First report of web blight of Lablab purpureus caused by Rhizoctonia solani AG‐5 in Bangladesh

Abstract

Lablab purpureus (country bean, hyacinth bean) is one of the important winter-season legume vegetables in Bangladesh. In October 2020, a new disease was observed in the vegetative stage of country bean crop in a research field of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Gazipur, Bangladesh. Several farmers’ fields were subsequently surveyed in districts of Gazipur for bean diseases and similar symptoms were observed. Initial symptoms included water-soaked lesions that expanded rapidly and coalesced into a scalded appearance. As the symptoms progressed, the leaves became brown and necrotic leading to premature defoliation (Figure 1). Near the margin and petiole of the diseased leaves, a whitish mycelial growth was observed (Figure 2). Microscopy of the infected tissue revealed the typical morphological characteristics of Rhizoctonia: colourless septate hyphae, a right-angled branching pattern, multinucleated hyphal cells, and constriction of the hyphae near their point of origin (Sneh et al., 1994). The prevalence of the disease in the surveyed area was 70–80%. For isolation of the pathogen, diseased tissues were surface sterilised using 70% alcohol for 30 seconds and 2% sodium hypochlorite for 30 seconds (Prova et al., 2018). Samples were then washed in distilled water and placed on water agar media and incubated at 27°C. After 24 hours, emerging mycelia from the margin of the diseased tissues were transferred to potato dextrose agar (PDA). After seven days, white to brown mycelial growth radiated over the entire culture plate, with the formation of a ring of micro-sclerotia on the edge of the plate within 14 days. Prominent dark brown sclerotia (0.9–3.1 × 0.4–2.5 mm) were observed. To determine the number of nuclei, hyphal cells were stained with an alkaline safranin solution (0.5% of safranin, 10 ml of KOH 3%, 5 ml of glycerin and 79 ml of distilled water) as described by Sneh et al. (1994). The isolate had multinucleate hyphal cells, hypha branched at right angles, and moniloid cell formation occurred before sclerotia formation (Figure 3). To confirm the identity of the causal agent, DNA was extracted from two representative fungal isolates and the internal transcribed spacer (ITS) region of ribosomal DNA was amplified using primers ITS4 and ITS5, and sequenced (Islam et al., 2021). The resulting ITS sequence had the greatest identity (97%) with Rhizoctonia solani AG-5 nucleotide sequences (GenBank Accession Nos. OM039414.1, OM039416.1, MZ379610.1 and MW999177.1) using BLAST, and was deposited in GenBank (ON553688 - ON553689). Five country bean plants with two-three trifoliate leaves (cv. Bu Seem 2) were used to test the pathogenicity of the isolates (Figure 4). Agar blocks from seven-day-old cultures were placed onto superficially wounded leaves. Uninoculated PDA blocks were used on control plants. Inoculated plants were incubated in a moist chamber and water-soaked necrotic lesions appeared both on leaves and stems three days after inoculation. Five days post inoculation, leaves showed severe necrosis and died, resembling the symptoms found in the field. The pathogen was re-isolated from the plants and the re-isolated fungal isolates were morphologically similar to Rhizoctonia solani. Although infection of anastomosis group (AG)-A and AG-4 of binucleate Rhizoctonia on different bean species (Glycine max, Phaseolus vulgaris and Vigna unguiculata) has been recorded (Yang et al., 2005; Yang et al., 2007; Quadros et al., 2021), this is the first report of Rhizoctonia solani AG-5 infection on Lablab purpureus to our knowledge in Bangladesh and worldwide. The authors are thankful to the Research Management Wing of Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh, for their support.