[ 265 ]

Trans. Br. mycol, Soc. 64 (2), 265-281 (1975)Printed in Great Britain

AIRBORNE SPORES IN PASTURES

By J. LACEY

Rothamsted Experimental Station, Harpenden, Herts., ALS 2JQ

(With 5 Text-figures)

Between 1962and 196,1-> fungus spores were trapped periodically from pasturesof different types at Rothamsted and from those elsewhere on which animalshad been affected by grass sickness or photosensitization. Usually, catches weregreatest between June and August with Cladosporium predominant, accountingfor up to 98 % of the total. Plant pathogens and other allergens were oftenabundant although the former were restricted by the distribution of their hostplants. Otherwise management had little effect on the relative abundance ofdifferent spore types, despite long-continued fertilizer treatments having causedlarge changes in the species compositions of some swards. No conclusive relation­ship between fungi and animal disease could be found. Pithomyces chartarum(Berk. & Curt.) M. B. Ellis seems unlikely to cause facial eczema in Britain butTetraploa aristata Berk. & Br. was abundant where one outbreak of photo­sensitization occurred and should perhaps be investigated further.

About half of Britain is covered by pasture. Of this about 6'4 millionhectares are rough grazing, 4'4 million ha permanent grass and 1'7 millionha temporary grass (Ministry of Agriculture Fisheries & Food, 1972).Fungi associated with pasture may grow as epiphytes and parasites onliving leaves, or as saprophytes on dead leaves and stems. Many speciesproduce airborne spores of which some are known to be allergens (Hyde,1972). Plant pathogenic fungi may decrease the yield, quality and palat­ability of the herbage while others may harm grazing animals by produc­ing toxic metabolities.

The spore production of pastures has seldom been measured. In Canada,Salisbury (1966) washed spores from autumn forage and plated dilutionson agar media, while in New Zealand, Brook (1959, 1963) related theoccurrence of facial eczema in sheep to the production of Pithomyceschartarum (Berk & Curt.) M. B. Ellis spores containing the toxic metabolitesporidesmin that causes the disease. Mackenzie (1971) estimated the effectof season and benomyl fungicide on spore production of five plant patho­gens and eight saprophytes including P. chartarum. There uave been nodetailed studies in England, although Pawsey (1964) noted that the sporecontent of air increased when grass nearby was being mown.

This investigation was begun after the discovery of P. chartarum inEngland (Lacey & Gregory, 1962; Gregory & Lacey, 1964) and aimed tomeasure its distribution, seasonal frequency and whether it could causefacial eczema of sheep as in New Zealand. Hepatogenous photosensitiza­tion of sheep, clinically resembling facial eczema and affecting up to 40 %of lambs, occurs regularly in Scotland and northern England, where it isknown as yellowses, plochteach or head grit, and occasionally elsewhere.

266 Transactions British Mycological SocietyAlthough Ender (1960) had suggested it was caused by saponins in the bogasphodel (Narthecium ossifragum (L.) Huds.) growing in marshy ground,this has not been confirmed (Ford, 1964) and fungus spores seemed apossible cause. Grass sickness of horses also seemed to be a disease thatmight be caused by fungal toxins and a neurotoxic factor has been reportedin blood of affected animals (Gilmour, 1973). To test these hypothesis,pastures where photosensitization and grass sickness had occurred weresampled for fungi.

The presence on the Park Grass Experiment and elsewhere at or nearRothamsted, of different types ofpasture, provided sites for measuring theeffects of different species on the production ofairborne spores throughoutthe growing season.

MATERIALS AND METHODS

Spore trapsConstruction

Two forms of spore trap were used. Each contained a rotorod sampler(Perkins, 1957) in a partially enclosed chamber through which passed aircontaining spores removed from grass that had been disturbed by wind orshaking. The chamber was made from aluminium sheet on a slottedaluminium framework. One, used for all sampling near Rothamsted and inScotland in 1962, was mounted on four wheels (Fig. 1a) and incorporatedat the front a small 12 V battery-operated fan inclined downwards atapproximately 450 to the vertical to give a standard airflow over thepasture. Hinged flaps at the front and rear and a polyethylene 'skirt'around the sides excluded the wind and directed air from the fan over arotorod sampler (enclosed in wire netting to prevent grass stems stoppingit rotating and damaging the trapping surfaces) to the back of the trap.The complete trap was pulled behind the operator at walking pace. Amodified version (Fig. I b) without fan and wheels was used in Scotland in1963. To investigate the occurrence of the thermophilic fungi and actino­mycetes in pastures, the wheeled trap was modified to carry a compressedair injector that provided suction of 251/min to an Andersen sampler(Andersen, 1958) in the sampling chamber.

Operation

The traps were usually operated over a standard time and distancewhere areas were being compared regularly. On Harpenden Commoneach traverse was approximately 100 m long and about 2 min duration,while on the Park Grass and Highfield Ley-Arable Experiments a Z­shaped traverse of each plot was made in about I min. On farms, traverseswere varied in length according to the abundance of spores so as toobtain a moderate catch without overloading. Air was sampled at about1201/min.

Spores in pastures.], Lacey

(a)

(b)

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I I

Fig. I. Pasture spore traps. (a) wheeled version; (b) wheel-less version. Key: A, fan;B, C, hinged flaps; D, rotorod sampler; E, battery; F, polyethylene side flaps;........, wire netting; ---, lines not in plain of section; ..-, direction of movement.

Rotorod preparation andcounting

Rotorod trapping surfaces were prepared with adhesive cellulose tape('Sellotape') and molten petroleum jelly as described by Gregory & Lacey(1964). After exposure the tape was stripped off and cut into four lengthswhich were mounted side by side on microscope slides using glycerol jelly.

The number of spores/m3 air was calculated from counts using a micro­scope with x 50 oil immersion objective on traverses usually 19'2 p,m wideacross the four tape segments from each arm. These catches relate only tothe numbers of spores temporarily present in the trapping chamber. Toavoid confusion with the concentrations ofspores present in the air outside,the catches by these traps will be referred to by an arbitrary index, thecatch, equivalent to the spore concentrationjm" air divided by 1000.

Spore classification

Spores, mostly illustrated by Gregory (1973), were classified by morpho­logy and colour. Occurrence of the most abundant types is tabulated.Many others were occasionally common or occurred regularly in smallnumbers,

Media

To isolate fungi, the Andersen sampler was loaded with Petri dishes of2 %malt-extract agar containing 20 units penicillin and 40 units strepto­mycin/ml medium. For actinomycetes, halfstrength 'Oxoid' nutrient agar

268 Transactions British Mycological Society

Fig. 2. Weekly mean daily sunshine, maximum and minimum temperatures and totalrainfall during sampling periods at Rothamsted.

containing 50 mg actidione/ml was used (Gregory & Lacey, 1963). Afterexposure, Petri dishes were incubated at 40 and 60 DC for actinomycetesand 40°C for fungi.

Performance of the traps

Both rotorod traps were tested along parallel strips of grassland invarious configurations. Operating the fan on the first trap decreased thenumber of spores trapped. It was concluded that most spores were releasedby knocking the sward as the trap was pulled forward and that the fanmerely diluted the air within the trapping chamber with air from outside.The polyethylene side flaps were necessary to decrease the rate at whichspores escaped from the sampling chamber.

With the second trap, most spores entered the sampling chamberthrough the bottom aperture. Repeated passages along the same traverseshowed that after the first traverse numbers decreased rapidly at first, butthen more slowly so that after ten traverses the catch was still a quarterof that during the first traverse.

Weather data

Fig. 2 shows weather data from the Meteorological Enclosure atRothamsted which lay between the three main sites in the Harpenden area.

Spores in pastures. J. Lacey

Trapping sites

Two areas on Harpenden Common selected in 1962, were sampledregularly between April and October 1963, and occasionally in 1964. Onearea was mown approximately monthly and the mowings returned to thesward while the other was cut once for hay which was removed afterdrying and baling. Both areas had a similar flora, but finer leaved grasses(Festuca rubra L., Agrostis stolonifera L., Lolium perenne L.) predominated inthe mown area, while tussocks of Dactylis glomerata L. and Arrhenatherumelatius (L.) J. & C. Presl were common in the hay area. Trifolium repens L.was more common in the mown area than in the hay area.

Samples were also taken from contrasting swards resulting from the long­continued fertilizer treatments of the Park Grass Experiment and fromplots receiving different grazing, cutting and fertilizer treatments on theHighfield Ley-Arable Experiment. The plots of the Park Grass experimentthat were sampled and their fertilizer treatment (see Fig. 5) have beendescribed by Warren & Johnston (1964). The Highfield Ley-ArableExperiment has been described by Rothamsted Experimental Station(1970). Access to both experiments was limited so that sampling waserratic.

During 1962 and 1963, visits were made to farms in Scotland andNorthern England where photosensitization of sheep had occurred. Inaddition, visits were made to farms in Wales and Southern England wheregrass sickness in horses or, once, photosensitization of sheep had occurred.Pastures were always sampled when dry, during the late morning orafternoon, except during brief visits to distant farms, when this was notalways possible.

RESULTS

Seasonal variations in spore production

The two areas of Harpenden Common were sampled regularly alongtwo traverses of the mown area and three of the hay area in 1963 (Fig. 3).Isolated samples taken in 1962 and 1964 were also compared with thoseof 1963.

Few spores were caught in either area in April 1963 ('catch' = 40-150).A maximum catch of more than 5000 occurred in July, but numbersdecreased to 800 in October. At first Cladosporium spores were few but afterthe first mowing in late May the catch increased to more than 4000 and itremained the most abundant spore type in the mown area. Catches ofCladosporium spores were again large about 2 weeks after other mowings,but declined to about 300 between mowings. More Cladosporium sporeswere usually caught from the hay area than from the mown, with amaximum of 4600 after the hay crop was cut in July.

The proportion of spores other than Cladosporium was greatest in Apriland May. Botrytis, Epicoccum, myxomycetes and Helminthosporium were allpresent in small numbers, but sometimes the largest component wasunidentified spherical brown spores in the mown area or small spheri­cal hyaline spores, usually in clumps, in the hay area. Late in May,

27° Transactions British Mycological Society

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Spores in pastures. J. Lacey

Hay area

Fig. 3 (b). Log mean catches of different spore type on Harpenden Common in 1963.

18 MYC 64

272 Transactions British Mycological Society

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myxomycetes became numerous in the mown area and catches then usuallyremained between 100 and 400 but in the hay area they exceeded 100 onlyin June. At least two types of myxomycete spores were recognized. Mostwere yellow-brown and spinulose but others were grey, or pinkish withreticulate surface markings.

Botrytis, Epicoccum and Helminthosporium all increased from late June tomaxima in July and August, but only Epicoccum increased after mowing(like Cladosporium), but possibly after a longer delay. Alternaria occurredonly after early July and was commonest (catch = 40) early in August.Pithomyces chartarum was restricted to only one traverse of the mown areawhere it increased slowly to a maximum catch of only 5'4 in late August,and then declined (Fig. 4).

Plant pathogens occurred seasonally. Ustilago was found only in Juneand July, when grasses flowered, with a maximum catch of430 just beforethe hay was cut. Another smut fungus, Urocystis, occurred between mid­June and late August. Polythrincium trifolii Fr. (stat. conid. of Cymadotheatrifolii Wolf) appeared in lateJune and increased to give a maximum catch(57-75) in August and September, while rust uredospores appeared onlyin late July reaching maxima in early September in the mown area andin October when hay was cut. Numbers ofErysiphe spores were greatest inthe hay area in July just before the crop was cut, then decreased greatlybefore again increasing with the growth of aftermath in September. FewErysiphe spores were caught from the mown grass.

Of the spores not listed in Fig. 3, unidentifiable types were often themost common. Tetraploa aristata Berk. & Br, occurred regularly in the areacut for hay where cocksfoot was common (maximum catch 3). Torulaoccurred seldom but reached a similar maximum count. Ascospores andbasidiospores were frequently trapped but catches were small and varied

Spores in pastures. J. Lacey 273spasmodically. Occasional constituents included Peronospora, Entomoph­thora, Coremiella, Humicola, Scolecotrichum, Penicillium and Fumago types. Algalcells were trapped until early May and sometimes cysts of Thecamoeba andother protozoa.

Mown and hay areas were both sampled on similar dates in July 1962,1963 and 1964 and in September 1963 and 1964. Catches from both areaswere similar in July each year (5000-10000), except that in July 1962where hay was lying the catch was nearly three times greater than fromthe mown area. In September, the catches were only about 1000, withCladosporium forming from 12 to 80 % of the total compared with 80­98 %in July.

Cladosporium usually dominated the catches but less numerous typessometimes behaved differently in the two areas. Botrytis gave a catch of570 in the hay area in July 1963, 70 times greater than the mown area,but in July 1962 the catch was only 3, compared with I I in the mown area.Many minor components were less common in July 1962 thanJuly 1963.Helminthosporium gave a similar catch each year on the hay area but catcheson the mown area were greater in 1963 than in 1962 and still greater in1964.

In September 1964, Botrytris, myxomycetes and Helminthosporium (hayarea only) were less numerous than in the previous year, while Epicoccum,Helminthosporium (mown area) , Alternaria and Pithomyces were morenumerous. The latter group may have been favoured by the warm, drysummer of 1964, although it is difficult to explain why Helminthosporiumwas affected differentially in the two areas. The catch of Pithomycesexceeded 100 on this occasion only.

Warm weather in 1964 also affected plant pathogens. Ustilago hadcompleted its season before sampling in July 1964, while uredospores weremore numerous than in previous years. Erysiphe and Polythrincium trifoliiwere most numerous in 1963.

Samples taken at different times from the Park Grass and HighfieldLey-Arable Experiments varied similarly to catches from HarpendenCommon. Exceptionally, Ustilago was abundant on plot 14 (unlimed) ofthe Park Grass Experiment with abundant Arrhenatherum elatius in July1964 and many rust teleutospores on several plots of the same experimentin May 1963. Ustilago was also surprisingly common on plots of the ley­arable experiment in September 1963.

Variation between different swards

Long-continued fertilizer treatments have caused large differences in theflora and productivity of different plots of the Park Grass Experiment. Theunmanured, unlimed plot 3 contains the greatest diversity of species whilethe unlimed half of plot I, receiving ammonium sulphate alone, hasbecome dominated by grasses tolerant of acid soil, particularly Agrostistenuis Sibth. and Festuca rubra. Plot 11/1 receives the most ammoniumsulphate with other minerals and the unlimed half is dominated by Holcuslanatus 1., while the limed half, dominated by Alopecurus pratensis 1. givesthe largest yields and best quality hay. Detailed studies of the flora of eachplot have been described by Brenchley (1958).

274 Transactions British Mycological Society

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Fig. 5 (a). Incidence of different spore types on plots of the Park Grass experiment.Nt>N2, Ns, Ammonium sulphate applied at 210, 420, 630lbjacre; N1*, N2*, sodiumnitrate applied at 275, 550 lbfacre; P, superphosphate applied at 392lbfacre; K,potassium sulphate applied at 500 lbfacre; Na, sodium sulphate applied at 100 lbfacre;Mg, magnesium sulphate applied at 100 Ibfacre.

Spores in pastures. J. Lacey 275

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Transactions British Mycological Society

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Limed and unlim ed halves of each plot were sampled separately on fouroccasions between August 1962 and August 1964. Th e mean catches fromeach half-plot are shown on Fig. 5-

The mean catch from different half-plots ra nged from 1500 to 3400 ofwhich 54-87 % (II OO-2700) were Cladosporium (Fig. 5). Air samples fromthe un manured, unlimed half of plot 3 gave an average catch of 2100 ofwhich 83 % were Cladosporium. Botrytris was second most abundant(catch = 186) but constituted only 9 %of the total and Epicoccum (67)accounted for only 3 %. Other spore types each accounted for less thanI %of the total. A single spore of Pithomyces chartarum was found in thecatch from August 1964 (when it was most abundant on HarpendenCommon). Plot 1 (without lime) gave a similar catch (2800) to plot 3 andthe proportion of Cladosporium (84 %) was almost identical. However,Botrytis (catch = 35, 1 %of the total) and Erysiphe (2; o·1 %) were fewer,while Epicoccum (170 ; 6 %), myxomycetes (46; 2 %), Ustilago (10; 0-4 %)and Helminthosporium (170; 6 %) were all more numerous.

Despite large differences in yield of grass and flora, the two halves ofplot I 1/1 gave similar total spore catches (2900 unlim ed, 2600 limed ), butthe limed half yielded fewer Cladosporium spores (69 %of total) than theunlimed (90 %), so more spore types contributed appreciably to the total.

Spores in pastures. J. Lacey 277Only Epicoccum (catch = 300) exceeded I % of the total spores on theunlimed half, but with a catch of 200 (8 %) of the limed half it was out­numbered by myxomycetes while Urocystis, Torula and Botrytris eachcontributed more than I %to the total.

Considering the large differences between the floras of half plots, sporecatches were very similar both qualitatively and quantitatively. Usuallythe differences were no greater than those already described for thecontrasting plots. However, Helminthosporium was usually much moreabundant in the acid, unlimed halves of plots (but not II / I) than in thelimed halves, and most occurred in the unlimed halves of plots I, 9 and10. Plant pathogens tended to be more irregularly distributed probablyreflecting differences in host distribution with Urocystis most in plot II/I(limed), Ustilago in plot 14 (limed) and rusts in plot 14 (unlimed).Polythrincium trifolii spores correlated closely with the occurrence ofTrifolium repens and was therefore more common on the limed plots.Erysiphe differed from other plant pathogens, occurring on nearly everyplot, particularly on the limed halves of plots 7 and 9 where legumes wereencouraged by the potassium and phosphorus applied.

Effects ofmanagement on spore production

There were few differences that could be attributed to management.The effect of mowing on Cladosporium and Epicoccum and of cutting for hayon Erysiphe spore catches has already been noted. Urocystis and myxomycetespores were also more common on the mown area of Harpenden Common,while Botrytis, Epicoccum, Helminthosporium and Alternaria were moreabundant in the hay area.

Average total spore catch under the management treatments of theHighfield Ley-Arable Experiment varied between 590 and 1360. Perma­nent and long-term reseeded grass plots (sown in 1949) gave similarcatches, but both yielded fewer spores than second- and third-year leytreatments. The third-year grass clover leys receiving little nitrogen thatwere grazed by sheep yielded more total spores, Cladosporium, Epicoccum,Ustilago, Helminthosporium, Polythrincium and Erysiphe, but fewer Botrytis,myxomycetes and rust uredospores than similar leys cut for silage. Secondyear grass clover leys lacking nitrogen and cut for silage yielded fewerCladosporium and more Botrytis spores than the older leys, but were other­wise intermediate between the grazed and conserved treatments.

Spore production ofpastures associated with animal disease

Outbreaks of grass sickness were sporadic and only five farms could bevisited. These usually yielded spore types similar to those of other pastures,with Cladosporium predominant. However, these were occasionally few andmyxomycetes were then most numerous. Fusarium-like spores were trappedin small numbers from two farms.

Photosensitization of sheep in Northern England and Scotland occursmostly in the spring, and pastures were sampled in 1962 and 1963 duringJune, where possible comparing nearby affected and unaffected pastures.

278 Transactions British Mycological SocietyThe pastures were mostly rough moorland grazing with Calluna vulgaris(L.) Hull, Trichophorum caespitosum (L.) Hartman, Erica cinerea L., E.tetralix L., Potentilla erecta (L.) Rausch., Molinia caerulea (L.) Moench andNardus stricta L. Among the most abundant species, Narthecium ossifragumwas widespread; it occurred in 70 %of random quadrats (30 em square)in affected areas and in 45 % of quadrats in unaffected areas, and hadbeen grazed in both.

Samples with a pasture spore trap were supplemented with largersamples using a carried or stationary basket trap (Gregory & Lacey, 1963).On more inaccessible pastures, the basket trap was the only one that couldbe used, but it gave smaller catches.

Owing to the time of year and unfavourable weather during 1962,catches were small. Even in Harpenden, catches were small before the endof May, while in Scotland, particularly in the north and during 1963,growth started later. Qualitatively, the pattern was similar to that furthersouth. Cladosporium was rarely outnumbered by other fungi, and Epicoccum,Alternaria and myxomycetes were usually the next most abundant sporetypes. Botrytis was usually rare, but helicoid conidia resembling Helico­sporium, four-celled brown ascospores resembling Sporormia and occasionallypill-box-like spores resembling Coremiella ulmariae (MacWeeney) Masonwere found. Algal cells and Thecamoeba cysts were also trapped.

Two affected localities were lowland pasture, permanent grass exceptfor some short-term leys on one farm. Narthecium ossifragum was absent fromboth. Cladosporium was abundant at both sites, together with Epicoccum,Tetraploa aristata, myxomycetes and other spore types. T. aristata wasparticularly abundant at the one site in the south of England where photo­sensitization was reported in July and where Dactylis glomerata wasabundant on pasture overlying chalk, giving a catch of up to 16.

Isolation oj thermophilic fungi andactinomycetes from pastures

Thermophilic fungi and actinomycetes are important in the mouldingof hay associated with farmer's lung disease. Festenstein et al. (1965)suggested that grass at haymaking carries a small, but uniform, inoculumwhich does not become obvious unless heating occurs.

Thermophilic and thermotolerant fungi were isolated from all samples(Table I). Most abundant were Aspergillus fumigatus Fres. A. nidulans(Eidam) Wint. and Absidia sp., but Humicola lanuginosa (Griffon &Maublanc) Bunce occurred regularly, in small numbers. Other speciesoccurred only occasionally.

Thermoactinomyces vulgaris Tsiklinsky was the most common of thethermophilic actinomycetes (Table 2) and was sometimes abundant onplates incubated at 60 cC but Micropoiysporafaeni Cross, Maciver & Lacey,the most potent source of farmer's lung antigens, grew only occasionally.However, colonies of M. faeni may have been obscured by the bacteriathat grew on isolation plates. Saccharomonospora (Thermomonospora) viridis(Schuurmans, Olson & San Clemente) Nonomura & Ohara, Nocardia spp.and Streptomyces spp., particularly those with grey aerial mycelium, oftengrew at 40 cC.

Spores in pastures. J. Lacey 279

Mean catch·Species

Table I. Occurrence of thermophilic and thermotolerant fungi on Rothamsted fieldsand Harpenden Common

Occasions trapped(out of 6)

Absidia spp.Aspergillus fumigatus Fres.A.flavus LinkA. nidulans (Eidarn) Wint.A. niger van Tiegh,Humicola lanuginosa (Griffon & Maublanc) BunceMucorpusillusLindtPaecilomyces varioti Bain,Penicillium piceum Raper & FennellTalaromyces thermophilus StolkThermaascus aurantiacus MieheT. crustaceus (Apinis & Chester) StolkOthers

462I

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5I

I

5I

I

I

2

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< 0'010·060'020'020'°30'01

< 0'01< 0'01

O'II

• Catch = colonies isolatedjm" x 10-3 ; see text p. 267.

Table 2. Occurrence ofthermophilic andthermotolerant actinomycetes on Rothamstedfields and Harpenden Common

0'107'g80'02

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444

4I

810

7

Occasions trapped

(out of 14)2

14I

Species

Isolated at 60°CMicropolyspora faeni Cross, Maciver & LaceyThermoactinomyces vulgaris TsiklinskyThermomonospora spp. (white)

Isolated at 40°CMicropolyspora faeniNocardia sp.Saccharomonospora viridis (Schuurrnans, Olson &

San Clemente) Nonornura & OharaStreptomyces albus (Rossi-Doria) Waksman & HenriciS. griseus (Krainsky) Waksrnan & HenriciStreptomyces spp. (grey)Thetmoactinomyces vulgarisOthers

'" Catch = colonies isolatedjrn" x 10-3, see text p. 267.

DISCUSSION

The pasture spore traps enabled the numbers and types of fungus sporesproduced on different pastures to be estimated and compared. By tempor­arily enclosing within the sampling chamber air bearing the sporesreleased from small areas of pasture, samples could be taken from neigh­bouring small plots with negligible cross-contamination. The traps werealso lighter and easier to make and use than the volumetric spore trapdesigned by Brook (1959), which impacts spores in a stream of air onto asticky microscope slide. However, air movement around the rods iscomplex and the process of spore deposition is not fully understood(Bainbridge, personal communication). Spores of saprophytes, especiallyCladosporium, were numerous and some plant pathogens with distinctivespores were easily recognized. Such traps might be useful for studyingthe epidemiology of forage diseases, particularly early in epidemics whenlesions may be too few to be assessed visually.

280 Transactions British Mycological SocietyThe survey, although limited, indicated that grassland of widely differ­

ent types constitutes a vast source of spores, because catches from airretained within the sampling chamber and close to the source of spores,were many times greater than spore concentrations estimated with auto­matic volumetric spore traps I m or more above ground where concentra­tions are usually less than 105 spores/m3.

The times and conditions when these traps were operated meant thatsome spore types common in the air only at certain times of day or inparticular weather were rare or not found. For instance, Sporobolomycesspores, often abundant at night and easily isolated from grass leaves, werenot found and ascospores, requiring moisture for their release, were few.The spore types caught more frequently were those known to be importantcolonizers of senescent and dead grass (Webster, 1956, 1957; Hudson &Webster, 1958; Webster & Dix, 1960). Cladosporium, Alternaria andEpicoccum are primary colonizers which spore most abundantly in thesummer of flowering. Helminthosporium and Tetraploa spore on the deadstem bases one year later. Plant pathogens were also an important com­ponent. The role of Botrytis is uncertain; the spores were common, but itwas not recorded by Webster and his colleagues as a colonizer ofmoribundor dead grass tissue.

Spore types varied seasonally, as would be expected from their occur­rence in the atmosphere generally in Britain (Hyde, 1972). Similar trendshave been found in New Zealand pastures (Mackenzie, 1971).

The fungus flora seemed little affected by management or fertilizertreatment of the grass, age of the sward and geographical location. Eventhe large variations in species composition between the Park Grass plotsmade few large differences. Northern hill pastures produced fewer sporesthan lowland pastures in the south, but with few exceptions the specieswere similar. Some small numbers probably resulted from the date ofsampling and from wet weather. Mowing produced a temporary effect,with increased Cladosporium and Epicoccum catches about 2 weeks later,although the overall yield of spores was less than that of the nearby areacut once for hay. Many of the spores trapped are important componentsof the air spora and well known as allergens. Cladosporium is the mostcommon (Hyde, 1972) but the thermophilic actinomycetes that causefarmer's lung have not previously been found in pasture close to haymakingtime, although their presence has long been presumed.

No fungus was conclusively related to animal disease. Even thoughPithomyces chartarum was found (maximum catch = 100) it seems unlikelyto cause photosensitization in Britain. Apart from one area of HarpendenCommon only isolated spores were found. One lowland outbreak of photo­sensitization was associated with many spores of Tetraploa aristata, anotherlarge spored dematiaceous hyphomycete. Possibly its role should beinvestigated, but as it occurred only sporadically and seemed unlikely tobe implicated in outbreaks on hill pastures, this was not done.

I thank Mrs Maureen E. Lacey and Mr D. R. Henden for technicalassistance and the many people who provided information on outbreaksof animal disease and assisted with arrangements for farm visits.

Spores in pastures. J. Lacey 281

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(Accepted for publication 29 August 1974)