Phytophthora hedraiandra de Cock & Man in’t Veld

California Pest Rating for
Phytophthora hedraiandra de Cock & Man in’t Veld
Pest Rating: B


Initiating Event:

Recently, Suzanne Latham, CDFA plant pathologist, detected Phytophthora hedraiaindra in Arctostaphylos pumila samples that were collected from a nursery in Monterey County during an inspection related to an earlier detection of P. tentaculata.   In December 2014, the pathogen had also been detected in Arctostaphylos plants propagated in a nursery in Alameda County.  This nursery had requested the CDFA Plant Pathology Laboratory to test some Arctostaphylos plants for Phytophthora spp. before they were to be released for planting.  The nursery baited the water flow-through the potted plants with Rhododendron and Oregano tissue that were provided by the CDFA Lab and returned the same for analysis.  Suzanne Latham, CDFA plant pathologist, confirmed the detection of Phytophthora hedraiandra in culture from Rhododendron leaf baits.  The collection of official samples and trace forward investigations are currently in process related to this detection.  Phytophthora hedraiandra was initially detected in San Francisco, California 2013 on Arctopstaphylos plants.  Arctopstaphylos native plants extracted from a natural site in San Francisco were propagated at three different nurseries in San Francisco, Berkley, and Santa Cruz.  Similar to the 2014 incident, the water flow-through of potted Arctopstaphylos plants was baited with plant tissue which was then sent to the CDFA Lab for testing.  Cheryl Blomquist, CDFA plant pathologist, identified P. hedraiandra in the samples.  The detection was confirmed by the USDA Lab in Beltsville, Maryland. All potted plants were eventually destroyed.  Phytophthora hedraiandra currently has a Q rating that is herein reassessed for the proposal of a permanent rating.

History & Status:

Background: Since the discovery of Phytophthora ramorum, causal organism for the Sudden Oak Disease, there has been an increase of surveys throughout the world, for Phytophthora spp. which resulted in the identification of several new species, including P. hedraiandraPhytophthora hedraiandra was first discovered in 2001 on leaf spots of Viburnum sp. in the Netherlands (de Cock & Lévesque, 2004).

Hosts: The full host range of Phytophthora hedraiandra is yet not known.  Presently, only certain species of Rhododendron (azalea) and Viburnum are reported as susceptible hosts (CABI, 2014; EPPO, 2014).  Fagus sylvatica (common beech) is also listed as a host (CABI, 2014; Hejna, et al., 2014).  According to CDFA Plant Pathology Detection Records (2014-2015), Arctostaphylos spp. appears to be a new host for this pathogen.

Symptoms:  Plant symptoms caused by Phytophthora hedraiandra infections may vary with the infected host.  Symptoms in Viburnum include wilting, leaf spots, stem cankers and root and collar rots, while symptoms in Rhododendron include leaf lesions and shoot dieback (Henricot & Waghorn, 2014; Schwingle, et al., 2006, 2007).  Symptoms in Fagus sylvatica include root rot, leaf chlorosis and wilting (Hejna et al., 2014).

Damage Potential: Currently, there are no reports on quantitative economic losses in plant production caused by Phytophthora hedraiandra. However, infestations may result in significant damage and loss in production and stands of host plants by causing root and collar rots of infected plants. Nursery ornamentals and plants grown in natural ecosystems are particularly affected.  In general for Phytophthora spp., young seedlings of trees and annual plants may be killed within a few day, weeks or months (Agrios, 2005).

Disease Cycle: Generally, species of Phytophthora that cause root and stem rots survive cold winters or hot and dry summers as thick-walled, resting spores (oospores and chlamydospores) or mycelium in infected roots, stems or soil.  During spring, the oospores and chlamydospores germinate to produce motile spores (zoospores) that swim around in soil water and roots of susceptible hosts. The pathogen infects the host at the soil line causing water soaking and darkening of the trunk bark. This infected area enlarges and may encircle the entire stem of small plants which wilt and eventually die.  On large plants and trees, the infected, necrotic area may be on one side of the stem and become a depressed canker below the level of the healthy bark.  Collar rot canker may spread down the root system. Roots are invaded at the crown area or at ground level.   Mycelium and zoospores grow in abundance in cool, wet weather causing damage where the soil is too wet for normal growth of susceptible plants and low temperatures (15-23°C) prevail (Agrios, 2005).

Transmission:  Infected soils, plants, nursery and planting stock, seedlings, run-off and splash irrigation and rain water, cultivation equipment and tools that may spread contaminated soil and plant materials to non-infected sites (Yang et al., 2012).

Worldwide DistributionEurope:  Italy, Netherlands, Slovenia, Spain, United Kingdom; North America: USA; Oceania: Australia.

In the USA, Phytophthora hedraiandra has been found in California, Minnesota, and Virginia (CABI, 2014; EPPO, 2014).

Official Control:  None reported.

California Distribution:  Alameda, Monterey and San Francisco Counties (see ‘Initiating Event’).

California Interceptions:  The pathogen has not been intercepted in quarantine shipments of plants.

The risk Phytophthora hedraiandra would pose to California is evaluated below.

Consequences of Introduction: 

1)  Climate/Host Interaction: Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

– Low (1) Not likely to establish in California; or likely to establish in very limited areas.
– Medium (2) may be able to establish in a larger but limited part of California.
– High (3) likely to establish a widespread distribution in California.

 Risk is High (3) Phytophthora hedraiandra has already been detected in few nurseries in California.  Within California, it is likely to establish in cool, wet climates in susceptible hosts.

2)  Known Pest Host Range: Evaluate the host range of the pest. Score:

– Low (1) has a very limited host range.
– Medium (2) has a moderate host range.
– High (3) has a wide host range.

Risk is Low (1) The full host range of Phytophthora hedraiandra is yet not known.  Presently, only certain species of Rhododendron (azalea) and Viburnum, and Fagus sylvatica (common beech) are reported as susceptible hosts.  

3)  Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest. Score:

– Low (1) does not have high reproductive or dispersal potential.
– Medium (2) has either high reproductive or dispersal potential.
– High (3) has both high reproduction and dispersal potential.

 Risk is High (3) Phytophthora hedraiandra is primarily spread artificially via infested soils, plants, nursery and planting stock, seedlings, run-off and splash irrigation water, cultivation equipment and tools that may spread contaminated soil and plant materials to non-infected sites.

4)  Economic Impact: Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.
B.The pest could lower crop value (includes increasing crop production costs).
C. The pest could trigger the loss of markets (includes quarantines).
D. The pest could negatively change normal cultural practices.
E. The pest can vector, or is vectored, by another pestiferous organism.
F. The organism is injurious or poisonous to agriculturally important animals.
G. The organism can interfere with the delivery or supply of water for agricultural uses.

– Low (1) causes 0 or 1 of these impacts.
– Medium (2) causes 2 of these impacts.
– High (3) causes 3 or more of these impacts.

Risk is High (3) – Although quantitative economic losses in plant production have not reported, the potential for infected plants to result in root and collar rot, canker, leaf lesions and shoot dieback could decrease stands on non-infected plants, increase production costs and cause loss of market of infected  nursery stocks. The capability of the pathogen to survive and spread in infected soils and irrigation water could require changes in normal cultivation practices of host plants.

5) Environmental Impact: Evaluate the environmental impact of the pest on California using the criteria below.

A.  The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B.  The pest could directly affect threatened or endangered species.
C.  The pest could impact threatened or endangered species by disrupting critical habitats.
D.  The pest could trigger additional official or private treatment programs.
E.  The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

– Low (1) causes none of the above to occur.
– Medium (2) causes one of the above to occur.
– High (3) causes two or more of the above to occur.

Risk is Medium (2) – Currently, the host range and geographic distribution of P. hedraiandra are not fully known.  The few known host plants (see ‘Hosts’ above) can be found in natural ecological habitats as well as in nursery environments.  Subsequently, under favorable climate conditions, natural plant communities and ecosystems, as well as home/urban gardening and ornamentals may be negatively impacted.

Consequences of Introduction to California for Phytophthora hedraiandra:

Add up the total score and include it here. (Score)

-Low = 5-8 points
-Medium = 9-12 points
-High = 13-15 points

Total points obtained on evaluation of consequences of introduction of Phytophthora niederhauserii to California = (12).

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

-Not established (0) Pest never detected in California, or known only from incursions.
-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
-High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation is Low (-1). To date, Phytophthora hedraiandra has been detected in three California coastal counties (Alameda, Monterey and San Francisco Counties) on the same host (Arctostaphylos spp.) under similar climate (coastal nurseries).

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score:  Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 11.


The full host range and in-state distribution of Phytophthora hedraiandra is not currently known.  To date, in California, the pathogen has only been detected from Arctostaphylos plants propagated in nurseriesContinued statewide surveys for Phytophthora spp. occurring in nurseries and natural ecosystems (e.g. restoration sites) will contribute to the present knowledge of this pathogen group as well as that of P. hedraiandra.  Consequently, the current proposed rating of P. hedraiandra may be affected.  

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Phytophthora hedraiandra is B.


Agrios, G. N.  2005.  Plant Pathology fifth edition.  Elsevier Academic Press, Massachussetts, USA.  922 p.

CABI.  2014.  Phytophthora hedraiandra datasheet (basic) report.  Crop Protection Compendium.

de Cock A. W. A. M. and C. A. Lévesque. 2004. New species of Pythium and Phytophthora. Studies in Mycology 50: 481-487.

EPPO.  2014.  Phytophthora hedraiandra (PHYTHD).  European and Mediterranean Plant Protection Organization PQR database.

Farr, D.F., & Rossman, A.Y.  2014.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved November 10, 2014, from .

Henricot, B. and I. Waghorn.  2014.  First report of collar and root rot caused by Phytophthora hedraiandra on Viburnum in the UK.  New Disease Reports 29:8.

Hejna M., K. Cerny, L. Havrdova, and M. Mrazkova.  2014.  First report of Phytophthora hedraiandra causing Rhododendron dieback and root rot of Common Beech in the Czech Republic.  Plant Disease 98:1,434.2.

NAPPO.  2006.  Phytophthora hedraiandra de Cock & Man in’t Veld First detection of Phytophthora hedraiandra in the United States and North America.  North American Plant Protection Organization’s (NAPPO Phytosanitary Alert System):

Schwingle, B. W., J. A. Smith and R. A. Blanchette, S. Gould, and B. L. Blanchette.  2006.  First report of dieback and leaf lesions on Rhododendrons sp. caused by Phytophthora hedraiandra in the United States.  Plant Disease 90:109.

Schwingle, B. W., J. A. Smith and R. A. Blanchette.  2007.  Phytophthora species associated with diseased woody ornamentals in Minnesota nurseries.  Plant Disease 91:97-102.

Yang, X. P. A. Richardson, S. R. Ghimire, P. Kong, and C. X. Hong.  2012.  Phytophthora hedraiandra detected from irrigation water at a perennial ornamental plant nursery in Virginia.  Plant Disease 96:915.3.

Responsible Party:

Dr. John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110,[@]

Comment Period:  CLOSED

The 45-day comment period opened on Thursday, April 9, 2015 and closed on May 24, 2015.

Pest Rating: B

Posted by ls