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Factors that Affect Spread of Pseudomonas syringae in the Phyllosphere

First author: Department of Plant Pathology and Plant Disease Resistance Research Unit, U.S. Department of Agriculture-Agricultural Research Service, University of Wisconsin, Madison 53706; second and third authors: Department of Plant Pathology, University of Wisconsin, Madison 53706; and fourth author: Departments of Plant Pathology and Statistics, University of Wisconsin, Madison 53706

Successful spread of an organism to a new habitat requires both immigration to and growth on that habitat. Field experiments were conducted to determine the relative roles of dispersal (i.e., immigration) and bacterial multiplication in spread of Pseudomonas syringae pv. syringae in the phyllosphere. To study spread, individual plots consisted of three nested concentric squares with the inner 6 m² planted to snap beans serving as the sink. Each sink, in turn, was surrounded by a barrier zone, usually 6 m wide, which was surrounded by a 6-m-wide source area. The source areas were planted with snap bean seeds inoculated with doubly marked strains derived from wild-type P. syringae pv. syringae B728a. The treatments were designed to test the effects of the nature and width of the barrier zone and suitability of the habitat in the sinks on spread of P. syringae pv. syringae. The marked strains introduced into the source areas at the time of planting were consistently detected in sink areas within a day or two after emergence of bean seedlings in the sources as assessed by leaf imprinting and dilution plating. The amounts of spread (population sizes of the marked strain in sinks) across barrier zones planted to snap bean (a suitable habitat for growth of P. syringae pv. syringae), soybean (not a favorable habitat for P. syringae pv. syringae), and bare ground were not significantly different. Thus, the nature of the barrier had no measurable effect on spread. Similarly, spread across bare-ground barriers 20 m wide was not significantly different from that across barriers 6 m wide, indicating that distance on this scale was not a major factor in determining the amount of spread. The suitability of the sink for colonization by P. syringae pv. syringae had a measurable effect on spread. Spread to sinks planted to clean seed was greater than that to sinks planted with bean seeds inoculated with a slurry of pulverized brown spot diseased bean leaves, sinks planted 3 weeks before sources, and sinks planted to a snap bean cultivar that does not support large numbers of P. syringae pv. syringae. Based of these results, we conclude that the small amount of dispersal that occurred on the scale studied was sufficient to support extensive spread, and suitability of the habitat for multiplication of P. syringae pv. syringae strongly influenced the amount of spread.