Zonal Soil Amendment with Simple Sugars to Elevate Soil C/N Ratios as an Alternative Disease Management Strategy for Rhizoctonia Damping-off of Sugar Beet.

Effects of monosaccharide-amended soils on suppression of Rhizoctonia damping-off of sugar beet were compared under controlled experiments. Suppressive effects of glucose, fructose, sorbose, and xylose were significantly (P < 0.001) greater than that of galactose or mannose but the effect of sorbose was reduced by soil treatments with antibiotics. Saprotrophic growth of Rhizoctonia solani in the laimosphere also was suppressed by glucose, fructose, sorbose, and xylose, whereas only sorbose repressed pericarp colonization. Sugar alcohols (mannitol, sorbitol, and xylitol) neither suppressed Rhizoctonia damping-off nor halted the saprotrophic growth of the pathogen. Seed germination was not affected by any of these six monosaccharides, whereas galactose and mannose inhibited seedling emergence significantly (P < 0.001) compared with the nontreated control or other monosaccharides. Soil fertilization with inorganic nitrogen at a C/N ratio of 20:1 negated the suppressive effects of glucose and fructose on both damping-off and saprotrophic colonization but improved seedling growth in carbonized soils. Obviously, microbial competition for mineral nitrogen was responsible for disease suppression; however, it delayed seedling growth after emergence. This paradox was resolved by adding glucose to the top 1-cm surface-soil zone at a C/N ratio of 50:1 or 125:1. This protected the laimosphere, resulting in effective disease suppression while complementarily enhancing seedling growth.