XIII 



DEPOSITION GRADIENTS AND ISOLATION 



Chapter V described efforts to formulate changes in concentration of the 

 spore-cloud while it diffuses and travels downwind. We must now discuss 

 the more complex phenomenon of deposition gradients — the decrease in 

 number of spores deposited with increasing distance from the source. 



The infection of a plant by an airborne spore is itself a complex process 

 of which physical transport is an important part. Infection may fail at any 

 one of a chain of stages; and, for a deeper understanding of the whole 

 process, it is necessary to understand the parts. Spore diffusion and 

 deposition are stages selected for special attention in this chapter. 



It is impossible to predict, from knowledge of the characteristics of 

 spore deposition gradients, how many infections will be acquired by a 

 plant at a given distance from a source of known strength, because con- 

 ditions may be unsuitable for infection; but there is a possibility of being 

 able to predict an upper limit, and to use this knowledge in choosing safe 

 isolation distances. This chapter deals with gradients measured up to 

 distances ranging from a few metres to a few kilometres from the source ; 

 long-distance dispersal is discussed in Chapter XIV. The discussion 

 assumes the simplified conditions described on page 47; but, even so, 

 deposition and infection gradients have complications. 



Factors Complicating Infection Gradients 



(i) Deposition coefficient. With sources at or near ground-level, the 

 diffusing cloud of spores, unlike a gas or smoke, is robbed by heavy 

 deposition close to the source. Concentration and area-dose at a point 

 downwind therefore depend on two factors — the diffusion history and the 

 deposition history which the relevant part of the cloud has experienced. 



In view of the evidence given in Chapter VI I, velocity of deposition 

 can provisionally be taken as proportional to terminal velocity, and the 

 importance of deposition will increase with spore size. 



(ii) Viability. It is assumed, in the absence of experimental evidence 

 to the contrary, that viability is not affecting gradients over the short 

 distances discussed here, though the future may show this assumption 

 to be an over-simplification (p. 190). 



(iii) Available sites. Deposition gradients do not necessarily give rise 

 to observable infection gradients. For instance, a fruit-body ofGanodernm 



162 



