With Plate I I and z Text-figures)A suction trap has been made in which the spores entering a narrow orifice, directed into the wind, are impacted on a Vaseline-coated microscope slide moved across the orifice at z mm./hr. Estimates of spore content of the air can be made, with higher efficiency than by previous traps, at different times of day and thus be more closely correlated with variations in weather.Wind-tunnel tests with spores of Lycopodium clavatum showed maximal and minimal efficiencies of 93.8 and 62.4% respectively, with a suction rate of 10.0 1./ min., in the range of wind speeds from 1-5 to 9.3 m./sec.

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SUMMARY:The air over an arable field at Rothamsted Experimental Station, Harpenden, was sampled from 1 June to 25 October 1952 at 2 m. above ground with an automatic volumetric spore trap. Each day's slide was scanned and all the spores counted on an area representing a sample volume of 41 1. of air. Spores were classified in 20 morphological groups and a miscellaneous one. Seasonal periodicities are presented as 6-day running means of the daily average number of spores/m.8 air, and then related to meteorological data. Cladosporium conidia accounted for 46 yo of the total catch; hyaline basidiospores (chiefly Sporobolomyces) for 31 yo; and pollen only 1 %. The relative frequency of various spore types differs from that recorded by earlier workers because the suction trap catches spores of all sizes with almost equal efficiency and is little influenced by external conditions. The results which should be representative of large rural areas of central and south England show that the major changes of spore concentration depend on the weather and the phenology of the local vegetation and its associated fungal flora. During 24 days in late June and July comparable estimates of spore concentration were made with another trap 24m. above ground. The spore concentration of the twelve commonest groups a t 24 m. totalled 82 yo of that at 2 m.The sampling of air continuously over long periods for air-borne microorganisms was first practised by Schoenauer Both slides and Petri dishes catch large spores more efficiently than small ones and the efficiency of both depends on wind speed; also dishes are exposed only for short periods and record only those organisms that grow to form macroscopic colonies. The data we present are derived from the catches by automatic volumetric spore traps, which are much less affected by changes in wind speed and which catch spores of greatly differing size with almost equal efficiency (Hirst, 1952). The catches, examined microscopically under an oilimmersion objective, are therefore thought to reveal the relative and absolute frequency of all air-borne spores larger than 8p. more accurately than was possible until now. The method is limited by the difficulty of classifying spores visually; more specialized methods will be required for further study of many groups.

S U M M A R YAirborne spores can be carried long distances, but little is known about the atmospheric transport processes involved or the rates at which spore clouds are depleted. Aircraft sampling is expensive and inevitably intermittent, and surface traps reveal only some of the processes involved. The best compromise is to combine surface and aircraft observations and to support both with detailed meteorological interpretation. Gravity slide traps exposed for r day indicate the arrival of spores less precisely than moving-slide impactors, which therefore provide a more accurate starting time for estimating the past track of spores from air trajectories. Catches of Puccinia graminis uredospores from continental European sources illustrated how immigration depends on the movement of atmospheric pressure systems and the gradients within them and suggested that in addition to surface air movement winds at the 700 and 500 mb. levels were important.Aircraft of the Meteorological Research Flight, using suction impactors which operated approximately isokinetically, sampled air in the lower troposphere, both to ascertain vertical spore profiles over land and to intercept immigrant Puccinia graminis uredospores over the English Channel. The vertical distribution of spores seemed to be determined in the same way as that of other aerosol particles; atmospheric turbulence was a major factor and there were indications that wind shear, precipitation and surface deposition might be important. However, most spores are liberated periodically and so encounter different degrees of atmospheric turbulence depending on the diurnal periodicity of their concentration near the ground. Concentrations of 1 0 4 s p~r e s / m .~ occurred at heights up to I O O O~. and h~ndreds/m.~ at 3000 m. In unstable air spore concentrations often declined roughly logarithmically with height, but layers of stable air were often associated with abrupt changes of concentration. Details of vertical spore profiles also depended on the history of both the temperature profile and the spore cloud. Such factors tended to affect all spore types similarly: but occasionally some components, e.g. P. graminis uredospores, showed unique vertical profiles. One such profile, characterized by preferential ' erosion ' of the spore cloud from air near the surface, may indicate travel remote from sources. Spores of plant pathogenic fungi were frequent in samples of air moving northward over the English Channel but their viability was not tested. I N T R O D U C T I O NBiological pollution of the atmosphere is chiefly by small organisms or propagules which can remain suspended in air, so most reports concern bacteria, insects or the spores of plants. This paper describes work done to study movements of plant pathogenic fungi, but pollen grains and the spores of saprophytic fungi are also mentioned.

SUM MARYThe prevalence of Venturia inaequalis ascospores in orchards was compared both in terms of the number of spores per volume of air (dose) and the number produced per area of dead leaf (productivity). The two parameters often gave divergent estimates, probably because dose depends on total leaf per unit area of ground whereas productivity does not. Differences in the amount of dead leaf surviving until bud-burst in different orchards or years were enough to explain the anomalies and suggested that because earthworms often removed over 90% of the fallen leaves by spring, they exercised an important control of ascospore number.Assessments of fruit surface scabbed, an accepted way of judging scab control, were not related to ascospore productivity the following spring, but estimates on the percentage of all leaves infected at leaf-fall were. Other natural variables, like the dates when leaves were formed or fell, seemed of minor importance, especially when compared with effects of earthworms or chemicals. Applying dinitro-ortho-cresol to dead leaves in spring decreased the number of ascospores liberated by at least 9076 and there is evidence of a similar effect from broadcast ammonium sulphate. The spring and summer spray programmes were also important. Measurements in several orchards during an %year period showed that dose and productivity both declined to less than a hundredth of the highest levels, but in a single wet year productivity in sprayed orchards increased several times and by 40-fold on unsprayed trees. Ways to prevent high ascospore doses occurring are discussed, together with possible causes of past severe attacks of apple scab at Wisbech.