First report of Fusarium solani (Mart.) Sacc. causing sugar beet seedling rot in Minnesota, USA

Authors: KHAN, MOHAMED - North Dakota State University; BHUIYAN, ZIAUR - North Dakota State University; DEL RIO MENDOZA, LUIS - North Dakota State University; Lakshman, Dilip; Ismaiel, Ed - Ed; BALDWIN, THOMAS - North Dakota State University; AZIZI, ABDOLBASET - North Dakota State University; AMEEN, GAZALA - South Dakota State University

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/12/2023
Publication Date: 10/6/2023
Citation: Khan, M., Bhuiyan, Z., Del Rio Mendoza, L., Lakshman, D.K., Ismaiel, A.A., Baldwin, T., Azizi, A., Ameen, G. 2023. First report of Fusarium solani (Mart.) Sacc. causing sugar beet seedling rot in Minnesota, USA. Plant Disease. https://doi.org/10.1007/s42161-023-01518-7.
DOI: https://doi.org/10.1007/s42161-023-01518-7

Interpretive Summary: Sugar beet (Beta vulgaris L.) is the leading sugar-yielding crop next to sugarcane and accounts for 25 % of the world’s sugar. Minnesota is the top sugar beet (Beta vulgaris L.) producing State in the United States. In a disease survey conducted in Moorehead, Minnesota, sugar beet seedlings with symptoms of stunting, roots with brown to black color, and depressed necrotic root lesions with rotten root tips were observed. Molecular and morphological analysis revealed that the disease was caused by the soilborne fungal pathogen Fusarium solani. The pathogenicity of the fungus was confirmed by artificial inoculation on healthy sugar beet seedlings. To the best of our knowledge, this is the first report of the fungus causing seedling rot in Minnesota. The discovery of this pathogen in sugar beet fields of Minnesota will prompt future research to screen for resistant varieties and initiate research to evaluate the disease’s impact on yield and quality. Those findings will be helpful to farmers, scientists, and extension agents.

Technical Abstract: ugar beet (Beta vulgaris L.) seedlings showing disease symptoms were collected from a growers’ field located in Moorhead (46.9190 N, 96.70610 W), Minnesota, USA. Affected plants were stunted, pale in color, and had less vigor with rotten root tips (Fig. 1a & b). Disease incidence was estimated approximately 15-20%. Symptomatic plant roots were surface sterilized in 1% NaOCl solution and about 2 cm root pieces were placed in water agar (WA) and amended potato dextrose agar (PDA) media and incubated for seven days at 25 0C in darkness. Single spores were sub-cultured on carnation leaf agar (CLA) media for observing morphological features. Colonies had uniform appearances and produced creamy white, sparse to abundant, thick, and floccose mycelia (Fig. 1c). Conidiophores produced from lateral hyphae were simple to branched. Phialides were slender, smooth, hyaline, and septate. Micro-conidia were abundant, single-celled, oval to ellipsoid in shape with smooth walls, and measured 3.4 to 8.8 µm X 1.3 to 4.3 µm (Fig. 1d). Macro-conidia had 3 to 5 septa, and were hyaline and moderately curved shaped, thick-walled, slightly blunted apical end and measured 12.5 to 30.2 µm X 2.2 to 3.6 µm (Fig. 1e). Chlamydospores were round, thick-walled, and produced in abundant numbers, singly or in chains (Fig. 1e). Based on morphological characteristics, the fungal isolates were identified as Fusarium solani (Mart.) Sacc. (Leslie & Summerell, 2006). Genomic DNAs of the fungal isolates were extracted and the internal transcribed spacer (ITS1/ITS4), translation elongation factor (EF1/EF2) (O’Donnell et al., 2009), second largest subunit of nuclear RNA polymerase II (5F2/7cR and 7cF/11aR) (Liu et al., 1999), and ß-tubulin (Bt2a/Bt2b) (Glass & Donaldson, 1995) genes were amplified and sequenced. Respective sequences of the isolates were identical and submitted to GenBank under accession numbers OP347082 (ITS), OP353550 (EF-1a), OP353551 (5F2/7cR), OP353552 (7cF/11aR), and OP353553 (Bt2a/Bt2b). BLASTN search against the NCBI database showed the obtained sequences have 100% homology with MT605584 (ITS/ITS), MN650097 (TEF-1a), MK441990 (5F2/7cR), MH582226 (7cF/11aR), and MH598820 (Beta- tubulin) of F. solani. Pathogenicity tests were conducted on ten days old sugar beet seedlings (cv. Marathon, SeedEX). Ten seedlings were immersed for five minutes in a spore suspension of 1x106 conidia/ml and transferred into pots (25x125x20 cm) with peat mix and the experiment was replicated thrice. After inoculation, all seedlings were incubated in greenhouse under 12-h photoperiod and a temperature of 25 0C for 21 days. At 21 days post-inoculation, all seedlings exposed to the pathogen wilted and roots turned brown and rotten; symptoms that were identical to those observed in the field (Fig. 1g & h) while sterile water mock-inoculated plants remained symptomless. The fungus was re-isolated from the inoculated plants and represented similar morphology to the isolate used for seedling inoculation. F. solani has been reported to cause seedling rot in many economically important crops worldwide. In the United States, F. solani was reported causing sugar beet stalk rot on the Pacific Coast (Hanson & Lewellen, 2007). To our knowledge, this is the first report of Fusarium solani causing seedling rot in sugar beet in Minnesota, USA.