Vol.:(0123456789) 1 3 https://doi.org/10.1007/s13314-021-00444-w Colletotrichum siamense causing anthracnose-twister disease of onion (Allium cepa) in Sri Lanka Indunil S. Herath 1  · Dhanushka Udayanga 1  · Shaneya Miriyagalla 1  · Lisa A. Castlebury 2  · Dimuthu S. Manamgoda 3 Received: 22 August 2021 / Accepted: 26 October 2021 © Australasian Plant Pathology Society Inc. 2021 Abstract Symptomatic samples of anthracnose-twister disease of cultivated onion (Allium cepa) were collected from heavily infected felds at Matale district, Sri Lanka during a severe outbreak that occurred through late July to August in 2020. The pathogen was identifed as Colletotrichum siamense based on morphology and combined gene sequence analysis of nuclear ribosomal internal transcribed spacers 1 and 2 with 5.8S region (ITS), glyceraldehyde-3-phosphate dehydrogenase gene (GPDH) and beta-tubulin (TUB) regions. C. siamense is a species with global distribution and a wide host range. This is the frst report of C. siamense infecting onion in Sri Lanka. Keywords Anthracnose · Molecular diagnostics · Onion diseases · Tropical pathogens Onion (Allium cepa), an important crop grown for its fa- vorful bulbs and leaves, is consumed globally. Large scale cultivation of onion in Sri Lanka accelerated in the early 1980’s. Overall, the extent of cultivation has increased since then with some annual fuctuations. However, in the year 2020 local cultivation did not meet even a quarter of the annual requirement of onion (DCS, 2020). One major fac- tor contributing towards the reduction of onion yield in the country was the impact of plant diseases that occur both in the nurseries and feld. Fungal pathogens are the causative agents of many severe diseases that reduce yield of onion worldwide. Among them, species of Alternaria, Colletotri- chum, Fusarium, Sclerotium, and Rhizoctonia are widely encountered (Conn et al. 2012; Mishra et al. 2014). Among the major onion producing regions in Sri Lanka, Matale district produces an estimated 50% of the total production of big onion (DCS, 2020). Through late July to August in 2020, a severe outbreak of anthracnose-twister disease was reported from several localities of onion culti- vations of Matale district (Central Province), in Sri Lanka leading to a signifcant yield loss. These afected onion felds in Matale district were reported to abandon because of this disease, causing a short-term scarcity of onion supply in the local market. The leaves and bulbs of onion plants showed typical anthracnose-twister disease symptoms (Alberto et al. 2019), such as yellow to dark spots of leaves and bulbs, irregular leaf yellowing, curling, twisting, chlorosis, abnor- mal elongation of the neck, formation of slender bulbs and ultimately the death of plants. Concentric rings of acervuli and orange conidial masses were abundantly observed on bulbs with anthracnose symptoms and twisted leaves of dis- eased plants. Multiple samples of infected onion plants with disease symptoms were collected from heavily infected felds in Bulanawewa, Galewela (7.84 N, 80.58E) and Sigiriya area (7.95 N, 80.75E) in Matale District. The single spore iso- lation method (Choi et al. 1999) was used to obtain the pathogen from diseased plant tissue onto Potato Dextrose Agar (PDA) plates. The inoculated plates were incubated at 25 °C under dark conditions. Characteristics of conidia, conidiophores, and appressoria were assessed under a Carl Zeiss compound light microscope, using an Axiocam digital camera. (Carl Zeiss Microscopy, Thornwood, NY, USA). In total, 10 isolates were obtained from the infected * Dhanushka Udayanga dudayanga@sjp.ac.lk 1 Department of Biosystems Technology, Faculty of Technology, University of Sri Jayewardenepura, Pitipana, Homagama 10200, Sri Lanka 2 Mycology and Nematology Genetic Diversity and Biology Laboratory, United States Department of Agriculture Agricultural Research Service, MD 20705 Beltsville, USA 3 Department of Botany, Faculty of Applied Sciences, University of Sri Jayewardenepura, Nugegoda 10250, Sri Lanka / Published online: 1 November 2021 Australasian Plant Disease Notes (2021) 16: 30