Fall 2014 Field Observations: Phytophthora tentaculata

by Steve Tjosvold

First identified in North America at a Monterey County nursery in 2012, Phytophthora tentaculata has since been found on nursery stock in Alameda, Butte, Placer, and Santa Cruz counties and on outplanted stock in restoration sites in Alameda County. Affected plants in California include Mimulus auranticus (sticky monkey flower), Frangula californica (California coffeeberry), Heteromeles arbutifolia (toyon) and Salvia sp. In 1993, the pathogen was first detected in Germany on Chrysanthemum sp., Delphinium sp. and Verbena sp. Since the first detection, the host list has increased to include Gerbera jamesonii, Origanum vulgare, Santolina chamaecyparissus, Lavendula angustifolia, Chichorium intybus, Auklandia lappa and Calendula arvensis.

Phytophthora tentaculata causes similar symptoms to those caused by other soil-or-water inhabiting Phytophthora species. Mimulus aurantiacus symptoms include root and stem rot, with the roots and stem collars developing necrotic, sunken lesions and few feeder roots. In Europe and China, the pathogen is reported to cause crown, root and stalk rot of nursery plants. Subsequently, aboveground symptoms include stunting, leaf russeting and yellowing to browning (chlorosis), defoliation and dieback of twigs, brown to black lesions girdling the basal stem, and eventually plant death.

These detections raise concern for our forest and wildland health. The infested California nurseries specialize in producing native plants for restoration purposes. Unfortunately plants move directly from these infested nurseries to wildlands, so risk of pathogen introduction to forests is very high. The USDA Forest Service, Pacific Southwest Research Station; Forest Health Protection, Washington Office; California Department of Food and Agriculture; and Phytosphere Research are cooperating on a survey to check restoration sites to determine the extent of introduced infestations. A few conservation nurseries will also be surveyed.

Steven A. Tjosvold
Farm Advisor, Environmental Horticulture
UC Cooperative Extension Santa Cruz County
1432 Freedom Boulevard
Watsonville, CA 95076-2796
(831)763-8013 phone, (831) 763-8006 fax
satjosvold@ucanr.edu
http://cesantacruz.ucanr.edu/