First report of Fusarium madaense as a cause of root and stalk rot on Sorghum bicolor in Australia

Sorghum (Sorghum bicolor) is mainly grown for stock feed in eastern Australia where significant economic losses result from diseases caused by Fusarium spp. (Petrovic et al., 2009; Kelly et al., 2017). During the 2018 sorghum growing season, 66 plants showing symptoms typical of Fusarium infection were randomly collected from three sorghum fields in Barramornie (27.08○N, 150.11°E), southern Queensland. Fusarium isolates were obtained from surface-sterilised sorghum stem or root pieces (1 cm) placed on Fusarium-selective PCNB media (Leslie & Summerell, 2008) and incubated at 23°C for five days. Ninety single-spore cultures were sub-cultured on potato dextrose agar (PDA) for molecular characterisation and 17 of these were deposited into the Queensland Plant Pathology Herbarium (BRIP, Brisbane, Australia). One isolate (BRIP 75161a) recovered from a diseased stem node was identified as F. madaense by DNA barcodes. Comparison of the partial gene sequences of RNA polymerase II largest subunit (rbp1), RNA polymerase II second largest subunit (rbp2), and translation elongation factor 1-alpha (tef-1α) (GenBank Accession Nos. OQ695493, OQ657300 and OQ657299, respectively) of isolate BRIP 75161a showed 100%, 100% and 99.7% nucleotide identities with F. madaense (ex-type: CBS 146669), respectively. The polyphasic identification tool on the Mycobank FUSARIOID-ID database (https://www.mycobank.org) using the same three genes confirmed the identity of the isolate as F. madaense with 99.9% identity to F. madaense (ex-type: CBS 146669). Phylogenetic analysis with partial sequences of rbp1, rbp2 and tef-1α, with five closely related species (F. andiyazi, F. madaense, F. mirum, F. musae, and F. verticillioides) (Costa et al., 2021a) resulted in isolate BRIP 75161a clustering with the ex-type strain and reference isolates of F. madaense in a well-supported clade (Figure 1). Colonies of F. madaense (BRIP 75161a) on synthetic nutrient-poor agar and carnation leaf agar (Leslie & Summerell, 2008) were white with infrequent areal mycelium without pigmentation (Figure 2), while on PDA they were powdery to floccose with abundant whitish aerial mycelia, becoming violet. Microconidia were cylindrical to narrowly ellipsoidal or obovoid, 0–4 septate, 6–16 × (1.5–) 2–4.5 μm, hyaline, produced on false heads (Figure 3) or long aerial chains. Pathogenicity was assessed on 10-day-old seedlings of S. bicolor cv. Resolute. One plant per pot was grown in sterile vermiculite inoculated with an agar plug (0.5 cm) of isolate BRIP 75161a (Kelly et al., 2017). Each experiment consisted of three replicates and the same experiment was conducted three times. Reddish to black discolouration in infected roots were observed (Figure 4). Control plants grown in sterile vermiculite inoculated with a non-colonised agar plug did not develop symptoms (Figure 4). Koch's postulates were completed by reisolating the fungus from the root tissues (10 days post inoculation) and by comparing cultural morphology on PDA. Fusarium madaense causes stalk rot in sorghum, maize, and millet, and pokkah boeng in sugarcane (Costa et al., 2021a). It was first isolated from groundnut and sorghum in Nigeria (Ezekiel et al., 2020) and later identified in several tropical countries (Costa et al., 2021b). This is the first report of F. madaense in Australia. This work was carried out as part of a project funded by Broadacre Cropping Initiative in Queensland, Australia.