Onion (Allium cepa L.) is an important crop worldwide, including Saudi Arabia. In November 2020, 2-month-old onion plants (cv. Redwing) in commercial fields within the Sajir area of the Riyadh region (; 1.4 ha) showed symptoms of yellowing, wilting, stunting, bulb discoloration, rot in the basal parts of the bulb, and a decrease in roots. In the advanced stages, the affected plants collapsed and died. The incidence of symptomatic plants ranged from 30 to 65% in the surveyed fields. To isolate the pathogen, symptomatic onion plants (n= 20) were sampled. Diseased tissues from roots and bulbs were cut into small pieces (4× 4 mm), sanitized with 1% sodium hypochlorite solution for 2 min, submerged in 70% alcohol for 20 s, and then rinsed with sterile water before plating on potato dextrose agar (PDA) medium. The plates were incubated at 25 C for 6 days. Subcultures of the mycelia grown out of the diseased tissues produced purplish-pink fungal colonies on PDA. On carnation leaf agar, cultures were characteristic of Fusarium oxysporum as described by Leslie and Summerell (2006), with the presence of unicellular microconidia (3.8 to 7.8× 1.7 to 2.5 µm, n= 50) without septa in false heads or short monophialides and slightly curved macroconidia (16.3 to 28× 4.2 to 6.1 µm, n= 50) with two to four septa. Older mycelia developed many chlamydospores that were single or in short chains. To further confirm pathogen identification, DNA was extracted from single-spore cultures of three representative isolates using the DNeasy Plant Mini kit (QIAGEN, Hilden, Germany). Three different fungal nuclear regions of internal transcribed spacer (ITS), translation elongation factor 1-a (TEF1-a), and the second largest subunit of DNA-directed RNA polymerase

II (RPB2) DNA were amplified by PCR and sequenced with the following primers: ITS4 and ITS5 (White et al. 1990), EF-1 and EF-2 (O, Donnell et al. 1998), and fRPB2-5F and fRPB2-7cR (Liu et al. 1999), respectively. Phylogenetic analysis based on the alignment of the ITS, TEF1-a, and RPB2 sequences using MEGA7 placed these isolates in the F. oxysporum clade. The ITS, TEF1-a, and RPB2 sequences of an isolate (FOC-OR9) were submitted to GenBank (OL721757, OL764494, and OL764495, respectively). To confirm the forma specialis cepae, a fragment of the Fusarium oxysporum f. sp. cepae gene Secreted In Xylem 3 (SIX3) was amplified by PCR (Kalman et al. 2020). The SIX3 amplicon (; 277 bp) was sent for sequencing, and the sequence was submitted to GenBank (OL828265). BLASTn analysis of the sequences showed 100% identity with F. oxysporum f. sp. cepae (KP746408). To fulfill Koch, s postulates, pathogenicity tests were performed with healthy onion bulbs ‘Redwing, of 100 to 150 g each. Prior to inoculation of onion bulbs, the dry bulb scales, one of the fleshy inner scales, and the roots were removed. Bulbs were then surface sterilized (as described above) and injected with 20 µl of a conidial suspension (106 spores/ml) into the basal plate of each bulb and approximately 1 cm deep into the tissue. Six bulbs were inoculated for each isolate, placed in a mesh bag, and incubated at 28 C in the dark. Six bulbs were injected with sterile water, and six noninoculated bulbs served as controls. At the 4th week postinoculation, necrotic rot symptoms and brown discoloration were observed on the basal plates of these inoculated bulbs (symptoms similar to those observed in the field), whereas control treatments showed no symptoms. The pathogen was reisolated from the basal plates onto PDA and identified morphologically and molecularly as F. oxysporum f. sp. cepae, thus fulfilling Koch, s postulates …