THE AIR-SPORA NEAR THE EARTH S SURFACE 



ejected. Spores of Ophiobolus graminis, the wheat take-all pathogen, do 

 not occur in the air during dry weather, but they reached a concentration 

 of 3,700 per cubic metre in air over wheat stubble within 2 hours of the 

 fall of 1-3 mm. of rain; a few ascospores were liberated by as little as 

 0-25 mm. of rain (Gregory & Stedman, 1958). Ascospores of some species 

 are evidently discharged when the ground is wet with dew, and these 

 types appear as part of the nocturnal air-spora. 



Little is known yet about the composition of the damp-air spora, or 

 about the occurrence of spores taking-off in rain-splash, and their study 

 awaits improved technique. 



Hamilton (1957) studied correlations at two centres (London and 

 Rothamsted) between spore concentration of twent\'-eight visual types 

 and the weather. Her main positive findings are as follows. Rainfall had 

 no effect on the atmospheric concentrations of hyaline basidiospores 

 (including those of Nolanea, Lactarius^ Tilktiopsis^ and possibly Sporo- 

 bolomyces). The concentrations of pollen and of most t}'pes of spore 

 decreased with rain, but all ascospore types and Helicomyces increased 

 with rain. In half of the types studied, concentrations were significantly 

 increased by increases in temperature, dew-point, or relative humidity. 

 The only significant decreases were in grass pollen (and possibly Ustilago 

 spores) with increased relative humidity, and in Nolanea with increased 

 dew-point. Sunshine had no significant effect except for positive corre- 

 lation with Ustilago and algal groups i^Gloeocapsa'). Increased wind 

 significantly decreased the concentration of Alternaria, some basidio- 

 spores (including Ganoderma, Tilletiopsis, and Sporobolomyces)^ Botrytis, 

 Cladosporium, Entomophthora, PuHularia, uredospores, insect fragments, 

 and Urtica pollen. By contrast, plant hairs and algal groups (^Gloeocapsa^) 

 were increased by increasing wind-speed — possibly because both are re- 

 leased by friction. Gustiness was associated with increases in Alternaria^ 

 filiform ascospores, and Ustilago. 



Conidia of Cladosporium., one of the best studied of the spore t}''pes, 

 show an interesting anomaly in relation to weather. Hamilton (1959) 

 found an appreciable decrease in their number during the hours when rain 

 was falling, but Ainsworth (1952), Hirst (1953), and Gregory (1954) 

 demonstrated a transient increase in concentration of Cladosporium spores 

 when rain started to fall. So far this phenomenon remains unexplained. 



BIOTIC FACTORS 



Human activity can also play a part in affecting atmospheric spore 

 concentration. Mowing and tedding of grass can produce a great and 

 immediate local increase in Cladosporium and Epicoccum spores, and 

 (with an apparent delay of 2 hours) of grass pollen (Sreeramulu, 1958). 

 Threshing of grain produces a local spore source (Heald & George, 191 8). 

 The role of overhead irrigation must not be overlooked, and spraying 



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