Histopathology of Rhizoctonia solani AG 2-2 infection of sugar beet

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

Submitted to: American Society of Sugarbeet Technologists
Publication Type: Abstract Only
Publication Acceptance Date: 1/1/2021
Publication Date: 3/1/2021
Citation: Minier, D.H., Hanson, L.E. 2021. Histopathology of Rhizoctonia solani AG 2-2 infection of sugar beet. American Society of Sugarbeet Technologists. 58:99-100.

Interpretive Summary:

Technical Abstract: Rhizoctonia root and crown rot (RRCR), primarily caused by Rhizoctonia solani AG 2-2, is a major disease of sugar beet that can cause substantial losses in most regions where the crop is grown. The disease is characterized by dark, shallow lesions on the root surface that eventually expand to involve the entire root. While no sugar beet varieties are known to be fully resistant to RRCR, several varieties show delayed or limited disease progression and are considered moderately resistant or tolerant. Observations indicate that resistant varieties primarily delay the rate at which the fungus penetrates into deeper tissues thus limiting disease progression. We are building on the previous work of Ruppel (1985) examining the infection process and making comparisons of susceptible and resistant varieties using some more advanced histopathological techniques. Sugar beet varieties C869 (susceptible) and SR98/2 (resistant) were inoculated at six weeks after planting with an isolate of R. solani AG 2-2 and sampled at 3, 4, 5, 6, 7, 9, and 11 days after inoculation. Diseased tissue was excised, fixed in 10% formalin, embedded in paraffin, and sectioned on a microtome. Sections were stained with toluidine blue (general structure), berberine-aniline (suberin), ruthenium red (pectin), or wheat germ agglutin / propidium iodide (fluorescent observation of fungal and plant tissue) and viewed on both a light/fluorescence microscope and a confocal microscope. We have observed apparent suberization in the resistant variety in response to infection that is not present in the susceptible variety. While infection cushions and direct penetration are commonly observed, hyphae that are associated with lateral roots and vascular tissue progress deeper into the tissue more rapidly in both varieties. Hyphae that are not associated with vascular tissue are generally limited to the outer cortex layer, advancing into deeper tissue primarily where there are breaks in the outer vascular ring. The advanced microscopy techniques employed here are expected to provide new insights into the infection process of R. solani AG 2-2 in sugar beet and may lead to developing innovative approaches to resistance breeding.