DEPOSITION GRADIENTS AND ISOLATION 



Further study will show whether certain groups of fungi are usually 

 dispersed only under low-turbulence conditions. If it is confirmed that 

 they are, a much smaller isolation distance could safely be permitted 

 between disease sources and healthy crops than is permissible with patho- 

 gens that are dispersed in normally turbulent air, because with a small 

 degree of turbulence the contaminated area is narrower and more intense. 

 Meanwhile it is safer for isolation purposes to choose for prediction the 

 line from the appropriate set which has been calculated for normal 

 turbulence, m = i-JS- 



The rapid decrease in deposition with increasing distance from the 

 source helps to explain the observations of Schmitt et al. (1959), who 

 showed that many small rust infection foci distributed over a wheat field 

 are more destructive to the crop than the same number gathered together 

 at a single focus. 



The fact that the slope of the curve is a characteristic of a particular 

 distance can be used in the field to locate an unknown source. If lesions 

 (or spores) are uniformly scattered over an area, we can assume that the 

 source is so distant that the place of observation is on the tail of one (or 

 many) dispersal gradients. Where counts increase rapidly, there is 

 evidence that a source is being approached. Stakman & Hamilton (1939) 

 located a large rust-infected barberry bush in western Minnesota, by 

 first identifying unusual rust races in the vicinit}' and then tracing the 

 telial stage of the rust on grasses leading up to the bush. 



Topographical Modification of Gradients 



Diflfusion has been treated so far under nearly ideal conditions, but 

 the literature contains information on the effects of topographical features 

 which modify the gradient. 



Pollen of wind-pollinated plants is distributed in typical gradients, as 

 shown, for example, by Roemer (1932), Jensen & B^gh (1942), Jones & 

 Newell (1946), and Bateman (1947a). During strong winds in open fields, 

 Jensen & B(/gh's catches of pollens of rye, ryegrass, cocksfoot, timothy, 

 sugar-beet and mangold, on sticky microscope slides, showed a steady 

 decrease with distance up to i ,200 metres from the source field. But they 

 found that hedgerows and plantations protected ryegrass, cocksfoot, and 

 mangolds, in proportion to the height of the obstacle. In tests near to 

 the source, a protection corresponding to an isolation distance of about 

 200 metres of open ground was obtained behind hedges, even so far 

 do\Miwind as 5 to 10 times their height (cf. Rider, 1952; Schrodter, 1952; 

 U.S. Weather Bureau, 1955; Cabom, 1957). 



Gene Dispersion 



The physical mechanisms of wind dispersal have a bearing on popu- 

 lation genetics. Theories of gene dispersion in populations at first assumed 



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