First Report of Fusarium equiseti Causing Leaf Yellowing and Stunting on Sugar Beet (Beta vulgaris L.) in Montana, USA

In May 2019, sugar beet (Beta vulgaris L.) seedlings were observed with stunted growth, yellowing of leaves, and dying seedling root tip tissue discoloration in Sidney (47.7167° N, 104.1563° W), Montana. The disease affected about 40% of the 10 ha commercial field. Roots of infected seedlings were surface sterilized with 70% ethanol for 30 seconds, rinsed with sterile distilled water and cultured on amended 50% potato dextrose agar (PDA) for 7 days at 25°C. The isolated fungus produced white to yellow aerial mycelium. Macroconidia (18.9 to 30.3×2.5 to 3.6 µm, 5-7 septate) exhibited dorsoventrally curvature. No microconidia were found. Chains of chlamydospores were observed in three weeks old culture. The morphological features of the isolates were consistent with Fusarium equiseti [1,2]. Genomic DNAs (NORGEN BIOTEK CORP, Fungi DNA Isolation Kit #26200) of three representative isolates were used for polymerase chain reaction (PCR) with the internal transcribed primers (ITS1-ITS4) [3]. The amplified PCR products were cleaned by E.Z.N.A ®Cycle Pure Kit, OMEGA and the three samples were sent for Sanger sequencing to GenScript (GenScript, Piscataway, NJ). Nucleotide Blast analysis showed 100% sequence alignment to Fusarium equiseti with GenBank accessions MN049482.1, MN049480.1, MK990145.1, MK764999.1, MK770797.1, MK962344.1, MK990137.1, and MK990143.1. The sequence was submitted to GenBank with accession no. MN128568. Pathogenicity testing was done using 10, 2-week old sugar beet seedlings (Hilleshog proprietary material) grown individually in pots (4˝×4˝×6˝). Seedlings were inoculated (conidial suspension, 104 conidia/ml for 8 minutes) by the root dip method [4]. Mock inoculated plants were dipped in sterile water. Inoculated and control plants were placed in the greenhouse at 25 ± 2°C, and 75% to 85% relative humidity. Two weeks later, all inoculated seedlings showed symptoms similar to those observed in the field and non-inoculated seedlings were symptomless. This study was repeated. The fungus was isolated from diseased roots and confirmed to be Fusarium equiseti based on morphological characters. Fusarium equiseti was reported in freshly harvested and stored sugar beet in Europe; however, it was not found to be pathogenic [5,6]. reported that F. equiseti was associated with root rot of sugar beet but did not report pathogenicity. This pathogen is reported in several crops including edible beans which is grown as a rotational crop with sugar beet in several production areas [7]. The most important Fusarium species reported to cause significant economic damage to sugar beet include F. oxysporum and F. secorum [8,9]. The presence of another pathogenic Fusarium species in sugar beet will require monitoring to determine how widespread it is and whether current commercial cultivars are resistant. To our knowledge, this is the first report of Fusarium equiseti causing disease on sugar beet seedlings in the USA.