DEPOSITION GRADIENTS ANT) ISOLATION 



between individuals ('under-dispersion' as understood by ecologists), 

 and K will then be less than unity. Aggregation ('over-dispersion'), on 

 the other hand, leads to K being greater than unit}". Aggregation may be 

 due to such causes as local spread of infection, progeny remaining near 

 parent, or to local differences in susceptibiHt}'. Plank (1946) has given a 

 useful test for detecting aggregation in the field. 



(v) Infection efficiency. Gradients may be observed either by directly 

 counting the numbers of spores deposited on equal areas at different 

 distances, or by counting some consequent effect such as colonies, leaf- 

 spots, or diseased plants. Usually these gradients will be the same when 

 numbers are plotted against distance, but the infection gradient will be 

 much lower than the deposition gradient. Even with a nearly loo-per-cent 

 viable spore suspension, the general experience in inoculation tests with 

 plant pathogens is that only a small proportion of the spores deposited 

 will give rise to a lesion — even when conditions for infection are as favour- 

 able as possible ; in unfavourable conditions the formation of lesions falls 

 to zero. The proportion of spores successfully infecting is termed 'infec- 

 tion efficiency' by Gaumann (1950, p. 1 57), and values recorded by various 

 workers under highly favourable experimental conditions include: 

 Phytophthora infestans 6-5 per cent, Alternaria solani 17 per cent, and 

 Septoria ly coper sici 0-2 per cent (all on tomato leaves, cf. McCallan & 

 Wellman, 1943); Botrytis sp. on Viciafaba 5 per cent (F. T. Last, un- 

 published); Peronosporu tabacina approximately i per cent (Waggoner & 

 Taylor, 1958). 



Rust fungi show relatively higher efficiencies. Thus Petersen (1959) 

 observed penetration by 30 per cent of uredospores of Puccinia graminis 

 tritici on wheat leaves; but, at the high concentrations tested, over 100 

 uredospores were required to produce one sporulating uredosorus {see 

 Durrell & Parker, 1920). \\'ith the same fungus, Rowell & Olien (1957) 

 obtained as many as eleven uredosori per 100 spores applied. McCallan 

 (1944) evidently obtained about 10 per cent efficiency from uredospores 

 of Puccinia antirrhini. R. H. Cammack (personal communication) ob- 

 tained 15 to 23 per cent efficiency when inoculating Puccinia polysora to 

 susceptible maize. 



Considering its importance in plant patholog}', it is remarkable 

 how little attention has been given to infection efficiency. Its value must 

 vary with dispersal conditions, but commonly the height of the infection- 

 gradient curve above the x-axis will be only about one-hundredth that of 

 the deposition curve, though the slopes of the two curves would be similar. 



(vi) Secondary spread. From turbulence theory we can predict only 

 primary dispersal gradients. As Waggoner (1952) points out: 'Because 

 proximity of a source is relatively more important than strength of the 

 source, spatial distribution of diseased plants becomes more uniform as 

 secondary infection progresses.' The infection gradient will therefore be 

 flattened if observed long enough after deposition for secondary spread 



165 



