Quantitative methods for assessment of the impact of different crops on the inoculum density of Rhizoctonia solani AG2-2IIIB in soil

Abstract

Rhizoctonia solani AG2-2IIIB is the causal agent of late crown and root rot in sugar beet. In a 4-year field study we analyzed the impact of different plant residue management systems of sugar beet and maize as well as of growing wheat (non-host) and different maize varieties on the soil inoculum density of R. solani. Sugar beet remains were either tilled or removed from the field; maize was then grown during the two following years and also tilled or removed. The soil inoculum potential of R. solani was studied using three different on- and off-site monitoring systems. A monthly assessment of root damage indices of maize and sugar beet and broad bean as an indicator plant was carried out. In addition, an indirect quantitative real-time PCR assay using quinoa seed baits was developed to analyze field soil samples for R. solani AG2-2 soil concentration at the end of each year. The results show that the non-host wheat as a pre-crop to sugar beet reduced the Rhizoctonia inoculum potential in the soil significantly. Additionally, the incorporation of host plant debris (sugar beet + maize) into the soil increased the Rhizoctonia soil inoculum potential and the incidence of sugar beet rot. Although the maize genotypes’ susceptibility to R. solani differed, their plant debris did not significantly influence growth and survival of R. solani in the soil. This work describes methods that allow elucidating the effect of agricultural practice on Rhizoctonia levels in the soil and on disease development in the field.