Rhizoctonia root rot represents an increasing problem in sugar beet production in many countries. The reasons for the spread of the disease caused by Rhizoctonia solani anastomosis group (AG) 2-2IIIB are not well understood. However, an observed increase in disease incidence and severity in regions with narrow sugar beet-maize crop rotations suggested maize as a possible host plant of R. solani AG 2-2IIIB. Therefore, greenhouse trials were conducted to investigate the pathogenicity of Rhizoctonia solani on maize. Trials revealed that the pathogen established well on maize and caused distinct root rot symptoms. In a successive greenhouse experiment, Rhizoctonia infected ground maize roots were used as inoculum source for sugar beet plants and successfully induced root and crown rot symptoms. The pathogen strains reisolated from these maize-inoculated sugar beet plants were identical in PCR analysis to those strains used to inoculate maize in the first experiment. Koch’s postulates were thereby completed. Maize rootstocks were used as a further inoculum source in a crop rotation experiment in the greenhouse to test the spread of Rhizoctonia solani from maize to sugar beet. Whole maize rootstocks (infected and healthy) were buried in the center of large pots. Sugar beet was sowed around these rootstocks. Within 5 weeks, a spread of Rhizoctonia solani was observed from one crop to the other. Additionally, Rhizoctonia solani was isolated from maize plants grown under field conditions, and isolates were compared to AG 2-2IIIB isolates from sugar beet by PCR. Isolates from maize were identified as R. solani AG 2-2IIIB. This is the first time, that anastomosis group 2-2 subgroup IIIB has been described as the causal agent of crown and brace root rot on maize in Europe. These results indicate that maize as a preceding crop in rotations may keep up the inoculum potential in the soil and therefore plays an important role in the epidemiology of Rhizoctonia root rot in sugar beet.