Cercosporella unguis-cati, the causal agent of the leaf spot of Dolichandra unguis-cati, reported from Paraguay

Keywords: aetiology, biocontrol, Cercosporella dolichandrae, invasive plant, Pseudocercospora unguis-cati

Cat's claw (Dolichandra unguis-cati; Bignoniaceae) is a perennial climbing plant, native to the Neotropics which, when introduced as an ornamental to other countries, has become an invasive forest weed. The potential for biological control of D. unguis-cati is being evaluated, with attention being given to fungal agents in its native range (Silva et al., 2012; Colmán, 2014) and a recently described cercosporoid fungus, Cercosporella dolichandrae, in South Africa (RSA) has attracted attention (Crous et al., 2014).

Silva et al. (2012) found leaf spots on D. unguis-cati in Brazil caused by Pseudocercospora unguis-cati. Lesions are subcircular, well delimited, dark brown surrounded by a purplish brown halo, 2-6 mm, coalescing, and infection leads to severe defoliation (Fig. 1). This was by far the most common disease of D. unguis-cati reported from surveys in Brazil and Paraguay (Silva et al., 2012; Colmán, 2014). Dried specimens from these surveys were deposited in the herbarium of the Universidade Federal de Viçosa (UFV, Minas Gerais, Brazil). Disease symptoms on D. unguis-cati were recognised as similar to leaf spots caused by C. dolichandrae (Crous et al., 2014), and questions emerged as to whether P. unguis-cati and C. dolichandrae are conspecific. A fresh specimen of the leaf spot was collected in June 2013 from General Higinio Morínigo (Caazapa Department, Paraguay) and deposited in the culture collection at UFV (Acc. No. COAD 3079). A pure culture was obtained by direct transfer of conidia from a sporulating lesion onto potato dextrose agar (PDA). The morphology was found to be identical to P. unguis-cati as described by Silva et al. (2012) (Figs. 2-3).

The identification was confirmed by molecular means. DNA was obtained from a single-spore pure culture grown on PDA with a Wizard Genomic DNA Purification Kit (Promega, USA). The ITS region was PCR amplified with the ITS1/ITS4 primer pair and the sequence was deposited in GenBank (Accession No. MW036753). A BLASTn search showed that the ITS sequence of the isolate from Paraguay had 99.8% identity (604/605 nt) to the type sequence of C. dolichandrae from RSA (NR156282.1; Crous et al., 2014). Cercosporella unguis-cati, originally collected on D. unguis-cati in Argentina and described by Spegazzini (1911), is morphologically identical to the fungal isolates from RSA, Brazil and Paraguay. The earlier name of Cercosporella unguis-cati Speg. takes priority over Pseudocercospora unguis-cati (Speg.) U. Braun and Cercosporella dolichandrae Crous & den Breeÿen, which are now considered synonyms.

The presence of Cercosporella unguis-cati in RSA appears to be due to an accidental introduction, likely with its plant host, from the Neotropics rather than an indigenous fungus having developed the ability to attack D. unguis-cati, as previously thought. It is now, fortuitously, acting as a classical biocontrol agent against D. unguis-cati in RSA (King, 2017).

The pathogenicity of C. unguis-cati against Australian biotypes of D. unguis-cati was demonstrated for isolate COAD 3079 and isolate IMI 507114 from RSA. Inoculation of D. unguis-cati leaves using mycelial plugs, taken from the actively growing margin of pure cultures of C. unguis-cati and subsequent incubation at 20°C and 100% RH for 48 hours resulted, after 28 days, in leaf spot symptoms identical to those originally observed.  Colonies with typical morphology of C. unguis-cati were obtained from these lesions, thus fulfilling Koch's postulates (Fig. 4). Our results clarify the identity of the pathogen and represent its first record from Paraguay.