Phenotypic and Molecular Characterization of Colletotrichum siamense Causing Onion Twister Disease in India
Manikchandra Sharanappa Patil *
Department of Plant Pathology, College of Agriculture, University of Agricultural Sciences, Raichur, 584104, Karnataka, India.
Amaresh Y. S.
Department of Plant Pathology, College of Agriculture, University of Agricultural Sciences, Raichur, 584104, Karnataka, India.
Gowdar S. B.
Department of Plant Pathology, College of Agriculture, Gangavathi, University of Agricultural Sciences, Raichur, 583227, Karnataka, India.
Yenjerappa S. T.
Department of Plant Pathology, College of Agriculture, Gangavathi, University of Agricultural Sciences, Raichur, 583227, Karnataka, India.
Arunkumar Hosamani
Department of Entomology, College of Agriculture, University of Agricultural Sciences, Raichur, 584104, Karnataka, India.
Y. Pampanna
Department of Horticulture, Main Agriculture Research Station, College of Agriculture, University of Agricultural Sciences, Raichur, 584104, Karnataka, India.
*Author to whom correspondence should be addressed.
Abstract
Onion twister disease, caused by diverse Colletotrichum species worldwide, is a major constraint on onion production and can lead to severe yield and post-harvest losses. Accurate identification within this diverse genus is essential for determining the pathogen associated with the disease in India. In the present study, four fungal isolates (Cs-1, Cs-2, Cs-3, and Cs-4) were obtained from symptomatic onion tissues and characterised using morphological, cultural, pathogenic, and molecular approaches. Morpho-cultural evaluation revealed circular colonies with floccose to velvety textures and colours ranging from creamy white to light grey; mean mycelial growth rates ranged from 8.17 ± 0.65 to 18.00 ± 0.54 mm day-1. Conidia were hyaline, unicellular, guttulate, and cylindrical to oblong, and the appressoria were globose to clavate. Molecular characterisation targeting the internal transcribed spacer (ITS) rDNA region yielded approximately 550-bp amplicons with 97.54-100% sequence identity to Colletotrichum siamense. Neighbour-Joining phylogenetic analysis placed all isolates within the C. siamense clade. Pathogenicity tests produced symptoms characteristic of twister disease, including anthracnose lesions, leaf twisting, abnormal neck elongation, dieback, and plant death. Anthracnose lesions appeared 3-10 days after inoculation (DAI), followed by leaf twisting at 5-12 DAI. Across isolates, disease incidence ranged from 40% to 100%, disease severity from 33.33% to 79.36%, and leaf twisting from 10% to 50%. Re-isolation from symptomatic tissues fulfilled Koch's postulates. These findings support C. siamense as a causal agent of onion twister disease in India. Additional protein-coding loci may be required as secondary barcodes for precise species identification within the C. gloeosporioides species complex.
Keywords: Onion anthracnose, twister disease, Colletotrichum siamense, Allium cepa, morpho-cultural characterisation, pathogenicity, ITS rDNA, phylogenetic analysis, Koch’s postulates