SPORE LIBERATION 



Stepanov (1935) was apparently the first to use a small wind-tunnel 

 to blow spores at controlled wind-speeds. Using either cultures or plants 

 infected with pathogenic fungi, he found that the minimum wind-speed 

 required to remove spores varied according to the organism being tested : 

 for Botrytis cinerea it was o-36-o-5o metres per sec; for Alonilia sitophila^ 

 Ustilago spp., uredospores of Piiccinia triticina, and Hehninthosporhim 

 sativum it was 0-5 1-0-75 rnetres per sec; for aecidiospores of Puccmia 

 coronifera and P. priugsheimia, o-'jb-i-o metres per sec; for Ciinning- 

 hamella sp., 1-5-1 -75 metres per sec. On the other hand, Phytophthora 

 hifestans and Fusarium cidnwrum spores were not removed at any speed 

 tested up to 3-37 metres per sec More spores were removed in turbulent 

 than in streamlined wind. 



A special structure facilitating blowing away is the 'wind-cup' 

 described by Brodie & Gregory (1953). Flow of air over a cup-shaped 

 structure produces a double eddy system which can effectively remove 

 dry spores contained in the cup, as shown by wind-tunnel experiments 

 with smoke and Lycopodium spores. Soredia were also removed from the 

 podetia oiCladonia at 1-5 to 2-0 metres per sec, and spores were removed 

 from the cupulate sporangia of certain Myxomycetes at 0-5 metres per sec. 



Certain Gasteromycetes, including the puffballs Lycoperdon perlatum 

 and L. pyriforme^ and the earth-stars {Geaster spp.), have a 'bellows' 

 mechanism consisting of a thin, flexible, waterproof wall covering the 

 spore mass. Indenting this wall forces out a jet of air laden with spores. 

 Contact with animals operates the bellows efficiently, but must be a 

 relatively rare event in nature. Raindrops or run-off drops from trees also 

 operate the bello^^ s mechanism, and as one fruit-body would be hit many 

 thousands of times in a season, rain is probably the most effective mech- 

 anism in the field (Gregory, 1949). In India W. H. Long & Ahmad (1947) 

 find that the bellows mechanism of Tylostoma is operated by wind-blown 

 sand grains in addition to raindrops. 



(iv) Mist pick-up. This is a mechanism that has only recently been 

 recognized. Dry, or even humid, wind fails to detach spores of some fungi 

 which are nevertheless readily removed from their conidiophores by 

 collision with minute droplets carried by mist-laden air. This method is 

 kno^\Tl to function with two important crop pathogens, Cercosporella 

 herpotrichoides (Gl}Tme, 1953), and Verticillium albo-atrum (R. R. Davies, 

 1959), and it may play a part also in the dispersal of Cladosporium. 



(v) Splash dispersal. Spores of some species are 'presented' in sticky 

 masses to which they adhere tenaciously in wind. However, spores may 

 become incorporated in splash droplets (Plate 3, and Fig. 4) which are 

 thrown up from the impact of a falling raindrop, or a drip from a leaf, 

 hitting a liquid film containing spores (Gregory et al., 1959). Rain splash 

 is thus another passive mechanism, quite different from mist pick-up 

 by which slime-spored fungi may become airborne in the smaller droplets. 

 Experiments suggest, however, that the larger splash-droplets, over 50 or 



35 



