In recent years, small avocado producers in coastal Colombia returned to their farms after decades of war to discover they must battle the pathogen Phytophthora cinnamomi devastating their trees. While this disease is well-described in the U.S., Australia, and Europe, no previous research has examined environmental correlates of P. cinnamomi in avocado in the diverse tropical agroecosystem of the Colombian Montes de Maria. I used agroecosystem inventory, in-field diagnostics and disease observations to describe avocado agroecosystem diversity and identify relationships among P. cinnamomi presence, root rot disease, slope, pH, soil compaction, elevation, and distance to household in remote smallholder farms in the coastal mountains of Colombia. I also evaluated accuracy of a locally accessible, low-cost bioassay for detecting the pathogen. Although P. cinnamomi proved to be ubiquitous in the region, soil compaction and increasing slope were both negatively related to disease incidence at the farm level, and some infected trees appeared healthy. Furthermore, the low-cost bioassay detected P. cinnamomi equally well as commercial immunostrips. As conflict reparations are negotiated in this remote region, small farmers should be compensated for retaining their highly diversified genetic stock, diverse cropping palettes, and indigenous techniques, as they may provide a refuge for avocado from the heavy disease burden in the steep and rocky growing terrain. v Acknowledgements I would like to thank my committee chair, Dr. Rachel O'Malley for her continued guidance, trust, and enthusiastic support throughout this process. I would like to acknowledge and thank my committee members, Dr. Will Russell and Dr. Jenny Broome for their support in designing this research and writing this thesis. This research would not have been possible without the support and guidance of the plant pathology team at Driscoll's and my in-country mentor, Lillian Hall as well as the Sembrando Paz organization, its employees, and all the farmers and their families who graciously allowed me to survey their farms, fed me and housed me. Lastly, I would like to thank my family and especially my fiancé, Dalton, for their continued love and support. vi Table of Contents List of Tables .