The spread of viral diseases can be very difficult to control, and damage to a crop can be severe. Insecticides are sometimes applied to control populations of aphids, but often have little impact on the spread of the disease. Scientists have discovered new genetic engineering methods that provide resistance to viral disease where options may have been limited before. This short strand of DNA or RNA contains instructions for the virus' coat and movement proteins, and a few other genes to assist reproduction. After a virus infects a cell, it removes its coating of proteins and then reproduces itself by tricking the cells they infect into manufacturing copies of their proteins and genetic material.
New viruses assemble themselves from the newly made parts and then escape to infect other cells.
Disease symptoms in the plant are the result of cellular damage caused by the viruses. GE resistance to the viral diseases is much better than conventional sources of resistance, but it is still somewhat susceptible to variations in pathogen strain and environment. Control of the disease was limited to careful observation to remove infected plants. Researchers in Hawaii and at Cornell University developed two varieties of GE papaya resistant to PRSV using coat protein-mediated resistance.