Relative Abundance and Deposition Gradients of Clusters of Urediniospores ofUromyces phaseoli

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Abstract
Singlets and clusters of urediniospores of Uromyces phaseoli were released naturally from a small area of infected plants within a 0.25-ha bean field. Deposition gradients were assessed by counting the number of singlets and clusters containing 2, 3, 4,...,N urediniospores deposited on sticky, horizontal microscope slides located at various distances downwind of the source and by counting the number of lesions that developed on the leaves of potted trap plants that were exposed in the field concurrently with the slides. Clusters accounted for more than 50% of the dispersal units and about 80% of the total number of spores deposited on the slides. Fully 30% of the clusters contained five or more urediniospores. Both the number of dispersal units deposited on slides and the number of lesions that developed on trap plants decreased approximately with the inverse square of distance from the source.