Effect of crop rotation and crop residues on the Rhizoctonia inoculum potential in soil

The soilborne fungus Rhizoctonia solani (Kahn) is the causal agent of Rhizoctonia crown and root rot, leading to enormous yield losses in infested sugar beet growing areas all over the world. Besides weather conditions disease occurrence and development depend on the inoculum potential of the pathogen persisting in soil. With the help of bait plants, a newly developed method for the detection of the inoculum potential, factors influencing the occurrence of root rot, were identified. With field beans as bait plants it was possible to relatively quantify the Rhizoctonia inoculum potential in crop rotation trials (sugar beet followed by two-year cultivation of silage-, grain-maize and winter wheat, respectively, followed by sugar beet) in the field and in soil samples under greenhouse conditions. Culture of Rhizoctonia host plants (maize) led to an increased inoculum potential compared to non-host plants (winter wheat). To date a disease promoting effect of harvest residues was only described in greenhouse studies. In this study it was shown now for the first time in field experiments that the harvest residues of the host plants grain-maize and sugar beet had a promoting effect on Rhizoctonia. Infestation level and white sugar yield of subsequently cultivated susceptible sugar beets confirmed the estimated inoculum potentials. Application of the bait plant test and removal of harvest residues for integrated Rhizoctonia control are discussed.