Histopathology of Rhizoctonia root and crown rot of sugar beet reveals differential responses in susceptible and resistant cultivars to invasion by Rhizoctonia solani AG 2-2

Authors: MINIER, DOUGLAS - Michigan State University; Hanson, Linda

Submitted to: Annals of Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/7/2026
Publication Date: 7/8/2026
Citation: Minier, D.H., Hanson, L.E. 2026. Histopathology of Rhizoctonia root and crown rot of sugar beet reveals differential responses in susceptible and resistant cultivars to invasion by Rhizoctonia solani AG 2-2. Annals of Botany. 00:1-16. https://doi.org/10.1093/aob/mcag201.
DOI: https://doi.org/10.1093/aob/mcag201

Interpretive Summary: Rhizoctonia root and crown rot is a serious disease of sugar beet. It is caused by the fungus Rhizoctonia solani anastomosis group (AG) 2-2. Little is known about the infection process on underground tissues like roots. Using several microscopy and staining methods we investigated the infection process for R. solani AG 2-2 in beet roots. Several differences were observed in the infection process compared to other crops, such as a lack of T-shaped branching or alignment with host cell walls. Infection cushions were formed but were not necessary for the fungus to penetrate the plant. The resistant variety showed fewer fungal penetration sites than the susceptible variety. In both types, the fungus spread in a shallow layer between the outer surface and the outermost vascular ring, with deeper penetration seen where the vascular ring was not complete. The root groove showed higher invasion levels than other root regions. A callose-type resistance response was observed in the resistant variety as well as a lower rate of breakdown of pectin in the tissue. Results indicate early infection is likely impacted by root anatomy. These indicate some potential areas that might be targeted for resistance breeding.

Technical Abstract: Rhizoctonia solani anastomosis group (AG) 2-2 causes Rhizoctonia root and crown rot (RRCR) of sugar beet and is a major constraint on production. Adult sugar beet roots possess distinctive anatomical features, including a corky periderm, a root groove region, and concentric vascular rings, that may influence infection outcomes for pathogens. Adult-plant resistance is known but the mechanisms are not known. This study provides an updated histopathology study of RRCR and compares responses in a resistant and a susceptible variety. Six-week old roots of a susceptible (C869) and resistant (SR98/2) variety were inoculated with R. solani AG 2-2. Roots were sampled starting at 3 days up to 12 days after inoculation. Tissues sections were examined with brightfield, epi-illumination, fluorescence, and confocal microscopy. Infection processes, tissue maceration and plant cell wall modifications were included. The invading hyphae did not produce structures reported on other hosts such as T-shaped branching but did form infection cushions. Direct penetration was observed with or without infection cushion formation and was reduced in the resistant variety. Hyphae spread laterally between the periderm and the outer cambium, with deeper penetration observed here the outer cambium was disrupted. Tissues within the root groove showed a higher susceptibility associated with structural discontinuities. Lesions showed a red autofluorescence. Lesions in the resistant variety were bordered by callose. There also was evidence for differential pectin degradation between the resistant and susceptible varieties. Results indicate early infection is likely impacted by root anatomy. A callose-associated response was observed in resistant roots. These results indicate some potential targets for resistance breeding.