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Spatial-Temporal Colonization Patterns of a Rhizobacterium on Underground Organs of Potato. J. B. Bahme, Former graduate research assistant, Department of Plant Pathology, University of California, Berkeley 94720, Present address: AgBioChem, Inc., 10795 Bryne Avenue, Los Molinos, CA 96055; M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 77:1093-1100. Accepted for publication 29 January 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1093.

The spatial-temporal colonization patterns of Pseudomonas fluorescens strain A1-B were determined at all stages of plant development on belowground parts of field-grown potatoes in two soil types. The populations of A1-B persisted on inoculated seed pieces during the season but declined from an initial density of 108 cfu per seed piece to about 106 by harvest. Strain A1-B was also detected throughout the season in nonrhizosphere soil in relatively high numbers. Irrigation caused a gradient of declining bacterial density beneath the seed pieces, but the downward dispersal was localized (46 cm in silty clay-loam and 1012 cm in sandy loam soils). Strain A1-B colonized roots, underground portions of shoots, stolons, and progeny tubers in a lognormal distribution pattern, with greatest populations on plant parts nearest inoculated seed pieces. Mean population densities on root segments before irrigation were about 1 log greater in sandy loam soil than in silty clay-loam. Spatial distribution patterns and population densities on roots were substantially altered after irrigation in contrast to those in nonrhizosphere soil. Mean population sizes of strain A1-B on sampled root segments were about 1 log greater after irrigation. The advancing root tips were sporadically colonized by strain A1-B with low population sizes, and this was affected by soil type. Colonization of root tips, underground portions of shoots, and stolons was greatest on those located nearest the source of original inoculum. Strain A1-B colonized developing progeny tubers; mean population sizes (log cfu per tuber) were about 35 times greater in the sandy loam (3.0) than in the silty clay-loam (1.5). Strain A1-B was not restricted to any particular root zone. Forty-one percent of A1-B cells were associated with rhizosphere soil, 54% were loosely adhered to the root surface, and 5% were tightly adhered or internal to the root surface. Samplings of root segments adjacent to seed pieces revealed that strain A1-B accounted for 0.16 and 12.58% of the total detectable aerobic and fluorescent pseudomonad bacterial component, respectively.