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Root Prints: A Technique for the Determination of the In Situ Spatial Distribution of Bacteria on the Rhizoplane of Field-Grown Plants. M. E. Stanghellini, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721; S. L. Rasmussen, Research assistant, Department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 79:1131-1134. Accepted for publication 22 May 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1131.

Visualization of the in situ spatial distribution of bacteria on the rhizoplane of sugar-beet roots was accomplished by imprinting excised portions of roots on agar medium. Up to 20 consecutive prints could be made from a single 1-cm2 section of rhizoplane. The number of colonies, which visually exhibited a random pattern of distribution, ranged from 48 to 113 (mean of 60) per square centimeter of rhizoplane. Total bacterial population size, estimated from dilution plating of washings from roots, ranged from 1.12 to 10.3 107 (mean of 5.1 107) colony-forming units/cm2 of rhizoplane. Bacillus spp. were identified as the dominant bacteria (greater than 57% of the colonies) on both root prints and dilution plates. Fluorescent pseudomonads, when present, accounted for less than 3% of the colonies on prints and less than 20% of the colonies on dilution plates. The root-print technique was not restricted to large taprooted plants. Visualization of the spatial distribution of bacteria on the rhizoplane of small cylindrical roots (at least 0.3 mm in diameter) also was accomplished by rolling the root on the agar surface.