An overview of cultural, morphological and molecular variability in Rhizoctonia spp. causing Cotton root rot

Abstract

Cotton is currently the leading plant fibre crop worldwide and is grown commercially in the temperate and tropical regions of more than 50 countries. During 2021-22, Haryana shares 6.47 area (lakh ha), 17.20 production (lakh bales of 170kg) with 452 kg/ha productivity. India is the largest country in world in terms of area under cotton is 133.50 lakh hectares which is 41% of the world cotton area. The cotton crop is grown extensively with a limiting factor, that is infected by fungal, bacterial and viral diseases like Anthracnose, Leaf blight, Wilt, Verticillium wilt, Ramularia leaf spot, Cercospora leaf spot, Target Leaf Spot, Myrothecium Leaf Spot, Rust of Cotton, Root rot, Angular leaf spot, Bacterial blight, Cotton leaf curl disease and Tobacco streak virus (TSV) etc. Out of all the diseases, root rot of cotton is the most devastating disease and now days this disease has become a major limiting factor in cotton cultivation. Cultural variability is commonly expressed through differences in colony colour, texture, growth rate, mycelial density, sclerotial formation, and pattern of hyphal growth on various nutrient media. Morphological variability further distinguishes Rhizoctonia species and their populations, particularly in hyphal characteristics such as branching pattern at right angles, septation near the branch origin, hyphal width, and the size, shape, and distribution of sclerotia. Techniques such as ITS-rDNA sequencing, RAPD, ISSR, AFLP, and SSR markers have been widely employed to differentiate species, anastomosis groups, and genetically distinct populations. The integration of cultural, morphological, and molecular approaches provides a comprehensive understanding of variability in Rhizoctonia spp., which is essential for accurate identification, epidemiological studies, and the development of effective disease management strategies in cotton.