OUTDOOR AEROBIOLOGY — GREGORY 451 



nisms underlying this characteristic "infection gi^adient" are probably, 

 in order of importance: (1) turbulent three-dimensional dilution of 

 the spore- or pollen-laden air mass by spore-free air as the impure air 

 travels downwind; (2) appreciable loss of particles from the spore 

 cloud by deposition on the ground, vegetation, or other surfaces, 

 especially in the early stages of travel when the cloud is concentrated 

 near ground level; and (3) loss of viability, which may or may not 

 affect the result. In reality, the source is not a point, and its magni- 

 tude and shape also affect the dispersal gradient; concentration is 

 higher, and falls off less rapidly, if the source is a sizable area rather 

 than a point. As would also be expected, raising the source above 

 ground decreases loss from deposition near the origin. 



Prediction of the concentration of the spore cloud after a given 

 distance of travel presupposes both an adequate theoretical treatment 

 of the very difficult problems of atmospheric turbulence and also an 

 adequate quantitative theory of deposition. Different theories now 

 current predict different concentrations at a given distance, but agree 

 generally with observation and experiment in predicting a rapid 

 decrease in concentration with increasing distance from source. For 

 instance, there is evidence that 90 percent of spores of the wheat bunt 

 fungus Tilletia tritici and the clubmoss Lycopodiwn, when liberated 

 just above ground level, are deposited within 100 meters of the source. 

 Theory suggests that smaller particles than these would be deposited 

 less rapidly, but there is little experimental evidence to support this. 



A paradox is apparent here. With such a high rate of deposition 

 near the source, the effect of a point source at distances greater than 

 a few hundred meters must be negligible, yet in spite of this the con- 

 centration of micro-organisms in the upper air and for some distance 

 out to sea is substantial. The paradox is probably to be explained 

 by the fact that although the distant tail of the distribution from a 

 single point source is indeed negligible, the quantity in the upper air 

 over the ocean is the sum of the tails of the distributions of all the 

 point sources present on the continent from which the wmd has 

 traveled. 



The pattern of windborne dispersal differs from a Gaussian fre- 

 quency distribution around a point source by having increased concen- 

 trations both very close to the origin and at great distances, balanced 

 by smaller concentrations at intermediate distances [1]. 



TERMINATION OF THE DISPERSAL PROCESS 



Infection gradients of some plant pathogens have been traced over 

 distances of tens or hundreds of kilometers. Spores of some of the 

 cereal-rust fungi migrate annually for many hundreds of miles in 

 India and in the Soviet Union, and over the North American Conti- 



