Investigating the management of potato and tomato late blight caused by Phytophthora infestans through host genetics and pathogen characters Gregorio Amílcar Sánchez Pérez Under the supervision of Dr. Amanda Gevens, Dr. Shelley Jansky, and Dr. Dennis Halterman University of Wisconsin-Madison Phytophthora infestans (Mont) de Bary, is the causal agent of late blight of potato and tomato. In 2009 to 2013, four P. infestans clonal lineages US-22, US-23, US-24, and US-8 were isolated in several U.S. states including Wisconsin. In the present research two components of late blight management were investigated. 1) Biology and ecology of four recent clonal lineages of P. infestans. A phenotypic and genotypic characterization was performed on P. infestans collected in 2013 in Wisconsin. Results revealed that two clonal lineages, US-23 (A1 mating type, mefenoxam sensitive) and US-8 (mating type A2 and resistant to mefenoxam) were present. In an assessment of the virulence and oospore production with US-22 (A2), US-23 (A1), and US-24 (A1) isolates, results revealed that potato and tomato plants transformed with the RB gene and tomato Mountain Magic, carrying Ph-2 and Ph-3 genes, were resistant to inoculation with all of the individual isolates and no oospores were detected in these plants. In an investigation of survivability and infectivity of oospores, it was discovered that oospores were able to survive and remain infective to tomato leaflets after being exposed to natural field winter conditions in Wisconsin, and 5 temperatures ranging from 22° C to -20° C. 2). Breeding for resistance. Resistance to late blight from the wild potato species Solanum verrucosum has been introgressed into five diploid hybrid clones. Two of these clones have resistance to early blight from the wild potato species S. commersonii. All five clones have acceptable tuber yield, do not produce 2n pollen grains, and produce viable pollen. Using molecular approaches, transient expression of 18 RB orthologous genes from 7 Solanum species were evaluated for the ability to recognize the IPI-O effectors from P. infestans. Our results revealed that none of the 18 RB orthologs were able to recognize the IPI-O effectors. The nucleotide sequence diversity observed between non-functional and functional RB genes can be used for future research to identify regions of importance for molecular characterization of the interaction between IPI-O effector and the RB gene