Dynamics, spread, and persistence of a single genotype of Pseudomonas syringae relative to those of its conspecifics on populations of snap bean leaflets.

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A rifampin-resistant strain of Pseudomonas syringae (R10) was introduced onto bean plants grown in field plots to examine the processes of growth, spread, and survival of a single genotype relative to the dynamics of its conspecifics on populations of individual leaflets. R10 was applied to four plots (400, 200, 100, and 50 m2), each of which was centered in a quadrant of a bean field (90 by 90 m). Population sizes of the species P. syringae and of R10 were determined on each of 25 individual leaflets collected from the largest plot (A) at approximately weekly intervals during a 10-week period following application. The spread of R10 from all plots was monitored by leaf imprinting of individual leaflets collected at sites along four transects, each of which bisected two of the plots. The introduced strain was a dominant component of the species for about 5 weeks postinoculation on leaflets from plot A. Although the population sizes of R10 remained at about 5.0 to 5.5 log10 CFU per leaflet, the strain became a progressively minor component of the species as the population sizes of its conspecifics continued to increase during the latter part of the growing season. In general, a positive correlation was found for the population sizes of R10 and its conspecifics on individual leaflets collected throughout the growing season. This result suggests that large numbers of R10 early in the growing season did not exclude the colonization of bean leaflets by its conspecifics. It is apparent that the species pool comprised genotypes that were more fit than R10 under the conditions that prevailed during the latter part of the growing season. By 6 weeks postinoculation, R10 was detected at all sites sampled within the unsprayed areas of the field. However, it was present as a minor component of the species. The persistence of R10 throughout the winter and into the following growing season was monitored in plot A, which was plowed and replanted with wheat in the fall. R10 was detected on some of the samples (wheat seedlings or soil) taken at approximately monthly intervals from November to June of the following year. In June, the field was plowed and replanted with beans. We could not detect R10 on emerging bean seedlings in plot A. The results demonstrate that the successful spread and persistence of an introduced bacterium do not necessarily lead to the establishment of large populations of the bacterium in adjacent untreated areas or on its host plant in subsequent growing seasons.

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