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New research has found that oleander, one of the most widely planted ornamental shrubs in the United States, can host the pathogen responsible for sudden oak death, one of California’s most destructive forest diseases.

Sudden oak death is a serious disease of native plants caused by the invasive pathogen Phytophthora ramorum. Since it was first detected in the 1990s, the disease spread along the California coast and killed many millions of trees, especially Coast Live Oaks and Tanoaks, changed whole ecosystems and increased the risk of wildfires.

Researchers at the �鶹��վ�б�’s National Ornamentals Research Site (NORS-DUC) have confirmed that the Phytophthora ramorum pathogen infects oleander. The study marks the first report of the pathogen occurring on oleander in the United States.

“” has been published as a short report in the peer-reviewed journal Plant Disease.

“Oleander is a very popular ornamental plant cultivated worldwide, and therefore our finding can have an effect for nurseries growing and selling oleanders, and on the official regulations by federal and state agencies,” said Dr. Wolfgang Schweigkofler, Research Professor and Lead Scientist at NORS-DUC.

Oleander is commonly planted along highways, in residential landscapes, and in commercial developments because it is fast-growing, drought-tolerant, and produces abundant flowers in different colors. California alone is estimated to have approximately 25 million oleanders lining roadways. Because infected plants may not show severe or lethal symptoms, they can act as silent carriers, spreading the pathogen to nearby wildlands through wind-driven rain and water runoff.

The disease, called ramorum blight when found in nurseries and sudden oak death in the wild, has more than 100 susceptible host plants, including such common garden ornamentals as camellias and rhododendrons. Once introduced into natural areas, the pathogen can infect native species, including tanoaks and coast live oaks, accelerating forest decline.

In May 2024, NORS-DUC researchers observed small brown necrotic leaf spots on residential oleanders in Mill Valley, California — an area where many California bay laurels also are infested with Phytophthora ramorum.
To confirm oleander as a host, researchers fulfilled Koch’s postulates — a standard scientific method used to establish a cause-and-effect relationship between a pathogen and a disease. Healthy oleander plants were inoculated with the pathogen and subsequently developed the same symptoms, confirming susceptibility.

The study’s authors, all research scientists with NORS-DUC in the Department of Natural Sciences and Mathematics at �鶹��վ�б�, are graduate student Dinh Hieu Pham, Huiying Liu, Tomas Pastalka, and Schweigkofler. 

“The findings underscore the importance of monitoring ornamental plants as potential reservoirs for invasive pathogens and may inform future nursery regulations and landscape management practices aimed at protecting California’s forests,” said Schweigkofler.

A collaboration between �鶹��վ�б�, the United States Department of Agriculture (USDA), and the California Department of Food and Agriculture (CDFA), the NORS-DUC research site studies emerging and invasive quarantine pathogens of ornamental and native plants in an open, nursery-like environment.

NORS-DUC focuses on diseases of ornamental and forest plants, especially those caused by members of the genus Phytophthora, such as P. ramorum and more recently P. tentaculata, a newly discovered plant pathogen in North America impacting native plant nurseries in California.

The research facility’s main focus is applied research, such as validation and development of best management practices (BMPs); development of remediation options for soil, water and infested plants; and development of monitoring and control strategies.