First Report of Geotrichum candidum Causing Postharvest Rot of Sugar Beet (Beta vulgaris) Roots in Minnesota and North Dakota

Minnesota (MN) and North Dakota (ND) together produce about 57% of U.S. sugar beets (Beta vulgaris L.) for sucrose extraction from its tap root. In October 2018, healthy sugar beet roots were harvested from two major production areas, Grand Forks, ND, and near Foxhome, MN, and kept in storage at 8°C and 80% relative humidity. After 1 week, water-soaked soft rot with brown irregular lesions 10 to 50 mm in diameter and thick foamy white mycelial growth were observed on the sugar beet roots, along with a smell of fermentation. About 10% of the roots from Grand Forks, ND, and 5% of the roots from Foxhome, MN, had symptoms that covered approximately 30% of the root surface. Small pieces (10 mm²) of lesion margins were excised, surface sterilized with 10% sodium hypochlorite for 1 min, rinsed twice with sterile distilled water, air dried, and transferred to corn meal agar (CMA) and clarified V8 and incubated at 25°C with a 12-h photoperiod for 7 days. Colonies were white, thin, flat, and feathery on both media. Hyphae were dichotomously branched and septate. Conidia were arthrosporous, terminal or intercalary, hyaline, subglobose, one-celled, often in chains and/or solitary. The dimension of conidia varied from 5 to 12 × 2 to 5 µm (Watanabe 2002). Based on cultural and morphological characteristics, the fungus was tentatively identified as Geotrichum species. Genomic DNA was extracted (Norgen Biotek Corp., cat. no. 27300) from eight isolates: four isolates each from ND and MN. The isolates from each state were identical. To confirm the identity of the fungus, the universal internal transcribed spacer (ITS) region was amplified using the ITS1/ITS4 primers (White et al. 1990). The amplified PCR products were cleaned and sent for Sanger sequencing to GenScript. The sequences for the ITS-5.8S rDNA region for the ND (GenBank accession MK436200) isolates had 99% sequence identity with G. candidum accessions JQ668739, KY103456, and KR815826. For the same genetic region, the MN (MK436203) isolates had 99% sequence identity with G. candidum accessions KF112070 and LC054542. In comparison with the G. candidum type strain CBS 178.71 (KF984491), the ND and MN isolates had 95 and 91% sequence identity, respectively. Pathogenicity was confirmed in the greenhouse. Twelve 20-week-old healthy sugar beet (cv. Crystal 101) roots were surface sterilized and scraped with a sterile cork borer, followed by plugging the wound (5 mm²) with 7-day-old G. candidum on CMA. Mock-inoculated sugar beet roots (12) were used as a control. All the roots were kept at 25°C and 80% relative humidity. One week postinoculation, all inoculated sugar beet roots had the same symptoms as the field-infected roots. No symptoms were observed in the mock treatment. The fungus reisolated from the diseased beet root had identical cultural and morphological characteristics to G. candidum. There are several reports of G. candidum as the causal agent for rot of fruits, including Mori fructus and peach, as well as storage rots of sweet potato (Alam et al. 2017; Holmes et al. 2002; Zhang et al. 2018). Root rot of sugar beet caused by Geotrichum sp. F373 was reported for roots collected from Oregon and Idaho (Strausbaugh et al. 2009). To our knowledge, this is the first report of G. candidum causing rot on sugar beet root in storage in ND and MN, U.S.A. Because roots may be stored for about 4 to 5 months in nonventilated piles before processing, it will be useful to investigate and develop strategies to limit losses that could be caused by G. candidum.