Phytophthora fruit rot caused by P. capsici is a devastating disease for cucumber. As young fruit are especially susceptible, we sought to identify quantitative trait loci (QTL) associated with young fruit resistance using QTL-seq analysis. A cucumber accession, PI 109483, was identified as a source of young fruit resistance, and a resistant breeding line, MSU109483-53, was developed. Crosses were made between the susceptible breeding line, Gy14, and two resistant MSU109483-53-derived lines: an S₆ line, B5, and a doubled haploid line, DH A4-3. P. capsici isolate Barley’s 1 was used in all experiments. In the summer of 2017, F₂ progeny of Gy14 x B5 (n=397), and in 2018 Gy14 x A4-3 (n=362), along with parental lines and F₁ were grown in the field. For accurate phenotyping, plants were trellised to reduce wounding due to soil removal during cleaning and reduce possibility of contamination from other pathogens. Replicate harvests providing 10-50 fruits per plant allowed replicated scoring for each individual. Normally distributed disease scores suggested that young fruit resistance is a quantitative trait. Individuals with extreme resistant and susceptible phenotypes were selected for QTL-seq analysis, performed using QTLseqR. QTL were identified on chromosomes 1, 5, and 6 from both experiments. To narrow the genomic regions and verify QTL, kompetitive allele specific PCR (KASP) markers will be used to genotype an F₂ population of Gy14 x A4-3 in summer 2019.