Antimycotic activity of Xenorhabdus bovienii (Enterobacteriaceae) metabolites against Phytophthora infestans on potato plants

Soluble organic metabolites produced by phase one of Xenorhabdus bovienii strain A2 were tested for their antimycotic activity against selected plant pathogenic fungi and for their phytotoxicity. The metabolites were extracted with ethyl acetate from the supernatant of 96-h-old X. bovienii cultures in tryptic soy broth. They were fungicidal to Phytophthora infestans at 0.1 mg/mL on rye agar but only partially inhibited Pythium ultimum and Rhizoctonia solani on potato dextrose agar in petri plate tests. In potted plant tests, the antimycotic metabolites were applied to the foliage of 4- to 5-week-old potato plants (cv. Norchip) as a protectant spray at concentrations of 5.0 and 10.0 mg/mL. The 10.0 mg/mL treatments inhibited (P greater than or equal to 0.05) blight development 7 d after inoculation with only 4.0% of the inoculated leaflets showing symptoms of successful fungal infection, compared with 100% in the controls. The percentage of inoculated leaflets with blight symptoms was reduced to 24.0% on plants treated with 5.0 mg/mL of antimycotic metabolites. Phytotoxicity was observed on plants treated with the antimycotic metabolites, and the phytotoxic damage increased with increasing concentrations. The visible phytotoxic responses on potato plants treated with up to 10.0 mg/mL did not spread, and subsequent growth appeared normal. The selectivity of the antimycotic metabolites of X. bovienii against the three test fungi, its phytotoxic properties and the potential of these metabolites for fungal disease control are discussed.