Lack of Evidence for In Situ Fluorescent Pigment Production byPseudomonas syringaepv.syringaeon Bean Leaf Surfaces

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Abstract
The fluorescent siderophore of Pseudomonas syringae pv. syringae (P.s. pv. syringae) is a chromophore effective in protecting the producing cells from exposure to ultraviolet light (UV) (.lambda. - 254) in culture. P. s. pv. syringae strain B728a, which causes brown spot disease of bean [Phaseolus vulgaris] produces a fluorescent pigment and grows on the iron-deficient King''s B medium (KBM) supplemented with ethylenediaminedi(o-hydroxyphenylacetic acid) (EDDA). Strain I-1, a nonfluorescent (Flu-) derivative of B728a obtained after ethyl methanesulfonate mutagenesis, did not grow on the KBM-EDDA medium. Strain I-1 was more sensitive than the parental strain B728a to UV irradiation in culture. LD50 values, calculate for UV killing curves, were 142.4 .+-. 1.4 and 100.8 .+-. 1.8 erg/mm2 for strains B728a and I-1, respectively. Growth rates, stationary epiphytic population sizes, number of brown spot lesions, and UV sensitivities of B728a and I-1 did not differ significantly on leaf surfaces of greenhouse-grown bean plants. No consistent differences were observed between four parental and four Flu- derivative strains with respect to their populations sizes on bean leaf surfaces over a 7-day period on field-grown bean plants. These indirect studies provide no evidence of in situ fluorescent siderophore production by P. s. pv. syringae or its contribution to the growth, durvival, or pathogenicity of P. s. pv. syringae on bean leaf surfaces.