Agronomic risks are defined as those related to quality or productivity of a modified crop. A number of issues and concerns emerge when agronomic consequences of using transgenic pest-protection strategies against viruses are considered. These center on emergence of new or novel viral strains, introduction of new transmission characteristics, and changes in susceptibility to heterologous viruses. Some of the concerns, such as the question of new virus emergence, also have relevance to ecological risks.
The emergence of strains of pathogens that overcome plant-genetic resistance or other disease-control methods has been and probably always will be a problem in agriculture. Indeed, this problem is common to all plant and animal hosts for which pathogens exist. Traditionally the problem has been managed by development of multiple strategies for disease control (genetic and other control measures), surveillance of pathogen activity and strain development, deployment of new pest-protected germplasm in response to emerging pest strains, and development and use of longer-lasting forms of pest-protection. With transgenic pest-protected plants that express a pest-protection gene transferred from another plant, the selective pressure for development of resistance-breaking strains should be qualitatively similar to the selective pressure associated with conventionally bred pest-protected plants. In section 2.9, these issues are discussed.
Recombination between a virus-derived transgene and a virus during plant infection has been suggested as a potential source of novel virus strains with enhanced virulence characteristics. From an evolutionary perspective, new viruses emerge through gradual accumulation of point mutations and major acquisition or deletion of genetic material. New genetic material can be incorporated by recombination with nucleic acids