Control of damping-off of organic and conventional cucumber with extracts from a plant-associated bacterium rivals a seed treatment pesticide

• Ethanol extracts of Serratia marcescens control disease on conventional or organic cucumber.
• Ethanol extracts of S. marcescens had at least a three month shelf life.
• Control of damping-off of cucumber was similar to that provided by a seed treatment pesticide.

Environmentally friendly control measures are needed for soilborne diseases of crops grown in organic and conventional production systems. We tested ethanol extracts from cultures of Serratia marcescens N4-5 and N2-4, Burkholderia cepacia BC-1 and BC-2, and Burkholderia ambifaria BC-F for control of damping-off of cucumber caused by the soilborne pathogens Pythium ultimum and Rhizoctonia solani; ethanol being an Organic Materials Review Institute (OMRI) -approved solvent for use in certain applications in organic crop production. Ethanol extracts from strains N4-5 and N2-4 inhibited mycelial growth and germination of sporangia of P. ultimum in vitro but those from strains BC-1, BC-2, BC-F, and the ethanol control did not. Ethanol extracts from strains BC-2 and BC-F inhibited mycelial growth of R. solani in vitro while ethanol extracts from strains BC-1, N2-4, N4-5, and the ethanol control did not. Thin-layer chromatography demonstrated that ethanol extracts from strain N4-5 contained prodigiosin while ethanol extracts from strains BC-2 and BC-F contained pyrrolnitrin; extracts from strains N2-4 and BC-1 did not contain either of these compounds. DNA sequencing confirmed the presence of a biosynthetic gene for prodigiosin in strain N4-5 and its absence in strain N2-4, while a biosynthetic gene for pyrrolnitrin was found in strains BC-2 and BC-F but not in strains N2-4, N4-5, and BC-1. Prodigiosin was previously implicated in inhibition of P. ultimum while pyrrolnitrin has been shown to inhibit R. solani. Certified-organic cucumber seed treated with an ethanol extract of strain N4-5 was the only extract treatment from any of these five microbial strains to effectively suppress damping-off caused by P. ultimum in growth chamber pot experiments. This ethanol extract provided suppression of P. ultimum on cucumber that was similar to that provided by a commercially available seed treatment pesticide and greater than that provided by a commercially available biocontrol agent for this pathogen. The inhibitory factor(s) in ethanol extracts of strain N4-5 was stable as a seed treatment for at least 14 weeks when incubated at 4 °C in the dark. No ethanol extracts applied as treatments of organic cucumber seed consistently suppressed damping-off caused by R. solani in growth chamber pot experiments. Experiments reported here suggest that certain natural products from microbial strains as seed treatments are promising alternatives for control of soilborne diseases in conventional or organic cucumber production systems.