Crop losses caused by viruses

Abstract

Several plant viruses are highly contagious and their effects on plants are often drastic. Many seriously reduce crop yield and quality. Numerous disorders are caused or triggered by viruses and a great deal of money is spent on preventing virus diseases from becoming disastrous. Losses caused by any disease cannot be measured directly because potential yield and damage are so variable. Experimental assessment of the losses attributable to virus diseases is particularly difficult because it is extremely hard to prevent contamination of healthy control plants and inoculation under vector-proof conditions may not reflect accurately what happens under natural conditions. The prevalence of the causal viruses or the extent of attack are not reliable guides to the amount of damage caused; this contrasts with the situation with invertebrate pests where losses are usually related to the population densities of the pests. In a few virus diseases, attempts have been made to establish the relationships between final yield loss and certain indicators such as incidence, severity and duration of disease, or between combinations of these, but the relationships seem to be valid only under strictly defined conditions. The main aims of this work have been to estimate actual losses inflicted on a farmer's crop or upon a larger area, to predict the magnitude of losses that may develop and to decide what control measures are justified. In practice, assessment of loss has been extrapolated from trials in which various infection severities and infection times have been used to simulate natural situations. Yield improvements with virus-free propagation material have sometimes been used for loss assessment. Losses caused by virus diseases depend on the cultivar involved: evaluation of the resistance of different cultivars may be further improved by assessing the effects of viruses on yield rather than by assessing the severity of symptoms. Losses is strictly financial terms are influenced by fluctuating prices for crop products and are even more difficult to asses, let alone to predict, than are losses of yield. Nevertheless, a better understanding of the way in which virus disease epidemics develop, and of the resulting biological and economic losses, is essential for effective crop protection, whether on individual farms or on a national scale.