ESTIMATION OF AND TEMPORAL CHANGES IN MEANS AND VARIANCES OF POPULATIONS OF PSEUDOMONAS-SYRINGAE ON SNAP BEAN LEAFLETS

Temporal changes in means and variances of populations of Pseudomonas syringae were examined in experimental field plots (30 m X 30 m) of snap bean cultivar Cascade, For each of two plantings, 30 individual leaflets were collected 5 days/wk from plant emergence to pod harvest and processed by dilution plating to estimate population sizes of P. syringae. Because population sizes of P. syringae were below the limit of detection for our method on some of the leaflets from several of the sampling times, the effect of censoring on data analysis was examined. A maximum likelihood (ML) method was compared to assigning censored values the limit of sensitivity (LS). If fewer than 10% of the values were censored, there was little difference between the mean or the variance estimated by either method. However, differences between the two methods increased as the amount of censoring increased, and at about 80% censoring, use of the LS method resulted in an estimated mean about 100 times larger, and an estimated variance about four to eight times smaller than the ML method. Temporal changes in population sizes of P. syringae differed substantially between the two plantings established about 1 mo apart when time was expressed as days after planting but not when time was expressed as calendar date. This suggests that the weather or other physical factors may have a relatively greater influence on the population size of this bacterium than the growth stage of the bean plants on which they reside. During periods when population sizes of P. syringae were relatively large, the bacterium was the dominant component of bacterial communities on bean leaflets. Changes in population size of P. syringae were relatively small (less than fivefold) in most 24-h periods but were very large (as great as 2,000-fold) an relatively fewer occasions. The largest change within 24 h corresponded to a doubling time of 2.1 h, which is of similar order of magnitude to the optimal doubling time for this bacterium in culture.