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.

SUMMARYLarge transient increases in the concentration of some dry airborne spores coincident with the start of rain suggested that the first raindrops to wet surfaces might disperse spores other than in splash droplets or by wetting fructifications. Experimental collisions between glass beads or water drops and spore-bearing surfaces showed that both rapid air movement in advance of radially spreading splashes and vibration can suspend spores in air. Removal by air movement is most effective when large drops collide with surfaces carrying spores that are loose or raised above the surface.

To measure the rate at which spore clouds were depleted over the sea, spores were collected with isokinetic suction impactors mounted in aircraft of the Meteorological Research Flight, Farnborough. Remote from sources able to replenish spore clouds, preferential deposition from the lower layers caused 'erosion' of the base of vertical profiles of spore concentration leaving maxima at heights between 500 and 1500 m. To determine vertical spore distributions throughout the largest possible distance downwind of the English coast, a saw-tooth flight plan of alternating ascent and descent was used. Of four flights, two encountered favourable weather, rain interfered with sampling on one and unexpected winds across the track converted another into an unintended but interesting cross-wind section of spore distribution.Pollens and Cladosporium spores were counted as examples of large and small spores liberated typically by day, and a composite group of spores liberated in damp air was chosen as an indicator of spores liberated mostly at night. In two flights in fine weather maximum spore concentrations occurred hundreds of miles off-shore. Diagrams showing height, distance from the coast and lines of equal spore concentration (' isospores ') demonstrated discrete clouds of each marker spore type. Pollen and Cladosporium clouds were centred at approximately the same distances from the coast but with the pollen about 500m. lower, probably because the pollen grains sedimented faster. Maximum concentrations o€ the damp-air group sometimes coincided and sometimes alternated with the day-liberated groups. Known periodicities of these spores over land, surface air trajectories and previous weather, suggested that the spore clouds which the aircraft overtook over the North Sea, were the residue of those produced from the British Isles on previous days or nights. Interpretation of the results was limited by meteorological uncertainties, the geographical complexity of probable source areas, and perhaps most by changes in the number of spores crossing the coast at different times of day, which prevented accurate measurement of rates of spore deposition.Spores of many species were recognized over the North Sea. The plant pathogens included established distant migrants such as uredospores of Puccinia graminis, which apparently originated east of the Baltic. The viability of the spores was not tested, but it seems safe to assume that distant transport is both frequent and extensive and probably important in temperate latitudes in summer.

SUMMARYKing Edward and Majestic seed tubers, selected as ‘clean’ (macroscopically symptomless), moderate and severe according to the extent of black scurf, were planted in field experiments at Rothamsted between 1964 and 1968. Seed infection sometimes delayed plants emerging but did not affect final plant populations. Crops from severely diseased seed yielded, on average, 7% less than ‘clean’ tubers (King Edward 6–8% less and Majestic 0–20% less). Seed infection affected tuber size distribution; compared with ‘clean’ seed, severely infected King Edward seed yielded slightly more chats (< 1 ½ in, 3.8 cm) and 1.5 ton/acre (3.8 t/ha) less large tubers (2 ¼–3 ¼ in, 5.7–8.3 cm). The effects were similar with Majestic although differences were smaller. However, total yields from diseased stocks (unselected) seldom differed significantly from the ‘clean’ tubers selected from them. Crops from moderately and severely diseased seed had more Corticium on stems and black scurf on tubers and usually less Oospora pustulans than from ‘clean’ seed.