Selection of Fluorescent Pseudomonads Antagonistic toErwinia carotovoraand Suppressive of Potato Seed Piece Decay

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Abstract
A procedure was developed for screening fluorescent pseudomonads from potatoes for suppression of the potato soft rot pathogen, Erwinia carotovora, based on antagonism in vitro (i.e., siderophore and/or antibiotic production) and suppression of preemergence seed piece decay. Strains were identified as either Pseudomonas putida or P. fluorescens, the majority being P. fluorescens biovar III. The pseudomonads (293 strains) were screened first on King''s medium B agar for ability to inhibit growth of E. carotovora subsp. atroseptica. On this medium, inhibition is due primarily to the production of siderophores. Siderophores having a diversity of iron affinities were produced; some were able to reverse iron deprivation of the producer strain at ethylenediamine-di-o-(o-hydroxyphenyl acetic acid) concentrations up to 5,000 .mu.g/ml. Antibiotics were produced by 56% of the pseudomonads on potato-dextrose agar. Candidate strains of Pseudomonas then were tested in vivo for suppression of preemergence seed piece decay. In greenhouse trials, plant emergence was significantly increased up to 64% and plant growth was increased nearly seven-fold compared to checks inoculated only with E. c. subsp. atroseptica. Strains that produced siderophores with high iron affinities were more suppressive as a group than strains that produced siderophores with low iron affinities. Furthermore, strains that produced both inhibitory siderophores and antibiotics appeared to give the strongest effect in greenhouse trials against E. c. subsp. atroseptica. In general, pseudomonads were generally less effective in suppressing seed piece decay caused by strain W3C105 of E. c. subsp. carotovora than strain W3C37 of E. c. subsp. atroseptica. The screening method, nevertheless, should prove useful in identifying potato rhizosphere pseudomonads with the greatest potential for controlling diseases caused by E. carotovora.