[From the December 2016 issue of the UC IPM Green Bulletin]

A new plant pathogen in the genus Phytophthora (pronounced Fie-TOF-ther-uh) has recently been found in several California native plant nurseries and habitat restoration sites. The pathogen, Phytophthora tentaculata, poses a risk of disease in wildlands, gardens and landscapes that use susceptible California native and non-native plants. Once introduced in these areas, the pathogen can generate disease for years to come, potentially causing lasting environmental and economic impacts. Because both native and non-native California plants from nurseries can carry new pathogens and other pests, it is important to remember that only healthy plant material should be used for planting.

What is Phytophthora?

Phytophthoras are microscopic, fungus-like organisms called water molds that produce spores and hyphae. Many are soilborne, attack plant roots and stems, and spread by the movement of infested soil, including soil stuck to tools, containers, or shoes. The genus Phytophthora is large, with over 100 described species, including the sudden oak death pathogen and other destructive pathogens of agricultural, ornamental, and forest plants.

Similar to other members of the Phytophthora genus, P. tentaculata releases swimming spores that move through water and are attracted to plant root exudates. Once infected, the pathogen can cause disease in susceptible plant roots. If susceptible stems are contacted, infection can occur there following water movement or splash, and stem disease can also result from the pathogen growing into the stem from the roots. Phytophthora tentaculata cannot be seen with the naked eye unless grown in a laboratory. However, it usually produces visible symptoms – stem cankers and root rots – on host plants.

Hosts, symptoms and detection

The pathogen was first described in 1993 from a nursery in Germany. In the United States, it was first found in 2012 in a nursery in Monterey County, and to date is present only in central California. Currently, seventeen plant species and two additional generaworldwide are thought to be susceptible to P. tentaculata, but the list may expand as we learn more. In California, eight native plant species and one additional genus have been found infected, all common in the native plant nursery trade and in wildlands. These include Artemisia douglasiana (mugwort), A. dracunculus (tarragon), A. californica (California sagebrush), Salvia species (sage), Ceanothus cuneatus (buck brush), Frangula californica (California coffeeberry), Monardella villosa (coyote-mint), and Heteromeles arbutifolia (toyon).

In California, P. tentaculata was first isolated from Diplacus aurantiacus (orange bush monkeyflower) where it was observed causing aboveground symptoms (Figure 1a) that included stunted growth, sparse and chlorotic foliage, stem collar lesions (Fig. 1b) and plant death. Root system symptoms included necrotic, sunken lesions, and few roots (Fig. 1c). Soon after, P. tentaculata was detected in the nursery trade in the U. S., the pathogen was recovered in central California wildlands, presumably having been released into the landscape via contaminated nursery plants used in restoration plantings. It has now been found in Alameda, Butte, Monterey, Placer, and Santa Cruz Counties.

Germany, Italy, Spain, and China have reported disease from P. tentaculata on several plant species including Apium graveolens (celery), Saussurea costus (costus root), Cichorium intybus (chicory), Chrysanthemum species (hybrids, marguerite, and oxeye daisy), Consolida ajacis (rocket larkspur), Gerbera jamesonii (Barberton daisy), Origanum vulgare (oregano), Santolina chamaecyparissus (lavender cotton), and Verbena species (vervain hybrids). Many of the above species are grown in California and should be considered at risk to disease.

Prevention and Management

Prevention: By far, prevention is the best possible method for dealing with any Phytophthora pathogen. Consider planting from seed as Phytophthora in general is rarely transmitted this way. If buying container stock of any of the above plant species, find out if the nursery is following best management practices for preventing Phytophthora (for example: tinyurl.com/zvmjyt3). Purchase plant material that has been grown in pasteurized soil and under proper sanitation procedure. Do not use/buy plants or material that has been in the nursery for an extended time, these can become contaminated with Phytophthora and other pathogens.

Avoidance: Avoid buying known host-plant container stock and do not purchase plants that appear unhealthy or otherwise potentially contaminated. At more advanced stages of disease, plants may exhibit symptoms as outlined above, but nearby plants may still look healthy even though they have been contaminated through soil or water movement. If a number of plants in the nursery block show symptoms of infection, do not buy those plants or their neighbors. The host species list is a work in progress; be aware that other, unlisted species may also be susceptible.

Quarantine: If you purchase host plants (or closely related species), consider setting them aside before planting. Give the plants time (4 - 6 weeks) to develop symptoms before planting them in your yard, and be sure soil and excess water from these plants does not flow into your garden soil. If symptoms develop, dispose of the plant, soil, and container according to disposal guidelines for your area. Do not home-compost this pathogen, as it may not be killed.

Remediation: If plants are already in the ground and exhibiting symptoms such as stunted growth and/or chlorotic foliage, check the root collar and stem for necrotic sunken lesions and/or stem rots. If possible, check root systems for abnormally large numbers of dead and dying roots, few healthy new roots, and necrotic spotting on roots that are still living. If the roots appear to be infected, do not move soil from the garden bed and nearby infected plants to other parts of the garden.

Change irrigation practices to reduce the potential for Phytophthora growth, as outlined in the UC IPM Pest Note: Phytophthora Root and Crown Rot in the Garden at ipm.ucanr.edu/PMG/PESTNOTES/pn74133.html. Clean your tools and boots before working another area of your garden. You may wish to contact your local Agricultural Commissioner or UC Cooperative Extension office to see if they can offer updates or further advice.

For more information, including references to original research and related articles, see the pages covering this pathogen at www.suddenoakdeath.org/diagnosis-and-management/nursery-information/phytophthora-tentaculata/.

~Laura Sims, UC Berkeley, simslaura@berkeley.edu; Igor Lacan, UCCE San Francisco and San Mateo counties, ilacan@ucanr.edu; Steven Swain, UCCE Marin County, svswain@ucanr.edu; and Matteo Garbelotto, UC Berkeley, matteog@berkeley.edu