POWER 9520676 The proposed research will link patterns of genetic variability in barley yellow dwarf virus (BYDV) with ecological interactions between the BYDV, and the virus' grass hosts and aphid vectors. We will measure genetic variability using serological and sequencing surveys of natural virus populations. Ecological interactions will be assessed with greenhouse experiments designed to distinguish between two ecological hypotheses explaining genetic structure in populations of the virus: genetic subdivision occurs as a result of different colonization opportunities for the virus or genetic subdivision reflects variability in the probability of a colonization resulting in a new infection. This research will improve our understanding of ecological factors affecting the evolution and epidemiology of barley yellow dwarf, a major disease of crop plants, and contribute to our understanding of the ecology of plant pathogens in natural communities. It will aid in management of this disease by providing information about the relative importance of different wild host species as sources of virus inoculum for crop plants.