A New Threat from an Old Foe: Impacts of Phytophthora on Oregon Nursery Production

Oregon’s nursery and greenhouse industry are the state’s top agricultural commodity, valued at $1.2 billion. However, Phytophthora species continue to pose significant threats to production. Three species—P. cinnamomi, P. plurivora, and the recently introduced P. austrocedri—are of particular concern. Dating back to the 1930s, P. cinnamomi has been a persistent issue in ornamental production, widely distributed across Oregon and the United States. However, recent research indicates that P. plurivora has become the dominant Phytophthora species in Oregon, a shift attributed to the emergence of fungicide-resistant isolates. Meanwhile, P. austrocedri, a newly detected threat in the U.S., has been identified in two nurseries in Oregon, leading to significant financial concerns for the industry. First reported in Argentina in 2007 and later causing widespread Juniperus communis dieback in Britain, P. austrocedri is a slow-growing oomycete associated with high mortality rates in the Cupressaceae family. For decades, Phytophthora species have been managed with broad-spectrum treatments, but increasing fungicide resistance and shifting pathogen populations have rendered many current management strategies ineffective. Our research aims to address these challenges by investigating key factors such as pathogen growth, sporulation, spread, and disease severity. By developing updated management strategies, we seek to provide Oregon growers with effective control measures while offering researchers a foundation for further study into these evolving threats.