Phytosphere Research

The exotic plant pathogen Phytophthora cinnamomi: A major threat to rare Arctostaphylos and much more

Tedmund J. Swiecki1, Elizabeth A. Bernhardt1, Matteo Garbelotto2, and Elizabeth J. Fichtner3

1 Phytosphere Research, 1027 Davis Street, Vacaville, California 95687;
2 Department of Environmental Science, Policy, and Management, Ecosystem Sciences Division, 137 Mulford Hall, University of California, Berkeley, California 94720;
3 Department of Plant Pathology, University of California, Davis, California 95616 (current address: Cooperative Extension Tulare County 4437-B S. Laspina St.. Tulare, CA 93274)

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Key words: Arctostaphylos myrtifolia,native habitat, nursery, Phytophthora cinnamomi, plant disease, rare plants, sanitation


Phytophthora cinnamomi, introduced into California by the early 20th century, is a serious root pathogen of many agricultural and horticultural species in the state. We determined that P. cinnamomi causes a lethal root rot of the threatened Arctostaphylos myrtifolia and is the cause of extensive A. myrtifolia mortality throughout the northern half of this plant's limited range in Amador County. The persistence of P. cinnamomi in soils renders pathogen-infested habitats unable to support A. myrtifolia. Consequently, A. myrtifolia conservation depends on stopping further pathogen spread. We subsequently found P. cinnamomi killing the endangered A. pallida and other native woody species in protected native habitat in the Oakland Hills, as well as Arbutus menziesii and Umbellularia californica in native stands in Marin and Sonoma Counties. Phytophthora cinnamomi has been isolated from container stock purchased from several native plant nurseries, suggesting nursery stock used for restoration projects or planted in the wildland-urban interface can provide a route for native habitat invasion. These and other findings indicate that P. cinnamomi is a developing threat to California native flora. Phytosanitary measures must be considered to prevent further introduction and spread of P. cinnamomi in many California plant communities.

Major support for this project was provided by the California Dept. of Fish and Game through a subcontract administered by University of California , Berkeley.