First report of Stromatinia gladioli causing corm rot of saffron ( Crocus sativus ) in the United Kingdom

Saffron (Crocus sativus L.) is cultivated for its dried stigmas, which are used as a spice. In June 2024, stunted and yellowing plants were observed in a two-acre commercial saffron field in Cheshire, England, UK, with approximately 30% of the field affected. Corms from affected plants were small and shrivelled, showing symptoms of internal dry rot with dark brown/black lesions when cut open. To identify the causal organism, pieces of diseased tissue were removed, surface-sterilised in 5% sodium hypochlorite for 45 seconds, rinsed twice with sterile water, and placed on potato dextrose agar (PDA) containing chlortetracycline (20 mg L⁻¹). After 3-7 days at 20 °C, a fungus with tan mycelium and small, roundish black sclerotia was consistently isolated from multiple corms. The sclerotia ranged from 176 to 300 μm in diameter, with an average of 219.8 ± 3.6 μm (n=80). DNA was extracted from fungal colonies, and PCR was carried out to amplify and sequence the internal transcribed spacer (ITS) and the large subunit (LSU, 28S) of the ribosomal DNA (rDNA) gene using the primers ITS1/ITS4 (White et al. 1990) and LR0R/LR6 (Vilgalys 1990), respectively. Following BLAST searches, the ITS and LSU sequences obtained showed >99% identity to Stromatinia (syn. Clarireedia) gladioli, respectively (GenBank accession nos. PX369403-PX369416 for ITS and PX394035-PX394038 for LSU). To evaluate the pathogenicity of four of the fungal isolates (AJ1011, AJ1014, AJ1016, and AJ1017), healthy saffron corms were surface sterilised by spraying with 70% ethanol and air-dried under sterile conditions. These were then inoculated with a 4 mm diameter agar plug from each isolate and placed onto a wound made on the basal plate of each corm. For control treatments, a sterile PDA plug was placed on the basal plate. Each isolate was tested on eight corms in two separate experiments. Corms were incubated in a moist chamber at 15 °C in the dark for four weeks, after which disease severity was assessed. Saffron corms inoculated with each of the four isolates developed severe symptoms identical to those observed previously, with >50% area of each corm affected. To fulfil Koch’s postulates, re-isolation from the inoculated corms resulted in fungal colonies with morphology consistent with S. gladioli, with identity confirmed by the formation of sclerotia and PCR and sequencing of ITS. Control corms remained symptomless, and no pathogen was isolated from them. To our knowledge, this is the first report of S. gladioli infecting Crocus sativus corms in the United Kingdom, representing a new threat to saffron production and expanding our understanding of the pathogen's geographic distribution. S. gladioli has previously been reported on saffron in Italy (Fiori et al. 2007) and Spain (Muñoz et al. 2020) and has caused dry rot of Gladiolus in the UK (Drayton 1934). None of the UK-registered fungicides have label permission for saffron; therefore, early detection, avoiding planting in infested fields, and implementing preventive management strategies will minimise the spread and disease impact.