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<i>Phytophthora infestans</i> is a highly aggressive pathogen that causes late blight, one of the most constraining factors in potato and tomato production. Three separate strategies are being explored to improve late blight management. First, 39 tomato varieties were evaluated for late blight resistance in field trials. Varieties with both <i>Ph-2 </i>and <i>Ph-3 </i>resistance genes, or were homozygous for <i>Ph-3 </i>only, along with three heirloom varieties, were highly resistant. In addition to host resistance, three DNA-based pathogen diagnostic assays were developed to enhance pathogen detection and identification. A loop-mediated isothermal amplification (LAMP) assay can detect <i>P. infestans </i>after a one hour isothermal reaction. A high-resolution melt analysis assay was also developed to differentiate three <i>P. infestans </i>lineages, US-8, US-11, and US-23. The third assay used locked nucleic acid technology to distinguish US-23 from non-US-23 individuals based on two SNPs. Because understanding pathogen diversity is critical to proper management, I am using genotyping-by-sequencing to identify population structure within four <i>P. infestans </i>lineages (US-8, US-11, US-23, and US-24). Preliminary results, based on neighbor-joining and principle component analyses, indicate that some isolates collected from the same geographic region in different years show high similarity. This supports the hypothesis that overwintering inoculum is responsible for initiating regional outbreaks.