LACK OF EVIDENCE FOR A ROLE OF ANTIFUNGAL METABOLITE PRODUCTION BY PSEUDOMONAS-FLUORESCENS PF-5 IN BIOLOGICAL-CONTROL OF PYTHIUM DAMPING-OFF OF CUCUMBER

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.