Sabine Ravnskov

The influence of the organic compounds tryptic soy broth, cellulose, glucose and chitosan on cucumber damping-off caused by Pythium aphanidermatum and biocontrol efficacy of the biocontrol agents (BCAs) Paenibacillus macerans and P. polymyxa were examined in a seedling emergence bioassay. Results showed that the organic compounds differentially affected both pathogen and BCAs. Tryptic soy broth, glucose and chitosan increased Pythium damping-off of cucumber, compared to the control treatment without organic compounds, whereas cellulose had no effect. Both Paenibacillus species had biocontrol effects against Pythium damping-off compared with the corresponding treatments with P. aphanidermatum alone, but the biocontrol efficacy depended on the type of organic compounds added. Both BCAs counteracted damping-off in treatments with TSB and chitosan. However, P. polymyxa counteracted damping-off in combination with either glucose amendment or without organic compound amendment, while P. macerans increased damping- off in combination with either cellulose amendment or without organic compound amendment. Dampingoff did not correlate with Pythium biomass estimated by ELISA, nor with background microbial communities as examined using biomarker fatty acids.