Sclerotinia sclerotiorum (Lib) deBary causes root rot and necrosis in sugar beet in Moorhead, MN, USA

Abstract

Sugar beet is a major sugar yielding crop in the states of Minnesota (MN) and North Da- kota (USA). Sugar beet root samples collected from Moorhead, MN in September 2020 had typical rot symptoms along with whitish mycelia growth and blackish sclerotia on the external surface of the root. Pure, sterile cultures were obtained from infected roots. Scle­ rotinia sclerotiorum was identified based on morphological features and further confirmed molecularly by sequencing of the Internal Transcribed Spacers (ITS) region and matching homology with reported ITS of the fungus. Pathogenicity of S. sclerotiorum was confirmed through mycelial inoculation of seeds and roots under laboratory and greenhouse condi- tions. Inoculated seeds showed a range of symptoms that included pre- and post-emergence damping off, wilting, black discoloration of roots, constricted collar regions and stunted seedling growth. Under laboratory conditions, roots were artificially wounded using a cork borer and inoculated by mycelial plug. This resulted in noticeable root decay and growth of whitish, cottony mycelia and sclerotia externally. Transverse sections of the diseased root showed brown to black discoloration and rotting of internal tissue. Root inoculation of 4-week old sugar beet plants was achieved by depositing pathogen colonized barley grains near roots in the greenhouse, resulting in brown to black lesions and necrosis of root tissue when evaluated at 28 days post inoculation. The S. sclerotiorum was re-isolated from inocu- lated roots showing infection and identical pure isolates of the pathogen were recovered from field samples. These findings could be useful for sugar beet growers in Minnesota, allowing better management of this pathogen under field and storage conditions before its widespread future occurrence.

Keywords

• disease management soilborne diseases
• fungal pathogen
• pathogenicity