Non-monotonic influence of biochar dose on bean seedling growth and susceptibility to Rhizoctonia solani: the “Shifted R_max-Effect”

Aims: Biochar affects the progress of plant diseases caused by soilborne pathogens, frequently featuring U-shaped biochar dose/disease response curves. This study tested this phenomenon in common bean (Phaseolus vulgaris L.) with several biochars. Methods: Four biochars prepared from two feedstocks (eucalyptus wood and greenhouse wastes) each at 350 and 600 °C were tested on bean seedling growth and infection caused by Rhizoctonia solani at concentrations of 0–3 % by weight. Biochar direct toxicity to R. solani was quantified in vitro. Results: In general, lower concentrations (≤1 %) of biochar suppressed damping-off, whereas higher concentrations (3 %) were ineffective at disease protection. Plant growth in the absence of the pathogen was generally improved at all doses by the four biochars. Maximum growth response (G-R_max) generally occurred at higher biochar doses than maximum disease reduction (D-R_max). Direct toxicity to the pathogen could not explain disease reduction. Conclusion: Inverted U-shaped biochar dose/plant growth and biochar dose/disease reduction curves are emerging as common patterns in biochar/crop/pathogen systems. Frequently, the inflection between growth promotion and suppression occurs at different doses than the inflection between disease suppression and promotion. We term this the “Shifted R_max-Effect”. As there is no simple rule-of-thumb for crop/soil/biochar/dose/pathogen combinations, the possible effects of biochar on plant pathogens should not be overlooked.