DEPOSITION GRADIENTS AND ISOLATION 



Waggoner (1952) gives two sets of data on the spread of potato late- 

 blight {Phytophthora infestans) at Clear Lake, Iowa. 'Point source' foci 

 were established in field plots by artificial inoculation. Hollow curves 

 were plotted, showing the proportion of leaflets diseased at various dis- 

 tances from the source on the ninth day after inoculation in 1949 and 

 the eighteenth day in 1950. Reading-off observed values from Waggoner's 

 graphs gives the values for 1949 and 1950 as plotted in Fig. 28; these 

 agree well with the slope of the predicted gradient (d) for dispersal from a 

 point source. (The highest incidence was between 7 and 8 per cent, so the 

 multiple-infection transformation is unnecessary.) 



^Q. 



- 3 



- 4 



10 



- 5 



1000 



source 



100 

 metres from 



Fig. 28. — Infection gradients of potato late-blight {Phytophthora infestans) observed by 

 Waggoner (1952) at Clear Lake, Iowa, in 1949 and 1950; compared with d = theoretical line for 

 deposition downwind from a point source, assuming m = 1-75, Cy = od (metres)i, p = 0-05, 

 and allowing for Q_x. 



(iii) Fixing limits for isolation. The formula for d (average deposition 

 at distance x in all directions around a point source) may be chosen when 

 spread of a disease within a field, or gene dispersal, is being considered. 

 For safe decision on isolating a healthy crop from contamination, the use 

 of dw may be preferable as it predicts maximum concentration do\\Tiwind. 



Suppose we wish to estimate the upper limit of the percentage of 

 those plants infected with Ustilago tritici to be expected at 50 metres 

 distance from a lo-metre-square plot of smutted wheat acting as a source — 

 knowing from past local experience that seed saved from the source plot 



M 



177 



