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Lack of Evidence for a Role of Antifungal Metabolite Production by Pseudomonas fluorescens Pf-5 in Biological Control of Pythium Damping-Off of Cucumber. Jennifer Kraus, USDA, ARS, Horticultural Crops Research Laboratory, 3420 NW Orchard Ave., Corvallis, OR 97330; Joyce E. Loper, USDA, ARS, Horticultural Crops Research Laboratory, 3420 NW Orchard Ave., Corvallis, OR 97330. Phytopathology 82:264-271. Accepted for publication 27 September 1991. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1992. DOI: 10.1094/Phyto-82-264.

Pseudomonas fluorescens Pf-5 protected cucumber from preemergence damping-off caused by Pythium ultimum and produced a number of antifungal metabolites in culture, including a fluorescent siderophore (pyoverdine), pyoluteorin, pyrrolnitrin, cyanide, and an uncharacterized compound termed antibiotic 3. Of these, the roles of pyoluteorin and pyoverdine in the biocontrol activity of Pf-5 against Pythium damping-off of cucumber were investigated. Tn5 insertion mutants of Pf-5 were screened for loss of pyoluteorin production (Plt) or pyoverdine production (Pvd). Thirteen Plt mutants, 13 Pvd mutants, and six antibiotic production (Apd) mutants, which were deficient in pyoluteorin, pyrrolnitrin, and antibiotic 3 production, were identified. Three Plt++ mutants, which produced greater quantities of pyoluteorin in culture than did Pf-5, were identified. The parental strain, Pf-5, inhibited hyphal growth of P. ultimum in culture, whereas the Plt and Apd mutants did not inhibit P. ultimum on certain growth media. In contrast, the Plt++ mutants produced inhibition zones against P. ultimum that were larger than those produced by Pf-5 on an agar medium. Nevertheless, the Plt, Pvd, Apd, and Plt++ mutants were similar to Pf-5 in suppression of Pythium damping-off of cucumber. These results are consistent with the suggestion that antifungal metabolite production by Pf-5 did not contribute substantially to its biocontrol activity against Pythium damping-off of cucumber.