ORCID
0009-0009-1579-7386
Graduation Year
2026
Document Type
Master's Thesis
Degree
Master of Science
Program
Biological Science
Program Director
Patti Culross, MD, MPH
First Reader
Wolfgang Schweigkofler, PhD
Second Reader
Erik Nelson, PhD
Abstract
Phytophthora is one of the most destructive genera of plant pathogens in California, posing significant threats to both nursery production and natural ecosystems. Phytophthora ramorum, the causal agent of Sudden Oak Death (SOD), has expanded its host range to include more than 100 plant species. In addition, Phytophthora elongata, previously isolated in Western Australia from Eucalyptus marginata in 2013, has recently been detected for the first time in the United States on Rhododendron species. This naturally raises concern that a new etiological species has entered the country. Therefore, the goal of this thesis study was three-fold: i) to identify potential new host plants of Phytophthora ramorum and Phytophthora elongata; ii) to carry out molecular identification and physiological characterization of the Phytophthora isolates; and iii) to analyze the optimal growth temperature range for Phytophthora to better understand its adaptation to new environments. To address these objectives, Koch’s postulates, PCR molecular identification techniques, metabolic activity measurements using Biolog EcoPlates™, and culturing techniques spanning a wide range of temperatures and media conditions were used. This research identified Ceratonia siliqua (carob) and Nerium oleander (common oleander) as novel hosts for Phytophthora ramorum and confirmed the detection of Phytophthora elongata on Rhododendron spp. within the United States. Moreover, unique substrates consumed by each pathogen were found that may indicate their infection strategies and how they may avoid plant defenses and also dictate their potential thermal zones in California. Our findings provide evidence that both P. ramorum and P. elongata can expand their host range and adapt to new environments, especially under changing climatic conditions.