated spores or the arrival of a spore-laden air-mass.' 



Before using an air sampler, its performance should be explored 

 experimentally. Consistency of performance alone is an unsatisfactory 

 criterion, because a trap may be consistently misleading. 



107 



