Abiotic soil properties and the occurrence of Rhizoctonia crown and root rot in sugar beet

Since 1993, Rhizoctonia crown and root rot (Rhizoctonia solani AG 2-2 IIIB) represented an increasing problem for sugar beet production in Germany. Up to now, the outbreak of the infection and the spread of the disease within a field cannot be predicted and effective countermeasures are not available. Although little is known about the living conditions of R. solani in soils, abiotic soil properties are likely to influence the disease occurrence. Investigations were carried out based on 60 pairwise comparisons, each consisting of a disease-affected and an adjacent nonaffected patch on farmers' fields in 2002 and 2003. Soil samples from the top soil layer (0-30cm) were collected before harvest, and eight of the most frequently mentioned soil properties potentially influencing Rhizoctonia crown and root rot infection were examined: bulk density, texture, carbonate carbon, potassium, phosphorus, organic carbon, total nitrogen, and pH. The occurrence of the disease was significantly related to the soil C : N ratio, indicating the influence of soil organic matter on the disease occurrence. Examinations of soil thin sections showed that organic-matter particles in the soil serve as a substrate for R. solani. All other soil physical and chemical properties examined did not differ between the disease-affected and nonaffected patches and seem to be of minor importance.