THE MICROBIOLOGY OF THE ATMOSPHERE 



itself is likely to produce only 5 per cent of diseased plants. We choose the 

 curve for deposition downwind of an area source, with liberation height 

 h=i metre, under standard conditions (w = 1-75), and p = o-i. 

 Assume that infection at i metre distant is also 5 per cent. From Fig. 24 

 we find that Q_x is o-88 Q^„ at 50 metres distance, and from Fig. 26, d^ 

 at 50 metres is only about 28 per cent, of the value at i metre. The 

 expected maximum level of infection at 50 metres is therefore 28/100 

 X 0-88 X 5 per cent = 0-12 per cent. Under similar conditions, Oort 

 (1940) recorded about half this decrease at 50 metres. At i km. down- 

 wind of a ID X 10 metre plot, it would be more appropriate to adopt 

 the curve for a point source and estimate the level at 1-5 X 10"^^ per cent. 



(iv) Relative contributions of near and distant sources. A susceptible 

 plant in a field is often exposed simultaneously to infection from near-by 

 and distant sources. The problem arises as to the relative importance of a 

 few diseased plants within the crop as compared with possible massive 

 sources of infection in neighbouring fields. Quantitative answers have to 

 be guessed to set limits of tolerance for issue of health certificates, and the 

 data from gradients can be used to improve such guesses. 



A simple case would be to inquire how many diseased plants at 1,000 

 metres distance would give the same amount of deposition as that on a 

 plot of 100 metres radius with a single diseased plant at its centre, assuming 

 that distribution by wind is uniform in all directions. From Fig. 25, 

 over the range x = to x = 1,000 metres (for m = i*75, p = 0-05, and 

 h = o-i metre), the total deposition is Q^^ — Q^^ (for x = 100 metres) 

 = 1-0 0^0 — o-o6 0^0 = 0-94 Q_o. From Fig. 27, the average deposition 

 from one plant at a distance x = 1,000 metres is 4 x io~^^ Q_o spores per 

 sq. cm. The circle of radius 100 metres around the single plant contains 

 3-14 X 10^ sq. cm., so a plant at 1,000 metres contributes: (4 x 10^^ Q^„) 

 X (3-14 X 10^) = 12-5 X 10^ Q_o spores. The number of plants i km. 

 away required to equal the contribution of one plant within the 100 metre 

 radius circular plot is therefore approximately: 



0-94 Q_o/i2-5 X 10^ Q^o =- 7,500 plants. 



Characteristics of Gradients* 



Some observed gradients fall off much more steeply than the line 

 calculated for m = 1-75. For example, the only gradients recorded for 

 aecidiospores oi Puccinia graminis (A. G. Johnson & Dickson, 1919; and 

 Lambert, 1929) follow closely the expected gradient for a strip source 

 with m = 1-24, suggesting that these spores are dispersed when tur- 

 bulence is small. Data on potato late-blight {Phytophthora infestans) 

 contributed by Limassct (1939) and Bonde & Schultz (1943), and for 

 Peronospora destructor on onions by Newhall (1938), also suggest dis- 

 persal under low-turbulence conditions. 



* Other properties of gradients are discussed by Plank (1960). 

 178 



