Fusarium brachygibbosum and Fusarium nygamai causing basal rot of onion (Allium cepa) in India
Year: 2024
Type: article
Abstract: Developing cultivars with broad-spectrum resistance to Fusarium basal rot is a major aim of onion breeding. Studies conducted between 2020–2023 to identify Fusarium spp. causing basal rot in Karnataka, India detected only F. acutatum and F. falciforme (Bhat et al., 2023). In September 2023, 34 onion bulbs with symptoms of Fusarium basal rot were collected from farmer's fields in the districts of Bagalkot, Chikkamagaluru, and Chitradurga in Karnataka (Figure 1). Single spore isolation of Fusarium from the diseased bulbs resulted in 64 cultures. Among them were 11 isolates with a colony morphology and colour on potato dextrose agar distinctly different from those previously described, prompting further investigation. Protocols used for isolation and characterisation of Fusarium were as described in Sarwadnya et al. (2023). Two isolates from the ochre-yellow (IBFB-35A and IBFB-35B) and dark purple (IBFN-41A and IBFN-41B) colour groups were cultured on synthetic nutrient-poor agar and carnation leaf agar media to study spore morphology. IBFB-35A and IBFB-35B produced microconidia with zero to one septum measuring 5.87-6.95 × 1.7-2.72 µm (n = 20) in size (Liu et al., 2023) and chlamydospores were round, single or in chains and were either terminal or intercalary in position, but macroconidia were not observed. Isolates IBFN-41A and IBFN-41B produced three-septate macroconidia, 17–32.9 × 1.43-3.71 µm (n = 20), microconidia were 6.07-7.88 × 1.43-2.69 µm (n = 20) in size and chlamydospores either solitary or in chains as described by Zhang et al. (2023) (Figures 2- 3). The internal transcribed spacer (ITS) and partial translation elongation factor 1-α (tef1) gene (O'Donnell et al., 1998) regions of the isolates were amplified and sequenced. Sequences of the four isolates (IBFB-35A, IBFB-35B, IBFN-41 and IBFN-41B) were submitted to GenBank, Accession Nos. PP981364, PP981365, PP981366, PP991501, and PP986950-PP986953 for the ITS region and tef1, respectively. Phylogenetic analysis placed isolates IBFB-35A and IBFB-35B with F. brachygibbosum (F. solani complex) and isolates IBFN-41A and IBFN-41B with F. nygamai (F. fujikuroi species complex) (Figure 4). The pathogenicity of the F. brachygibbosum and F. nygamai isolates, along with previously characterised F. acutatum (IBFA-17) and F. falciforme (IBFF-10) isolates, (Bhat et al., 2023), were tested on a susceptible onion line 16/7Y GR3. Twenty-eight-day-old seedlings, grown in sterilised cocopeat, were removed and washed with sterile water and immersed in a conidial suspension (1×104 conidia/mL) for 15 minutes before transplanting into a pre-sterilised soil mix (red soil: farmyard manure: cocopeat, 2:1:1 v/v/v). Each seedling was then treated at the base with 5 mL of the conidial suspension. Control plants were mock-inoculated with sterile water. Pots were kept in polytunnels with natural light (c. 13 hours daylight) and a day/night temperature of 27–33/16-18°C. All inoculated onion plants were dead by 40 days post-inoculation (Figure 5). However, when compared with F. acutatum and F. falciforme, the symptoms caused by F. brachygibbosum and F. nygamai took longer to develop, and inoculated plants took longer to wilt (Table 1). Fusarium brachygibbosum causing basal rot of onion has been reported in Mexico (Tirado-Ramirez et al., 2021).This is the first report of the pathogen infecting onion in India. Fusarium nygamai was reported as an endosymbiont of onion in Egypt (Sayed et al., 2021) but this is the first report as a pathogen of onion in India or globally.
Source: New Disease Reports
DOI: 10.1002/ndr2.12311
Authors: A. Rajakumara, G. Bhat, G. Shivakumar, B. B. Madalageri, P. Noojibail, R. Anandalakshmi
Language: English
Subfield: Cell Biology
Domain: Life Sciences
SDG: Zero hunger
Open Access status: diamond
OpenAlex ID: W4403236619