Deposition Gradients Near to a Point Source in a Barley Crop

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Abstract
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.