Impact of Virus and Viroid Diseases on Crop Yields

Abstract

Plants are important to human beings as they are the source of food, cloth and shelter. Any disturbance in plant life will lead to misery of human beings. Virus and viroid diseases are one of several factors accounting for loss in crop yields. Crop loss assumptions and loss estimates are necessary before the economic impact of a disease is established and the economic advantages of particular control strategies are determined. Comparisons can be made between disease plants or crops grown in different locations provided that the locations have similar environmental conditions. In general it is difficult to obtain the data on the actual losses due to virus diseases because the virus diseases are frequently less conspicuous than those caused by fungal or bacterial diseases. In general the data is available on reduction in growth, vigor and reduction in market values, because of size, shape, and fruit color. The viruses of greatest importance are those affecting the crops that feed the greatest number of people rather than those destined for export. For example, in African countries both cassava and sweet potato are prime food crops, as similarly in South east Asian countries where rice is the primary food. The economic returns from a crop decrease because of reduced yields and also because of cost of agricultural activities undertaken to reduce the damage to crop or both. The amount of losses caused by plant diseases will vary with plant, location, environment and plant product. It is much more difficult to provide the loss estimates at global level as the loss estimate data is available for small comparative trials. In a majority of cases the data on age of the plant, virus strain, and time of infection are frequently unavailable. In spite of all these limitations, yield loss estimates are determined in certain virus-host combinations and in this chapter, and data have been compiled in different crops to get an idea about the impact of virus and viroid diseases on crop yields.