SUMMARY: Airborne 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 1 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 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 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 10 spores/m. occurred at heights up to 1000 m. and hundreds/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. 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.


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