Farmers find it difficult to keep an eye on each and every plant in the cultivation area to detect manifestation of any infection. Many an image processing approaches has been developed in recent years to serve the purpose of a watch-dog, ie detecting the growth of a disease at an early stage, determining the type of the disease the leaf is infected with and suggesting necessary actions. This paper will go through most of the research work that has been done in the field of Image Processing and Artificial Neural Network for the purpose of detection and diagnosis of disease in a plant leaf from its image.