SUMMARY
Serial designs, balanced for effects of neighbours, were used in 1975–1977 to investigate interactions between plots of spring barley given different treatments for the control of powdery mildew (Erysiphe graminis DC. f. sp. hordei Mérat). Differences in amounts of disease, between similarly treated plots, could be related largely to the treatments applied to the neighbouring plots and to wind directions. Amounts of disease in plots were usually increased if the upwind neighbour was untreated but untreated plots themselves were also affected by neighbours, having least disease where the upwind neighbour was sprayed early.
Differences in mildew due to treatment were reflected in grain yield. Yields also provided evidence of interactions between plots. Interference, as well as altering the average response to treatments, can also contribute to variability in experiment results and lead to substantial losses in efficiency. The yields in 1975 and 1976 provided strong evidence of profiles of fertility. In the analyses, adjustment by covariates, employed to allow for these profiles, gave substantial net gains in accuracy.
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
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.
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