Sporobolus (Poaceae) is a genus of about 200 grass species in tropical and subtropical parts of the world, including Africa, temperate and tropical Asia, Australasia, North and South America (Simon and Jacobs 1999; TPL 2013; WCSP 2017). In Australia, 18 species are endemic, and a further six species are naturalised weeds (AVH 2017). The genus Sporobolus has been divided into a number of sections of closely allied species (Peterson et al. 2014; Simon and Jacobs 1999), including the S. indicus complex, which comprises of 23 to 25 species (Clayton 1965; Peterson et al. 2014). In Australia, the S. indicus complex includes five introduced weedy species, S. africanus (Parramatta grass), S. fertilis (Giant Parramatta Grass), S. jacquemontii (American Rat’s Tail grass), S. natalensis and S. pyramidalis (Giant Rat’s Tail Grass), as well as five native species, S. blakei, S. creber, S. elongatus, S. laxus and S. sessilis (Clayton 1965; Peterson et al. 2014; Simon and Jacobs 1999).

The weedy Sporobolus grasses are of serious concern to the grazing industry of eastern Australia, having the potential to infest 60% of Queensland and 30% of Australia over a range of soil types, where the annual rainfall is greater than 500 mm, at an estimated cost of $60 million per annum (Bray and Officer 2007). In Australian rangelands, Sporobolus species are not considered desirable pasture grasses and are indicative of a degraded grazing system in which these grasses completely dominate pastures at the exclusion of most other grass species (Burrows et al. 1988; Shrestha et al. 2003). The few native species regarded as favourable fodder species, S. actinocladus, S. caroli, S. mitchellii and S. virginicus, have high protein-content when fresh, but do not provide bulk (Simon and Jacobs 1999). A further two of the native species, S. disjunctus and S. partimpatens, are considered rare (Simon 1993), and S. pamelae is listed as endangered in Schedule 2 of the Queensland Nature Conservation Act 1992.

A leaf smut was found by one of the authors (JSV), on several plants of S. natalensis at a property at Taunton, central Queensland in Feb. 2017, during a survey for endemic pathogens on native and introduced Sporobolus species. A second collection of U. sporoboli-indici was made on leaves of S. natalensis from a grazing property at Conondale, south-east Queensland in June 2017. The two locations are 350 km apart. Specimens were deposited in the Queensland Plant Pathology Herbarium as BRIP 65466 and BRIP 66039, respectively.

Symptoms on S. natalensis were distorted or misshapen inflorescences and black striae on the leaf blades that produced masses of black powdery spores (Fig. 1a and b). Infected plants had fewer inflorescences compared to adjacent healthy plants. Anecdotal evidence from the landowner suggested that the fungus had been present on the Taunton property for at least five years. The smut was morphologically identified as Ustilago sporoboli-indici, which is characterised by yellowish-brown spores (Fig. 1c) that are either globose, subglobose, ovoid or ellipsoidal, 6.5–11.5 × 5.5–9.5 μm, with finely punctate or moderately densely verrucose-echinulate walls uniformly 0.5–-0.8 μm thick (Vanky 2003). Vanky (2003) reported 25 Sporobolus species infected by 17 smut fungi belonging to the genera Anthracocystis (2), Jamesdicksonia (3), Macalpinomyces (3), Tilletia (1), Tranzscheliella (1) and Ustilago (7). Five Sporobolus species, S. africanus, S. elongatus, S. indicus, S. natalensis and S. pyramidalis, have been recorded as natural hosts of Ustilago sporoboli-indici in Africa, China and the Philippines (Vanky 2012). Other species, S. fertilis, S. natalensis, S. sessilis and S. scabridus, were shown as hosts in artificial inoculation tests in South Africa (Yobo et al. 2009).

Fig. 1
figure 1

Ustilago sporoboli-indici (BRIP 65466). a Inflorescence and leaves of S. natalensis infected with leaf smut. b Sori, and c Teliospores. Bar = 2 cm (a), 500 μm (b) and 10 μm (c)

Full size image

The morphological identification of U. sporoboli-indici was confirmed by comparing the DNA sequence of the Australian specimen BRIP 65466 with a South Africa specimen, BRIP 39706 (duplicate of HUV 20154) found on S. pyramidalis. Fungal spores were macerated with 0.5 mm glass beads in a Tissue Lyser (Qiagen). Genomic DNA was extracted with the DNeasy Plant Mini Kit (Qiagen) according to the manufacturer’s instructions. The primers V9G (de Hoog and van den Ende 1998) and ITS4 (White et al. 1990) were used to amplify the internal transcribed spacer (ITS) region, and the primers LR0R and LR5 (Schoch et al. 2012) were used to amplify partial region of the large subunit (LSU) of the nuclear ribosomal RNA. All loci were amplified with the Phusion High-Fidelity PCR Master Mix (New England Biolabs). The PCR products were purified and sequenced by Macrogen Incorporated (Seoul, Korea). The ITS and LSU sequences, BRIP 65466, (GenBank accession MF716451, and MF716450, respectively) and BRIP 39706 (GenBank accession AY772736, and MF716449) were identical.

To date, there are 33 endemic or introduced species of Ustilago recorded in Australia (Shivas and Vanky 2003; Shivas et al. 2014). This paper reports an additional Ustilago, the first record of U. sporoboli-indici in Australia. Ustilago sporoboli-indici was investigated in South Africa as a potential biological control agent for weedy Sporobolus species in Australia (Yobo et al. 2009). The systemic smut fungus produced sori in the leaves, leaf sheaths and stems, rendering the infected plant shoots sterile (Cunnington and Shivas 2006). The ability of this smut to infect a great number of shoots indicated that it had potential as a classical biocontrol agent in Australia.

However, U. sporoboli-indici was found to infect four Australian native Sporobolus species, S. creber, S. elongatus, S. scabridus and S. sessilis, and was rejected as a potential biological control agent due to its non-selectivity (Palmer 2008).