Rhizoctonia solani suppression and plant growth promotion in cucumber as affected by biochar pyrolysis temperature, feedstock and concentration

• Feedstock and concentration of biochar affect diseases caused by Rhizoctonia solani in cucumber.
• Hormesis effects (low dose disease suppression, high dose disease promotion) are observed.
• Influence of biochars on plant growth and on R. solani diseases are not well correlated.
• Impact on diseases caused by soilborne pathogens should be considered in agronomic uses of biochar.

The soil application of biochar, the solid carbon-rich product of biomass pyrolysis, may improve soil quality and crop productivity. Recently, soil applied biochar was found to reduce severity of foliar diseases and soilborne diseases in various crops. Biochar characteristics are expected to affect its disease suppression capability, as there is a profound variability in the physical and chemical properties of biochar depending on the initial feedstock and production parameters. The aim of this research was to explore the influence of biochar produced from different feedstocks and at different production temperatures on its ability to suppress the soilborne pathogen, Rhizoctonia solani, in cucumber (Cucumis sativus L.). Biochar prepared from two feedstocks (Eucalyptus wood (EUC) and Greenhouse waste (GHW)), each produced at 350 and 600 °C were tested for their suppressive ability against damping-off at concentrations of 0–3%. In general, biochar addition at relatively lower concentrations enhanced plant growth performance and suppressed damping-off by R. solani. However, at higher concentrations, biochar was ineffective or even increased the disease incidence and severity as compared with the control, forming a U-shape response curve versus biochar concentration. Biochars produced at both low and high temperatures were equally effective against various disease parameters but suppression of disease was affected by the feedstock type. There was a significant interaction between feedstock and concentration for the final damping-off incidence and other disease parameters, indicating that each feedstock had an optimum concentration for disease control. The most effective dose for suppressing disease was 1% for EUC biochar and 0.5% for GHW biochar. In conclusion, R. solani suppression in cucumber induced by biochar soil amendment is feedstock and concentration dependent.