First report of Talaromyces pinophilus causing postharvest rot of sugar beet (Beta vulgaris) in Minnesota, USA

In August 2018, sugar beet roots with dark brown to blue lesions was observed (5% incidence) in Moorhead, Minnesota (46.8738° N, 96.7678° W). Infected sugar beet roots were collected and stored in a cold room at 8 ±2°C, with 78% relative humidity. A week later, dark greyish to blue mould was found on 10% of sugar beet root surface. In the second week, blue-green mycelial growth was observed on the surface (Fig. 1).

Infected root tissues were dissected and small pieces (5 mm²) were disinfected using 70% ethanol for 1 minute, flushed thrice with sterile water, air dried, transferred to 50% potato dextrose agar  and incubated at 25°C with a 12-hr photoperiod for seven days (Khan et al., 2019). The fungal colony was observed with blue-green velvety and white margins on the periphery (Fig. 2). Conidia were hyaline, globose, and conidiophores densely penicillated (Fig. 3). The morphological characteristics of the fungus suggested a Talaromyces species (Yilmaz et al., 2014). Five isolates were developed by the single spore isolation method and genomic DNA was extracted. For the PCR assay, the ITS4/ITS5 primers were used to amplify the ITS genomic region. PCR products were cleaned using an E.Z.N.A® Cycle Pure Kit (Omega Bio-tek, USA) and sent for Sanger sequencing by GenScript (GenScript, Piscataway, USA). A Blastn analysis of the ITS sequences of the five isolates showed 100% alignment with Talaromyces pinophilus (Penicillium pinophilum), GenBank Accession No. AB455516.1, with an E-value of 0. The amplified sequence (539 bp) was submitted to NCBI (MK757839.1).

A pathogenicity assay was done by spraying a conidial suspension (1 × 10⁶ conidia/ml) on the surface of healthy sugar beet (cv. Marathon) and kept in a cold room at 8°C and 80% relative humidity. Twelve 22-week-old roots with two replications were used. Mock-inoculated sugar beet roots were sprayed with sterile water. Three weeks post-inoculation, all inoculated sugar beet roots were observed to have similar symptoms to those described previously (Fig. 4). The mock-inoculated beets remained disease free. Microscopic analysis of the fungus isolated from inoculated beets revealed a similar morphology to that used for inoculation, thus fulfilling Koch's postulates.

Recently a Penicillium-like Talaromyces sp. has been reported to cause sucrose loss in sugar beet piles (Strausbaugh & Dugan, 2017; Strausbaugh, 2018). In this present study, the pathogenicity of Talaromyces pinophilus has been demonstrated in sugar beet. This is the first report of T. pinophilus causing postharvest rot in sugar beet in the USA.