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Donald E. Aylor

Abstract

Deposition of ragweed pollen released from a point above a corn crop was directly observed on foliage, and the number of particles deposited was related to the concentration of pollen in the air, the wind within the crop, and the foliage characteristics. At winds of 1–2 m s−1 deposition on the foliage both at the top and at the middle of the plants was primarily by sedimentation; while at winds of 3–4 m s−1, deposition was enhanced at the top about threefold. Since 20 μm particles impact inefficiently on corn plants for usual wind speeds within the crop, they should penetrate far through the canopy before settling out by gravity.

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Donald E. Aylor and Thomas K. Flesch

Abstract

Practical problems in predicting the spread of plant diseases within and between fields require knowledge of the rate of release Q of pathogenic spores into the air. Many plant pathogenic fungus spores are released into the air from plant surfaces inside plant canopies, where they are produced, or from diseased plant debris on the ground below plant canopies, where they have survived from one growing season to the next. There is no direct way to specify Q for naturally released microscopic fungus spores. It is relatively easy to measure average concentrations of spores above a source, however. A two-dimensional Lagrangian stochastic (LS) simulation model for the motion of spores driven by atmospheric turbulence in and above a plant canopy is presented. The model was compared 1) with measured concentration profiles of Lycopodium spores released from line sources at two heights inside a wheat canopy and 2) with concentration profiles of V. inaequalis ascospores measured above ground-level area sources in a grass canopy. In both cases, there was generally good agreement between the shapes of the modeled and measured concentration profiles. Modeled and measured concentrations were compared to yield estimates of spore release rates. These, in turn, were compared to release rates estimated independently from direct measurements. The two estimates of spore release rate were in good agreement both for 1) the 30-min artificial releases of Lycopodium spores [significance level P = 0.02 (upper source) and P = 0.02 (lower source)] and for 2) the daily total release of V. inaequalis ascospores (P < 0.002). These results indicate that the LS model can yield accurate values of Q (or, conversely, of concentration). Thus, LS models allow a means of attacking a nearly intractable problem and can play an important role in predicting disease spread and in helping to reduce pesticide use in disease-management decisions.

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Matthew T. Boehm, Donald E. Aylor, and Elson J. Shields

Abstract

The widespread adoption of genetically modified (GM) crops has led to a need to better understand the atmospheric transport of pollen because of concerns over potential cross-pollination between GM and non-GM crops. Maize pollen concentrations were modeled by a modified Lagrangian stochastic (LS) model of the convective boundary layer (CBL) and were compared with concentrations measured by airborne remotely piloted vehicles (RPVs) flown from directly above to 2 km from source fields. The turbulence parameterization in an existing CBL LS model was modified to incorporate the effects of shear-driven turbulence, which has an especially large impact near the surface, where maize pollen is released. The modified model was used to calculate concentrations corresponding to the RPV flight tracks. For the most convective cases, when at least 95% of the pollen came from sources near the RPV flight track for which source strength measurements are available and the results are less sensitive to uncertainty in wind direction since most of the pollen came from directly beneath the flight track, the geometric mean of the ratio between the modeled and measured concentrations was 0.94. When cases with larger contributions from more distant fields were included, the overall geometric mean decreased to 0.43. The scatter of the measured concentrations about the modeled values followed a lognormal distribution. These results indicate that the modified model presented herein can substantially improve the description of the near-source dispersion of heavy particles released near the surface during convective conditions.

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DONALD E. AYLOR, H. A. MCCARTNEY, and A. BAINBRIOGE

Abstract

Theoretically, the aerial transport and deposition of fungal spores near to their source dependon the way in which they become airborne. Spores of some fungi are injected into the air, independentof wind speed, whereas others are blown from the host mainly by gusts. We calculated the collectionefficiency of vertical, sticky cylinders for spores released continuously and steadily, and for spores released intermittently when the wind exceeded a certain speed. We tested these calculations in the fieldby trapping on cylinders of different diameters Lycopodium spores released steadily from an artificialsource, spores of Erysiphe graminis blown from a barley crop and Lycopodium spores blown from a fiatplate or a short crop.

The collection efficiency E of vertical sticky cylinders depends on wind speed, and the averageefficiency for an experiment was obtained by averaging E over all speeds during spore release. Thecatch of the spores released continuously can be predicted adequately from E at the mean wind speed. Incontrast, the catch of spores blown from a crop or a plate can be predicted only when speeds below athreshold are excluded.

When spores in crops are released by gusts, deposition by impaction will be greater than thatcalculated from the mean wind speed. This effect of the mode of spere release needs to be taken into accountin modeling spore transport in crops.

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Kirk M. Ducharme, David R. Miller, and Donald E. Aylor

Abstract

Cylindrical, platinum-coated, hot-film anemometers were struck with a series of individual drops of water while immersed in controlled airflows with velocities ranging from 0.3 to 5.0 m s−1. Subjecting the sensor to water drops caused slight, but permanent, changes in calibration. In an effort to overcome calibration changes following drop impacts, a Teflon-coated sensor was also tested. A filtering algorithm was devised to remove drop-caused spikes in the recorded time series. An average spike duration of 0.32 s per drop impact was found, and maximum record loss was estimated to be 1.7% for rainfall rates less than 30 mm h−1.

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Roger H. Shaw, David P. Ward, and Donald E. Aylor

Abstract

The removal from leaves of small particles such as pollen, dust or spores often requires gusts of wind having speeds that greatly exceed the mean wind speed. To determine the frequency of occurrence of such fast gusts we analyzed wind data that were measured inside a corn canopy using a fast-response (50 Hz) anemometer to obtain the probability density distribution of the total wind speed Q = (u 2 + v 2 + w 2)½ and of the temporal change in wind speed ΔQ for several sampling intervals. Gusts of wind with speeds exceeding the local mean wind speed by a factor of 3 or more were more frequent near the middle than near the top of the canopy and, for winds averaging 0.92 m s−1 for 1 h at mid-canopy, speeds as great as 4.4 times the mean occurred for about 0.8 s during the hour. We estimate that this relatively high speed occurred on at least 10 separate occasions during the hour-long observation period.

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Donald E. Aylor, Matthew T. Boehm, and Elson J. Shields

Abstract

The extensive adoption of genetically modified crops has led to a need to understand better the dispersal of pollen in the atmosphere because of the potential for unwanted movement of genetic traits via pollen flow in the environment. The aerial dispersal of maize pollen was studied by comparing the results of a Lagrangian stochastic (LS) model with pollen concentration measurements made over cornfields using a combination of tower-based rotorod samplers and airborne radio-controlled remote-piloted vehicles (RPVs) outfitted with remotely operated pollen samplers. The comparison between model and measurements was conducted in two steps. In the first step, the LS model was used in combination with the rotorod samplers to estimate the pollen release rate Q for each sampling period. In the second step, a modeled value for the concentration C model, corresponding to each RPV measured value C measure, was calculated by simulating the RPV flight path through the LS model pollen plume corresponding to the atmospheric conditions, field geometry, wind direction, and source strength. The geometric mean and geometric standard deviation of the ratio C model/C measure over all of the sampling periods, except those determined to be upwind of the field, were 1.42 and 4.53, respectively, and the lognormal distribution corresponding to these values was found to fit closely the PDF of C model/C measure. Model output was sensitive to the turbulence parameters, with a factor-of-100 difference in the average value of C model over the range of values encountered during the experiment. In comparison with this large potential variability, it is concluded that the average factor of 1.4 between C model and C measure found here indicates that the LS model is capable of accurately predicting, on average, concentrations over a range of atmospheric conditions.

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