The research findings of my M.sc (Plant Pathology) work has been presented in this book. The research was conducted at CCS Haryana Agri. University in Hisar, Haryana, India. The key objectives of the research was to study the effect of the fungus Sclerotium rolfsii on seed germination and seedling vigour of sunflower hybrids and secondly to manage the disease collar rot caused by fungus Sclerotium rolfsii through host resistance, fungicides and bioagents. Sunflower is an oil producing crop and is cultivated all over the world. Sunflower oil is preferred among the consumer's due to its health appeal worlwide. Sclerotium rolfsii Sacc. is a well-known polypagous, ubiquitous, omnivorous and most destructive soil borne pathogen and causes one of the most destructive disease of sunflower worldwide known as collar/stem rot which highly results in yield loss. Therefore, every effort has been taken to study the few key aspects in relation to the pathogen and its effect on sunflower plant growth and yield losses and possible effective ways of managing the fungus to minimize the losses.