THE AIR-SPORA NEAR THE EARTHS SURFACE 



(iv) The afternoon pattern develops from noon to i6-oo hours and 

 contains the majority of the day-time spore forms, notably: Cladosporium, 

 Alternaria^ conidia of Erysiphe, many other Fungi Imperfecti, uredospores 

 of rusts, and brand spores of smuts. 



(v) The evening pattern. The early part of the evening, from lyoo 

 to 21-00 hours, forms another interlude with few well-defined maxima, 

 until the nocturnal spora starts at about 22-00 hours, lasting until dawn. 



All these diurnal periodicities are based on mean records for a number 

 of days. Weather on a particular day may disturb the normal rhythm : 

 for example, Sporobolomyces may persist to 10-00 hours or later; anthesis 

 of grasses may be suppressed in dull weather, when pollen concentration 

 may remain low. In the dry season in Nigeria, certain typically afternoon 

 types may occur in the forenoon (Cammack, 1955). One other feature is 

 shown by many organisms, namely, the rise from a low value to the 

 maximum is often steep, whereas the fall is relatively slow; with others, 

 such as Sporobolomyces., the reverse is often true. 



Panzer et al. (1957) refer Leptosphaeria, Epicoccimi., Piricularia., 

 Chidosporiimi, Diplodia, and Ophiobolus, to the 'night spora' with maxima 

 between 18-00 and 09-00 hours, and Nigrospora, Penicillium, Alternaria, 

 Curvularia, and Tricoconis to the day spora, with maxima between o8-oo 

 and 17-00 hours. Some discrepancies with findings in Britain, such as the 

 occurrence of Chidosporium and Epicoccum at night, suggest that local 

 conditions favouring a high concentration in air may be found by day in 

 one climate and at night in another climate. 



The causes of these diurnal fluctuations are complex. Hirst suggests 

 that those spores which are commonest in the forenoon depend on hygro- 

 scopic changes during drying to liberate their spores ; commonest in the 

 afternoon are often those pollens and spores which are passively dispersed 

 by shaking and wind erosion from dry surfaces, and conditions favour 

 their liberation in the afternoon. Some nocturnal forms, such as Sporo- 

 bolomyces., Tilletiopsis, and some ascospores, depend on dew\ High night- 

 concentrations need not always be interpreted as resulting from an increased 

 number of spores being discharged at this time. We do not yet know 

 whether the basidiospore diurnal rhythm (Fig. 20) reflects a diurnal 

 spore-production rhythm, or whether spore emission is relatively constant 

 during the 24 hours, being diluted through the considerable height of the 

 turbulent boundary layer by day, but remaining concentrated near the 

 ground during the night — when eflfects of frictional and thermal turbulence 

 are usually small. 



SEASONAL CHANGES 



Seasons aflfect the air-spora profoundly. Cladosporium and Alternaria 

 show pronounced seasonal periodicity, as of course do the pollens and 

 spores of mosses, pteridophytes, and plant-pathogenic fungi. On the 

 other hand PenicilUum may show little seasonal change, whilst in cities 



119 



