Competition between viruses in a complex plant-pathogen

Interactions among viruses, vectors, and host plants may influence the spread and success of plant viruses. Major factors include direct competition within host plants, direct competition within vectors, differences in transmission rates, and virus influences on vector behavior and population dynamics. The aphid-transmitted barley yellow dwarf viruses (BYDVs), which infect a broad range of grasses worldwide, represent a model system for addressing questions about the outcome of direct and indirect competition between viruses. Historical shifts in the relative prevalence of BYDV strains document the apparent displacement of one virus strain (PAV) by another (MAV) over 20 yr. In the barley yellow dwarf system, transmission rate appears td play an important role in determining the outcome of competition between viruses. Moreover, the interaction between transmission rate and vector behavior may be particularly important. PAV is the stronger competitor within hosts, where double infections occur more often than in insect vectors. PAV also has significant advantages due to higher overall transmission rates than MAV. In addition, vector aphids show a strong nonpreference for PAV that may lead to greater rates of virus spread.