First report of Colletotrichum siamense causing anthracnose-twister disease of onion (Allium cepa) in Sri Lanka

Authors: HERATH, INDUNIL - University Of Sri Jayewardenepura; MIRIYAGALLA, SHANEYA - University Of Sri Jayewardenepura; MANAMGODA, DIMUTHA - University Of Sri Jayewardenepura; Castlebury, Lisa; UDAYANGA, DHANUSHKA - University Of Sri Jayewardenepura

Submitted to: Australasian Plant Disease Notes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/26/2021
Publication Date: 11/1/2021
Citation: Herath, I.S., Miriyagalla, S.D., Manamgoda, D.S., Castlebury, L.A., Udayanga, D. 2021. First report of Colletotrichum siamense causing anthracnose-twister disease of onion (Allium cepa) in Sri Lanka. Australasian Plant Disease Notes. 16(30). https://doi.org/10.1007/s13314-021-00444-w.
DOI: https://doi.org/10.1007/s13314-021-00444-w

Interpretive Summary: Fungi in the genus Colletotrichum occur on many different plant hosts. Because these fungal species have similar microscopic appearances, they are often difficult to identify correctly. In this study, a species of Colletotrichum was isolated from onions in Sri Lanka and characterized using DNA sequences and cultural characteristics. Results revealed that it was a species never before reported from Sri Lanka, C. siamense. New images and a description of this species is provided. This research will be used by extension agents, plant breeders, plant pathologists, and plant quarantine officials to implement management strategies and to accurately determine the presence of this species in Asia and countries in other parts of the world to enhance the control of onion diseases.

Technical Abstract: During late July to August in 2020, a severe outbreak of anthracnose-twister disease was observed in big onion (Allium cepa L.) cultivations of Matale district (Central Province), in Sri Lanka leading to severe losses. Among the major big onion producing regions in Sri Lanka, Matale district produces an estimated 50% of the total production of big onion. A majority of the farmers were reported to abandon their crops altogether because of this disease, causing a short-term dearth of big onion supply in the local market. The leaves and bulbs of onion plants showed typical anthracnose-twister disease symptoms such as leaf yellowing, curling, twisting, chlorosis and ultimate death of plants, abnormal elongation of the neck, and formation of slender bulbs. Concentric rings of acervuli and orange conidial masses were observed on bulbs and leaves of diseased plants. Multiple samples of infected onion plants with disease symptoms were collected from a heavily infected field in Bulanawewa, Galewela in Matale District. Based on the morphological features, four isolates were tentatively identified in the Colletotrichum gloeosporioides species complex. DNA sequencing identified the isolates as C. siamense. Koch's postulates were confirmed by re-isolating the pathogen from symptom-bearing plants. C. siamense had been previously found affecting onion plants in southwest India causing anthracnose. To our knowledge, this is the first report of C. siamense affecting A. cepa in Sri Lanka. More studies are required to understand the epidemiology of this disease and foster disease management programs in Sri Lanka to avert potential outbreaks in future.