What causes Sudden Oak Death in trees and plant life? Sudden Oak Death, or SOD, is caused by the pathogenic fungus Phytophthora ramorum. This waterborne mold pathogen infiltrates plants and trees through contaminated irrigation water, wind-blown rain, infected plants, and contaminated pots and soil mixes. This fungus is also associated with other tree wellness issues like Ramorum leaf blight, Ramorum dieback, and Phytophthora canker.