First report of Colletogloeum sp. as the causal agent of marginal leaf blight on Heliconia rostrata in India

Heliconia rostrata (false bird of paradise, Heliconiaceae) is native to South America but is an important herbaceous perennial ornamental plant in India and globally. In December 2016, a foliar blight was observed on six ten-month-old H. rostrata plants growing outdoors in Kolkata, West Bengal. Symptoms included a marginal blight, light brown to ash in colour, with a yellow halo (Fig. 1), and disease incidence ranged from 20-50%.

Blighted areas of the leaves were viewed under a microscope (×200) and acervuli were found in a concentric orientation around the diseased tissue. Conidiophores from the acervuli were pale brown, smooth, branched and simple, up to 13 µm long and 4 µm wide, with 1-2 percurrent proliferations. Conidia (n=30) were straight, irregular, sigmoid, tapered markedly to the apex, smooth, thick walled, 5-8 euseptate and 38 × 5 µm in size. Affected parts of ten diseased leaves were kept in a plastic box with wet filter paper and absorbent cotton to induce conidiation. Conidial masses were suspended in 250 µl sterilised distilled water on sterile glass slides and dropped onto 2% (w/v) water agar containing 0.5 mg/l of chloramphenicol. After 24 hours incubation at 25°C, individual germinating conidia were selected and transferred directly to potato dextrose agar and subcultured on peptone salt agar (10 g peptone, 5 g sodium chloride, 0.1 g calcium chloride, 20 g agar per litre). In both media, conidia and conidiophore sizes were similar to those observed on infected leaf tissues. Based on these morphological features the fungus was identified initially as a member of the genus Colletogloeum (Sutton & Mehrotra, 1982).

Genomic DNA was extracted from mycelia of the isolated fungus using the CTAB method (Doyle & Doyle, 1990) and the internal transcribed spacer (ITS) region and LSU ribosomal genes of rDNA were amplified using ITS1-F/ ITS4 (White et al., 1990) and LR1/ LR4 (Vilgalys et al., 1994) primers and sequenced (GenBank Accession Nos. MN644508 and MN644486 for ITS and LSU, respectively). BLAST analyses revealed that sequences from the present study had 99-100% identity with the type species of Colletogloeum sp. FG2.2 (FJ425194 and FJ031987). Based on the morphological characteristics and the molecular data, the causal agent was identified as Colletogloeum sp. (Hemnani et al., 2008).

The pathogenicity of the fungus was tested on six leaves (disinfected by spraying 90% ethanol followed by three rinses with sterile distilled water) from a single five-month-old H. rostrata plant grown in the greenhouse at 25°C with a 12/12-hr photoperiod and 100% relative humidity. Ten millilitres of a 3 × 10⁶ conidial suspension per ml was applied to each leaf. Another set of six leaves were sprayed with sterile distilled water as non-inoculated controls. After seven days, only the inoculated leaves showed leaf blight symptoms resembling those observed on naturally infected H. rostrata leaves. The pathogen was consistently re-isolated from the infected leaves thereby completing Koch's postulates.

To our knowledge, this is the first report of Colletogloeum sp. causing H. rostrata leaf blight in India and worldwide (Farr & Rossman 2019). The pathogen may pose a threat to H. rostrata production. These findings will be useful for the development of effective control strategies and further research.