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.

This article is the seventh in the series 'Taking Stock'N earlier article (Hogg 1964) has dealt with meteorology and agriculture.A The meteorological factors in horticulture are in some respects common to those in agriculture, but a grower is likely to be more conscious of them than a farmer, because of his intensive methods of production. Working on a smaller scale he is often able to modify his environment, reduce weather hazards and increase productivity. In the present article a number of topics have been selected to illustrate the range of problems in horticultural meteorology; if these have been mentioned in connection with agriculture, they are dealt with briefly here, with notes of any change of emphasis. IRRIGATIONThe principles of irrigation for outdoor horticultural crops are the same as for agriculture, and irrigation is well on the way to becoming an essential in many branches of competitive horticulture. Its use helps to obtain a rapid crop succession, thereby making maximum use of the land, and to encourage rapid growth, high quality, high yields and an even stand of crops; harvesting is often simplified if water can be applied a t the right time. A recent report (Anon. 1962) has shown that the yield of many crops may be increased by some 30-60 per cent; in monetary terms the increased value per acre may reach L300 for self-blanching celery, @50-160 for summer cauliflower and lettuce, and LIOO for dessert apples on shallow soil. As a result, fruit, vegetables, potatoes and peas justify irrigation with water a t the high price of 4s per 1,000 gallons.The main problem is to supply the water needed, and recently many farmers and growers have started to store winter rainfall on their land by constructing farm reservoirs, some of which have a capacity of I to 5 million gallons. Even using high-value land, the profit to be made from storing water may be many times the value of the crops which would otherwise have been grown there.In horticulture, as in agriculture, balance sheets may be used to determine the long-term water requirements for a crop or cropping sequence, but apart from bush and tree fruits, very few horticultural crops are in the ground for the whole growing season; adjustment must therefore be made for the shorter growing period. Also, because of differences in rooting depth, there is a wide variation in the permissible soil-moisture deficits, from about half inch for self-blanching celery to 3-5 inches for well-established fruit trees on good soils. As with agricultural crops, soil factors are as important as weather and Winter and Blackwall (1962) have quoted exceptional results of early potatoes which did not respond significantly to irrigation, although there was 234

No abstract

No abstract

No abstract