Agriculture is a most important and ancient occupation in India. As economy of India is based on agricultural production utmost care of food production is necessary. Pests like virus fungus and bacteria causes infection to plants with loss in quality and quantity production. There is large amount of loss of farmer in production. Hence proper care of plants is necessary for same. This paper presents an overview of using image processing methods to detect various plant diseases. Image processing provides more efficient ways to detect diseases caused by fungus bacteria or virus on plants. Mere observations by eyes to detect diseases are not accurate. Overdose of pesticides causes harmful chronic diseases on human beings as not washed properly. Excess use also damages plants nutrient quality. It results in huge loss of production to farmer. Hence use of image processing techniques to detect and classify diseases in agricultural applications is helpful.