ABSTRACT OF THE THESIS Rice Blast Control and Polyvarietal Planting in the Philippines: A Study in Genotype by Environment Biogeography by DANIEL J. FALVO Thesis Director: Dr. George H. Nieswand Rice blast disease is the principal and most adaptable disease of the rice plant. Our current understanding of why blast dispersal is less efficient within ricefields planted with mixtures of multiple rice varieties than within fields planted with a single rice variety remains incomplete. In this thesis, I will provide a more complete understanding of this phenomenon by examining it from a new approach to biogeography that could be called genotype by environment biogeography. According to genotype by environment biogeography, biogeographical phenomena result from effects that the biotic and abiotic environments produce on the physical functioning of organisms in addition to the effects that the abiotic environment produces on an organism's molecular genetic and biochemical functioning. I observed in a case study of an upland ricefield in the Philippines that farmers planting mixtures of three rice varieties resulted in an effective form of controlling blast dispersal that could be called intrafield gene deployment. According to this form of blast control, planting a varietal mixture reduces the efficiency of blast dispersal by decreasing the probability of rice plants that are potential hosts within a field. Whether or not a rice plant is a potential host is determined by the variety of the plant and also by how the environmental conditions of the plant's location affect the plant's biochemical ability to prevent a blast infection. In this case study, host potential was influenced by the variety of the rice plant and by how the plant's biochemical ability to prevent blast infection was affected by the soil moisture conditions of the plant's location within the field.