Effect of plot geometry on epidemic velocity of wheat yellow rust

The size and shape of field plots can impact on both the cost and outcome of epidemiological experiments. In previous studies, epidemic velocity of yellow rust (caused by Puccinia striiformis f.sp. tritici ) on wheat ( Triticum aestivum ) has been examined in long, narrow plots (6·1 m by 73–171 m). The present study compares spread in square, 61 × 61 m plots versus narrow, 6·1 × 61 m plots at two locations. The objective was to test whether plot shape has a substantial impact on spatiotemporal spread of yellow rust. Velocity increased curvilinearly with time for both plot shapes and at both locations. Curves of epidemic velocity versus time were nearly identical in square versus narrow plots in both the upwind and downwind directions. Contrary to expectation based on simulations, the results did not indicate faster disease spread in square plots, though the plot sizes studied may be beyond that at which there is a rapid change of disease increase with increasing plot area. Velocity also increased curvilinearly in all eight compass directions of the square plots. Results indicate that narrow plots, which are substantially less costly than equidimensional plots, may be justified for studying the spatiotemporal spread of wheat yellow rust and other diseases with similar epidemiological characteristics.