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Gradients in Horizontal Dispersal of Cereal Rust Uredospores. A. P. Roelfs, Research Plant Pathologist, Cooperative Rust Laboratory, Department of Plant Pathology, University of Minnesota, St. Paul 55101; Phytopathology 62:70-76. Accepted for publication 1 August 1971. DOI: 10.1094/Phyto-62-70.

Gradients of uredospore dispersal for Puccinia graminis f. sp. tritici, stem rust, and P. recondita, leaf rust, were determined within and around a 72-m-diam source plot of wheat, Triticum aestivum ‘Lee’. Spore samples were obtained using 5-mm-diam rod impaction traps 15 cm above the crop canopy stationed on annuli inside and outside the plot. No gradients in numbers of spores/cm² occurred between annuli inside the source 9 or more m. Numbers of spores on the downwind axis inside the plot increased within the first 9 m from the upwind edge of the source to 80 to 90% of the number trapped over the downwind edge of the source. Gradients outside the source differed with the methods of expression. The greatest gradient in numbers of spores was observed when the gradient was expressed as the average trapped at each annulus. This gradient predicts that an average of 3 and 6% of the initial number of spores trapped over the source will be trapped at 100 m, for stem and leaf rust, respectively. The regression equation of the form log Y = log a + b log X approximately described these gradients to an annulus. The omnidirectional horizontal gradient calculated from the sum of the spores per unit width at annuli 36 and 72 m from the source compensated for horizontal dilution as the circumference of the annuli increased with distance from the source. This calculation showed that 14 and 24% of the initial number of stem and leaf rust spores, respectively, were still in this horizontal plane at 100 m. The gradient for the number of uredospores/cm² downwind from the source showed that 10% of the spores of each rust species reached a point 100 m downwind. The regression equation of the form log Y = log a + bX described the downwind movement of both rust species.