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

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. Four biochars prepared from two feedstocks (eucalyptus wood and greenhouse wastes) each at 350 and 600 A degrees 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. In general, lower concentrations (a parts per thousand currency sign1 %) 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. 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.