Dispersal potential of spores and asexual propagules in the epixylic hepatic Anastrophyllum hellerianum

Dispersal ability is of great importance for plants, which commonly occupy spatially and temporally limited substrate patches. Mixed reproductive strategies with abundant diaspore production are favoured in a heterogeneous landscape to ensure successful colonisation at different distances. In bryophytes, long-distance dispersal has been thought to take place primarily by spores, while asexual propagules are important in local dispersal and in the maintenance of colonies. In the present study, we investigated the dispersal potential of two equally sized propagules, sexually formed spores and asexually produced gemmae in the dioecious, epixylic hepatic, Anastrophyllum hellerianum, which inhabits spatially and temporally limited substrate patches. We trapped propagules at different distances (0–10 m) and directions from the source colonies in two experiments: one in a natural habitat within a forest and another involving an artificial set-up in an open habitat. Spore dispersal showed only slight distance dependence both in the open and the forest habitats, presumably as a consequence of wind affecting the dispersal pattern. Gemma dispersal was more strongly distance-dependent in the open habitat than in the forest sites. Considerably more gemmae were deposited during rainy than dry periods, possibly because of the effect of rain drops on gemma release. However, weather conditions had no effect on the dispersal patterns of spores or gemmae. In A. hellerianum, the combination of occasional spore production and practically continuous, massive gemma production facilitates dispersal both on local scale and over long distances. Unlike previously assumed, not only spores but also the asexual propagules may contribute to long-distance dispersal, thus allowing considerable gene flow at the landscape level.     

Dispersal of Septoria nodorum Pycnidiospores by Simulated Rain and Wind

The influence of wind on the splash dispersal of Septoria nodorum pycnidiospores was studied in a raintower/wind tunnel complex with single drops or simulated rain falling on spore suspensions or infected stubble with windspeeds of 1.5 to 4 m/sec. When single drops fell on spore suspensions (depth 0.5 mm, concentration 7.8 × 10⁵ spores/ml) most of the spore-carrying droplets collected on fixed photographic film between 0–4 m downwind (windspeed 3 m/sec) were >200 μm in diameter. However, most spores were carried in droplets with diameter > 1000 μm, 70 % of which carried more than 100 spores. When simulated rain fell on infected stubble most of the spore-carrying droplets collected beyond 1 m downwind (windspeeds 1.4 and 4 m/sec) were <200 μm in diameter and none were >600 μm; most of these droplets carried only one spore. The distribution of splash droplets (with diameter >100 μm) deposited on chromatography paper showed a maximum at 40–50 cm upwind of the target but many more droplets were deposited 20–30 cm downwind, when single drops fell on a spore suspension (concentration 1.2 × 10⁵ spores/ ml) containing fluorescein dye with a windspeed of 2 m/sec; droplets were collected up to 3 m downwind but not more than 70 cm upwind. With a windspeed of 3 m/sec, numbers of sporecarrying droplets and spores collected on film decreased with increasing distance downwind; most were collected within 2 m of the target but some were found up to 4 m. When simulated rain fell on infected stubble, increasing the windspeed from 1.5 to 4 m/sec greatly increased the number of spores deposited more than 1 m downwind. At 1.5 m/sec none were collected beyond 2 m downwind, whereas at 4 m/sec some were collected at 4 m. A few air-borne S. nodorum spores were collected by suction samplers at a height of 40 cm at distances up to 10 m downwind of a target spore suspension on which simulated rain fell.     

Vertical Concentrations of Fungal Spores above Wheat Fields

Kramer-Collins volumetric spore samplers were used to measure concentrations of Puccinia graminis, P. recondita, Erysiphe graminis, Cladosporium, and Alternaria spores and fungal hyphal fragments within the canopy and at 1, 3, 6 and 14 m above ground level over wheat fields near Manhattan, Kansas, USA. The largest numbers of spores of each of the named fungi and hyphal fragments were trapped during hours when free moisture was not present on host leaf surfaces. As wind velocity increased, the number of spores and hyphal fragments trapped at all heights increased. Airspora trapped at the various sampling heights under all combinations of biometeorological conditions were calculated as ratios. Location and severity of infection or frequency of occurrence of the parent fungi greatly affected the percentage of propagules released in the canopy and escaped into the atmosphere. Less than 40% of P. recondita urediniospores released from tillering plants in the fall and trapped at 20–25 cm were trapped at 1 m, while only 6% of those trapped at 10 cm were trapped at 1 m. Ratios for the average number of spores trapped at 3 m: 1 m during the entire sampling period were similar for P. recondita (0.54), Alternaria (0.57), and E. graminis (0.48). However, the higher ratios that occurred with spores of P. graminis (0.72) and Cladosporium (0.76), and hyphal fragments (0.77) indicate considerable mixing of locally released airspora with airspora from exogenous sources.     

Some components of the air spora in Jamaica and their possible medical application

A knowledge of the pollen and fungal spores which comprise the air spora is useful as a preliminary approach to the problem of respiratory allergy. Therefore, this study of the qualitative and quantitative aspects of the air spora was done. Fungal spores were found to be numerically dominant, comprising 97.73% whilst pollen comprised 0.40% of the total material observed. A small number of types made up the majority of the fungal air spora, namely, Cladosporium, the Sporobolomycetaceae group, Diatrype, Glomerella, hyaline and coloured basidiospores, and septate fusiform spores. Seasonal periodicity studies on twenty-five fungal types showed that a high number of spores were trapped for sixteen during wet months, four during cooler months, and that five showed no seasonal trends. Mean diurnal periodicity studies for the year on the same twenty-five spore types showed that all had a maximum number of spores trapped at some time during the day. Investigation of the effect of rainfall on the numbers of spores released showed that the amount and duration of rainfall, the time of day rain occurs, and the length of the dry period preceding rain were of varying importance to particular spore types.     

Wind Dispersal of Alternaria alternata, a Cause of Leaf Blight of Cotton

Introducing Alternaria alternata, the cause of blight disease of cotton plants, into a field of young healthy plants growing in rows cross-wind, yielded disease foci which were spread downwind up to 7 m from the infection sources. Only light disease incidence was found in the remainder of the field. When the disease was introduced into a field of mature cotton plants grown in rows cross-wind, randomly scattered disease foci occurred. In mature plantations where rows were parallel to the average wind direction, only limited size disease foci developed downwind, up to 16 m from the source. These foci did not developed further during the season. The number of air-borne spores of A. alternata was significantly increased by the presence of diseased cotton plants, being highest close to the diseased plants. The spores were transferred to a distance of at least 20 m. However, the number of air-borne spores significantly decreased 6 m from the infection source. Periodical trapping of air-borne spores of A. alternata in a cotton growing region for 2 years, revealed that their air dispersal is local, probably at the field level. A. alternata in a cotton growing region for 2 years, revealed that their air dispersal is local, probably at the field level. A. alternata air-borne spores were also trapped in rather low numbers regardless of the presence of infected cotton plants. However, the number of the air-borne spores trapped was dependent mainly on the average wind direction and on the Alternaria blight epidemics occurring in the fields twice a year. It is suggested that A. alternata spores are transferred by wind for short distances but are constantly present in small numbers in the atmosphere throughout the whole year. The two peaks recorded for the number of spores present in the air above cotton crops correlate with the annual two outbreaks of Alternaria blight epidemics. In addition, both wind and plant row direction affect disease development in the fields.     

Dispersal of Erysiphe graminis and Lycopodium clavatum spores near to the source in a barley crop

In experiments to study dispersal of spores in a crop of barley, a 4-m wide strip of the cultivar Zephyr (a mildew-susceptible variety) was a source of mildew (Eyrsiphe graminis) conidia. Small suction traps, previously calibrated in a wind tunnel, were used to measure spore concentration within and above the crop. Large concentrations of conidia were measured in the crop at the downwind edge of the Zephyr strip but these decreased rapidly with distance downwind. At 1 m concentrations were halved and by 4 m they were no greater than the background concentration measured in the crop upwind of the source. Next to the source, concentrations were much greater within than above the crop and net spore movement (flux) was upwards out of the crop; by 4 m downwind concentrations were greater above the crop and spore flux was reversed. Lycopodium clavatum spores were released in the same crop from a line of point sources. Concentrations also decreased rapidly downwind but, with no background of spores, numbers remained greater within than above the crop further from the source than for E. graminis. Even so by 7 m downwind concentrations in the crop had declined to less than those above. Deposition of L. clavatum spores onto horizontal glass slides in the crop agreed with that expected by settling. However, impaction onto vertical cylinders among plants was much greater than predicted. The reason is not known although turbulent air-flow around plants may, in some way, enhance impaction. Many E. graminis conidia near the source were deposited in clumps. This prevented any accurate prediction of deposition rates as fall speeds of clumps (necessary for prediction) were not known. Not surprisingly, deposition on horizontal slides often exceeded that expected from settling of single spores although it was not always greatest where clumps predominated. The proportion of spores deposited on vertical cylinders and horizontal slides located among plants ranged from 0–02 to 0–27 and from 0–019 to 0–127 of the area dose, respectively. Although these may seem to be small trapping efficiencies, the same deposition rates in a crop with many leaves and stems would rapidly filter most spores from air in the crop and can explain why concentrations were observed to decline so rapidly.     

Seasonal changes in primary and secondary inoculum during epidemics of leaf blotch (Rhynchosporium secalis) on winter barley

Seasonal changes in numbers of conidia of Rhynchosporium secalis on debris from previous barley crops infected with leaf blotch (primary inoculum) were monitored in 1985–86 and 1986–87. In 1986–87, changes in numbers of conidia on leaves of plants in the new winter barley crop (secondary inoculum) were also recorded. The greatest increases in production of primary inoculum were in early spring after rain, when temperatures were increasing after periods of sub-zero temperatures when there was little conidial production. Subsequently, more conidia were recovered from this debris after cycles of drying and rewetting than when it remained wet. After January 1987, amounts of secondary inoculum produced on the crop were much greater than amounts of primary inoculum on debris. Most spores were produced on the basal leaves and more spores were present on the September-sown than on the November-sown crop. Thus, while primary inoculum was a source of disease when plants were emerging, secondary inoculum on basal leaves was the main source of disease at stem extension, especially on early-sown crops.     

Micro-organic colonization of forest soil after burning

Summary and conclusions The results show that for some time, fire sites remain largely free of many of the micro-organisms that normally inhabit the soil and are then gradually colonized by species characteristic of burnt sites. Colonization was most pronounced at the margins due presumably to the invasion of the nutrient-rich soil of the fire site by mycellia from surrounding unburnt ground. The micro-organisms isolated from the centre of the burns during the early stages of succession, probably arise from spores brought in the wind and washed down by rain water.     

A rain tower and wind tunnel for studying the dispersal of plant pathogens by rain and wind

The rain tower/wind tunnel complex at Rothamsted consists of a rain tower (height 11 m, cross-section 1 m²) linked to the upwind end of a wind tunnel (length 12 m, cross-section 1 m²), which may be operated in either an open or a closed configuration. At the top of the rain tower, water drops with diameters of 2.5 to 5 mm are produced by a drop generator, which can be fitted with different nozzles. Simulated rain with a drop diameter of 1 to 3 mm is produced at a rate of 8 to 12 mm h^-1 by a rain generator with an area of 52 × 67 cm. The rain tower may be operated in conjunction with the wind tunnel in an open configuration. The windspeed can be decreased from a maximum of 8 m s^-1 by decreasing the speed of the fan. The wind tunnel has its own internal lighting. When the wind tunnel is in a closed configuration, temperature and humidity can be controlled in the range 12 ^oC (62–80% r.h.) to 35 ^oC (22–50% r.h.). Data presented illustrate the use of this rain tower/wind tunnel complex to study dispersal of plant pathogen spores by rain-splash or wind.     

Deposition of Erysiphe graminis Conidia on a Barley Crop

Naturally released Erysiphe graminis conidia were trapped (on horizontal slides, on vertical sticky cylinders and in suction traps) in a barley crop infected with powdery mildew and the numbers of single spores and of clumps of different sizes deposited on the traps were counted. The efficiencies of impaction calculated from deposits and wind speed measurements were higher than expected from mean wind speed measurements. The values were consistent with the hypothesis that spores were removedpredominantly in gusts. More than half the conidia were removed in clumps of two or more spores. The measurements suggest that clumps were more effectively deposited than single spores. The measurements demonstrate that spore release mechanisms can influence spore deposition significantly, especially close to the source.     

Oviposition-site shift in phytophagous mites reflects a trade-off between predator avoidance and rainstorm resistance

Predators can reduce prey population densities by driving them to undertake costly defences. Here, we report on a remarkable example of induced antipredator defence in spider mites that enhances the risk to rainstorms. Spider mites live on the undersides of host plant leaves and usually oviposit on the leaf undersurface. When they are threatened by predatory mites, they oviposit on three-dimensional webs to avoid egg predation, although the cost of ovipositing on webs has not yet been clearly determined. We prepared bean plants harbouring spider mite (Tetranychus kanzawai) eggs on either leaf surfaces or webs and exposed them to rainstorms outdoors. We found that fewer eggs remained on webs than on leaf surfaces. We then examined the synergistic effect of wind and rain by simulating both in the laboratory. We conclude that ovipositing on webs comes at a cost, as eggs are washed off the host plants by wind and rain. This may explain why spider mite populations decrease drastically in the rainy season, although they inhibit leaf undersides where they are not directly exposed to rainfall.     

Production and Pathogenicity to Wheat of Pseudocercosporella herpotrichoides Conidia

Weekly estimates of numbers of Pseudocercosporella herpotrichoides conidia on naturally infected wheat straw, made from February to July 1982, showed there were most conidia (8.1 × 10⁶ per straw) in February and least (1.9 × 10⁴ per straw) at the end of June. The viability of these spores remained high throughout this period, with an average of 85 % germination after 24 h.
After removal of spores produced in the field, straws were incubated at 5, 10, 15, 20 or 25°C and subsequent sporulation assessed after 3 or 5 weeks. The optimum temperature for spore production was 5°C and very few spores were produced at 25°C. There was no difference in viability between spores produced at different temperatures.
Wheat seedlings placed amongst infected straw collected and retained spores on the upper and lower surfaces of all leaf blades and on outer leaf sheaths. Both naturally dispersed spores and spores sprayed on to plants were not removed by subsequent rainfall.
When wheat seedlings were inoculated between the coleoptile and outer leaf sheath with different numbers of P. herpotrichoides spores, lesion development was most rapid in seedlings inoculated with the greatest numbers of spores. However, after incubation for 12 weeks visible lesions were present on all plants inoculated with > c. 10 spores.     

The epidemiology of anthracnose disease of mango: inoculum sources, spore production and dispersal

Anthracnose disease spreads within mango trees by water-borne conidia of Colletotrichum gloeosporioides var. minor. Conidia were produced in lesions on leaves, defoliated branch terminals, mummified inflorescences and flower bracts. Conidia were trapped from these sources in the orchard during periods when anthracnose disease was developing both in flush growth and in flowers. The majority of conidia were trapped from lesions in young leaves. Conidia were produced in the laboratory from acervuli in leaf lesions over a wide temperature range (10–30°C) both in wet and humid (95–97% r.h.) conditions. Conidia would be present for dispersal within the tree throughout the entire season. Large numbers of conidia were trapped during prolonged periods of rain, and when these occurred during active growth or flowering, severe outbreaks of disease were recorded. No conidia were trapped following dews. Ascospores of Glomerella cingulata var. minor were not trapped while the disease was active in the orchard. These spores do not appear to contribute to the infection cycle of mango anthracnose.     

An analysis of rain-mediated dispersal of Drechslera teres conidia in field plots of spring barley

In two separate trials rain-mediated dispersal of Drechslera teres conidia was observed in field plots of cv. Beatrice spring barley. Within the crop spore number was greater towards the base reflecting both the downward movement of conidia and the greater availability of spores on older leaves. From the edge of the crop half as many conidia were trapped at 100 cm as at 25 cm. In both trials the cumulative spore catch total lagged behind the increase in net blotch infection levels on the top two leaves. The number of spores sampled appeared closely related to rainfall intensity.     

The effect of weather on the concentration of pollen within sugar-beet seed crops

The concentration of pollen in the air within diploid open-pollinated sugar-beet seed crops at Broom's Barn Experimental Station increased between 05.00 and 09.00 G.M.T. as relative humidity became less than 90%, was greatest between 09.00 and 11.00, when relative humidity was c. 75%, and gradually decreased towards evening. The average pollen concentration during 24 h periods ranged from 170 to 12400/m³ being greatest on fine, windy, dry days after periods of cooler weather. Rain during the morning washed pollen out of the air and damaged developing anthers, but rain in the late afternoon following a sunny morning seemed hardly to affect the pollen catch, while rain at night sometimes caused an immediate temporary increase in pollen concentration. Most pollen was released between 27 June and 31 July in all years (1965-7); more in the first than in the second half of July. 1965 was cool and damp, 1967 warm and dry, 1966 warm and dry early, and cool and wet late. The total pollen catch in 1965 was 83% and in 1966 31% of that in 1967 but the percentage germination of seed harvested in the 3 years was similar. The total pollen catch on a trap 230 m east of the 1965 crop was c. 1% of the catch within the crop on days with gusty westerly winds and the catch on a trap c. 46 cm above the 1966 crop averaged 78.6% of that at the level of most flowers.     

Departure of migrating European robins, Erithacus rubecula, from a stopover site in relation to wind and rain

Migratory birds replenishing their fuel stores have to decide when to leave their stopover site for the next flight bout. We studied whether the decision to leave a stopover site depends on wind and rain conditions. From capture-recapture data of 1153 European robins collected during three autumns at a stopover site in Switzerland, we estimated the daily emigration probability with a newly developed multistate capture-recapture model that accounts for the occurrence of transients. We tested whether the variation in the daily emigration probabilities can be explained by wind speed, wind direction (both on the ground and 300 m above ground) or rain. Variation in emigration probability was largely explained by variation in wind at 300 m and rain. The emigration probability was highest (0.5) during nights with no or weak (<1.5 m/s) winds at 300 m and no rain, intermediate (0.15-0.2) on nights without rain and with medium wind (>1.5 m/s), and on nights with weak winds (<1.5 m/s) and rain; and almost zero during nights with rain and strong winds at 300 m. Wind direction at 300 m and wind conditions (speed and direction) on the ground had no influence on departure decision. We suggest that birds may consider cues other than wind speed at ground level to predict wind speed at higher altitudes, and that they consider wind direction only when aloft by selecting an optimal flight altitude. Wind speed aloft and rain appeared to be significant factors that synchronize bird migration spatially and temporally.     

Deposition Gradients Near to a Point Source in a Barley Crop

By measuring deposits of droplets downwind from a source in a barley crop during crop growth, gradients of deposition were established. Droplets were generated using a May spinning disc, which approximated to a point source. Droplets were labelled with thiabendazole so that deposit could be measured photometrically. Droplet diameter was approximately 20 micrometres, a size similar to spores of barley powdery mildew, an important foliar pathogen. Gradients of deposition were influenced by the density of the crop, by wind speed and by air turbulence. At any one time these three factors could interact to change the gradient substantially. Exponential and power law equations fitted the data equally well although due to experimental variation neither gave a very good fit. It is suggested that over the first few metres of dispersal from the source an exponential equation can be used to model the gradient. This has the advantage that it can be extrapolated to give an estimate of “self-infection” i.e. the spores deposited on the plant on which they were produced.     

Quantification of soil-to-plant transport of recombinant nucleopolyhedrovirus: effects of soil type and moisture, air currents, and precipitation.

Significantly more occlusion bodies (OB) of DuPont viral construct HzSNPV-LqhIT2, expressing a scorpion toxin, were transported by artificial rainfall to cotton plants from sandy soil (70:15:15 sand-silt-clay) than from silt (15:70:15) and significantly more from silt than from clay (15:15:70). The amounts transported by 5 versus 50 mm of precipitation were the same, and transport was zero when there was no precipitation. In treatments that included precipitation, the mean number of viable OB transported to entire, 25- to 35-cm-tall cotton plants ranged from 56 (clay soil, 5 mm of rain) to 226 (sandy soil, 50 mm of rain) OB/plant. In a second experiment, viral transport increased with increasing wind velocity (0, 16, and 31 km/h) and was greater in dry (-1.0 bar of matric potential) than in moist (-0.5 bar) soil. Wind transport was greater for virus in a clay soil than in silt or sand. Only 3.3 x 10(-7) (clay soil, 5 mm rain) to 1.3 x 10(-6) (sandy soil, 50 mm rain) of the OB in surrounding soil in experiment 1 or 1.1 x 10(-7) (-0.5 bar sandy soil, 16-km/h wind) to 1.3 x 10(-6) (-1.0 bar clay soil, 31-km/h wind) in experiment 2 were transported by rainfall or wind to cotton plants. This reduces the risk of environmental release of a recombinant nucleopolyhedrovirus (NPV), because only a very small proportion of recombinant virus in the soil reservoir is transported to vegetation, where it can be ingested by and replicate in new host insects.     

The effect of three herbicides on the number of spores of Rhynchosporium secalis on barley stubble and volunteer plants

Single sprays of paraquat, glyphosate or dinoseb-in-oil, were applied to barley stubble or volunteer plants and their effects on the number of spores of Rhynchosporium secalis were studied weekly. Spore concentrations were measured by washing samples of stubble or volunteer plants and counting spores using a haemocytometer. The number of spores recovered from stubbles was relatively small; both paraquat and glyphosate caused some reductions but the results were not consistent. Treatment with paraquat generally caused a transitory increase on volunteer plants; this effect was not found after glyphosate. Both herbicides caused persisting reductions from 4 wk after treatment. Volunteer plants treated with dinoseb-in-oil had fewer spores for 6 wk after treatment, but the plants were not killed and when new growth became infected, spore production recovered to equal that on untreated plants. The fungicide captafol caused some reductions in spore number on stubble but results were not consistent, applied after paraquat it had no effect. It was generally ineffective when applied to volunteer plants; applied after paraquat it tended to increase spore number, but the differences were seldom significant. Spores washed from volunteer plants treated with paraquat or glyphosate were inoculated to pot-grown barley seedlings; neither herbicide had any effect on spore viability. Viable spores were recovered in February from stubble or volunteer plants treated the previous autumn with paraquat or glyphosate respectively.     

广西靖西西南桦天然林种子雨的时空动态

以一片西南桦(Betula alnoides)天然林和一个西南桦独立单株为研究对象, 通过收集散种期内与林分或母树不同距离的种子以及测定风速和风向, 研究了西南桦群落和个体水平上种子雨的时空动态及其与风速和风向的关系。结果表明: 群落水平上, 西南桦种子散布的初始期、高峰期、消退期分别历时11天、32天和40天, 而个体水平上则为9天、25天和26天。高峰期内群落和个体水平的散种量分别占其总量的83.1%和68.7%, 而且白天的种子雨密度高于夜间; 西南桦个体白天种子雨密度最大的时段为12:00-16:00, 与此时段内风速较高有关。在个体水平上, 距离母树0-30 m范围内散落的种子占总散种量的79.6%; 而在群落水平上, 距离林缘0-45 m范围内集中了总散种量的81.2%。西南桦种子散布具有方向性, 无论个体还是群落水平上不同方向间种子雨密度差异极显著(p < 0.01), 与散种期内的主要风向有关; 而且种子雨密度与风速亦呈极显著正相关关系。研究结果将有助于揭示西南桦天然更新动态和更新机制, 亦为开展西南桦人工促进天然更新提供理论依据。