Antagonistic potential of forestry compost bacteria on Sclerotinia sclerotiorum (Lib.) de Bary, causal agent of carrot white mould

Composts are known to contain beneficial bacteria, which may be antagonistic to plant pathogens. This work evaluated whether carrot white mould, causal agent Sclerotinia sclerotiorum, can be reduced using antagonistic bacteria isolated from forestry compost. In vitro and in vivo experiments demonstrated that bacteria from the genera Pseudomonas and Bacillus can inhibit mycelial growth and reduce white mould. Bacillus subtilis strains F9-2 and F9-12 and Pseudomonas arsenicoxydans strain F9-7 showed the highest inhibitory properties. Three cyclic dipeptides (diketopiperazines) were characterized from the antifungal culture filtrates of P. arsenicoxydans F9-7. When assayed against S. sclerotiorum, the diketopiperazines showed the following inhibitory activity, in increasing order: cyclo-(l-Pro-l-Val), cyclo-(l-Pro-l-Phe) and cyclo-(l-Pro-l-Leu). The combination of these diketopiperazines indicated additive and, occasionally, synergistic antifungal effects. These results indicated a potential for some bacteria to inhibit the growth of S. sclerotiorum and reduce its associated disease on carrots postharvest.