Disease and pathology

Plant virus infections can affect photosynthesis, respiration, nutrient availability, and hormonal regulation of growth, with morphological outcomes that include mosaic for- mation, yellowing, molting or other color disfiguration, stunting, wilting, and necrosis. These conditions, resulting from cell and tissue damage, can significantly reduce the yield and commercial value of crops and the impact of plant viruses globally are estimated to cost $70 billion annually. Plants and crops of all types can be affected; for example, swollen shoot disease devastates cocoa trees and is a serious and ongoing threat to the cocoa bean industry across large parts of western Africa. Rice tungro disease is the most important rice crop disease in Southeast Asia and is caused by a co-infection of two viruses. Disease symptoms are due to infection by rice tungro bacilliform virus (Family Caulomoviridae), while transmission between host plants relies on rice tungro spherical virus (Family Sequaviridae) and an insect vector, the leafhopper. Despite such obvious signs of infection, the way in which plant viruses cause cell damage has been difficult to understand. Plant viruses code for very few proteins, but as is the case with many viral proteins, they are usually multifunctional. Many of the nonessential (and as yet unknown) functions of plant virus proteins may interact detrimentally with host proteins, resulting in the observed pathologies.