Phytophthora parvispora Scanu & Denman, 2013

California Pest Rating for
Phytophthora parvispora  Scanu & Denman, 2013
Pest Rating: B 

Initiating Event: 

On August 19, 2016, non-official samples of pear baits of effluent collected from the bottom of four pots containing diseased Mexican orange blossom (Choisya ternata) plants, were sent by a private company to the CDFA Plant Pathology Lab for identification of the cultured pathogen.  The private company had prepared the pear baits after collection of the effluent drained from the four diseased plants that were contained in a commercial nursery in San Francisco County.  The associated pathogen was identified by Suzanne Latham, CDFA plant pathologist, as Phytophthora parvispora on September 9, 2016.  Subsequently, on September 22, 2016, San Francisco County Agricultural inspectors collected official samples of the same four symptomatic Mexican orange blossom plants originally sampled in San Francisco County.  The diseased plants exhibited root and crown rot symptoms and were sent to the CDFA Plant Pathology Lab for diagnosis.  On November 22, 2016, Suzanne Latham officially identified Phytophthora parvispora after baiting and culturing it from the roots of one of the four Choisya ternata plant samples (Latham, 2016).  This detection marked the first report of P. parvispora in the USA and therefore, a culture of the pathogen was sent from CDFA to the USDA APHIS CPHST in Beltsville for confirmation.  On November 15, 2016, the identity of P. parvispora was confirmed by CPHST.   Consequently, the pathogen was assigned a temporary ‘Q’ rating by CDFA.   The risk of introduction and establishment of this pathogen is assessed here and a permanent rating is proposed.

History & Status:

Background: Phytophthora parvispora was originally recorded from the stem bases of Beaucarnea sp. nursery plants grown in a greenhouse in Germany and was regarded a variety of the polyphagous fungal species, P. cinnamomi based on its morphology and temperature required for growth (Kröber & Marwitz, 1993).  However, Scanu et al., (2014) stated that P. parvispora was apparently isolated from citrus in Taiwan (Ann & Ko, 1985) prior to Kröber & Marwitz’ report and was suggested by several researchers to belong to a distinct species different from P. cinnamomi.   In subsequent years, studies based on molecular and phylogenetic analyses, combined with morphological characters, temperature-growth relations, and pathogenicity experiments, demonstrated that P. cinnamomi var. parvispora significantly differed from P. cinnamomi and therefore, was elevated to species status to become Phytophthora parvispora (Scanu et al., 2014).

Since its first records from Germany and Taiwan, P. parvispora has been also reported from Australia, isolated from potting mixes of nursery plants and an irrigation channel surrounded by cultivated agricultural fields; from South Africa, isolated from Arbutus unedo plants; from Brazil, isolated from the rhizosphere of cowpea; from Italy, isolated from Mandevilla sp. and A. unedo plants; and from Portugal isolated from potted Pinus pinea nursery plants (Scanu et al., 2014).   The current record of its detection in Choisya ternata in California marked its first detection in the USA.  As almost all reports of P. parvispora are associated with trade of plants intended for planting, the latter is considered a major pathway for introduction of the pathogen into ornamental, landscape and natural environments.  Choisya ternata is native to Mexico and not naturalized in the USA (NPGS).  The pathogen has never been recorded from natural environments (Scanu et al., 2014).  The origin of P. parvispora is considered to be south-east Asia as the earliest record came from Taiwan (Ann & Ko, 1985; Scanu et al., 2014).

Hosts: Although few plant hosts have been reported, they belong to six monocot, dicot, and coniferous plant families.  Rutaceae: Agathosma betulina (buchu) and Citrus sp.; Ericaceae: Arbutus unedo (strawberry tree); Asparagaceae: Beaucarnea recurvata (elephants foot, ponytail palm), Beaucarnea sp.; Apocynaceae: Mandevilla sanderi (Brazilian jasmine), M. splendens, Mandevilla sp., Mandevilla x amabilis; Pinaceae: Pinus pinea (Italian stone pine, umberella pine); Fabaceae: Vigna unguiculata (cowpea) (Farr & Rossman, 2016; Scanu et al., 2014).  Choisya ternata (Mexican orange blossom, Rutaceae; CDFA Pest and Damage Record, November 22, 2016) is included here as a newly reported host.

Symptoms:  Host plants infected with Phytophthora parvispora exhibit symptoms of root and collar rot, leaf chlorosis, shoot dieback and plant decline.  Two to three-year-old infected seedlings of Arbutus unedo showed symptoms of shoot tip dieback and root and collar rot (Scanu et al., 2014).

Disease Cycle:  Generally, species of Phytophthora that cause root and stem rots survive cold winters or hot and dry summers as resting spores (oospores and chlamydospores) or mycelium in infected roots, stems or soil.  For P. parvispora, it is suggested that the pathogen survives long terms in moderate dry conditions between consecutive rains as mycelial aggregations and selfed oospores than as chlamydospores as the former structures are produced in solid agar and in water, while chlamydospores are thin-walled and infrequently produced, thereby indicating short-term survival (Jung et al., 2013; Scanu et al., 2014).  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, 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).   The temperature range for the development P. parvispora in culture is from 10-11°C to 36-37°C, the optimum temperature being 16°C to 32°C (Kröber & Marwitz, 1993).  With high cardinal temperatures for growth, P. parvispora is well adapted to tropical and subtropical climates and greenhouse conditions (Scanu et al., 2014).

Dispersal and spread: Like most Phytophthora species, P. parvispora is soil-borne and water-borne and may be spread to non-infected sites through infected plants, nursery and planting stock, seedlings, pathogen-contaminated soil, run-off and splash irrigation and rain water, and contaminated cultivation equipment and tools.

Damage Potential:  Phytophthora parvispora causes root and crown rot on woody and semi-woody hosts.  At particular risk are nursery-grown plants for plantings in commercial and private garden, landscape, and horticultural environments.

Worldwide Distribution: Asia: Taiwan; Africa: South Africa; Europe: Germany, Italy, Portugal; North America: USA (California); South America: Brazil; Australia (Farr & Rossman, 2016; Scanu et al., 2014).

Official Control:  Presently, Phytophthora parvispora has a temporary, quarantine status and ‘Q’ rating by the CDFA.

California Distribution: San Francisco County.

California Interceptions: None.

The risk that Phytophthora parvispora would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: Phytophthora parvispora may be able to establish in cool to warm (10-11°C to 36-37°C, or 16-32°C optimal) and wet climates within California. Its in-state establishment is likely to be large but limited in accordance with the distribution of its hosts under climates favorable for infection.  Its hosts range from ornamentals such as Mandevilla which are grown in cool to warm coastal and inner valley regions, to citrus and stone pine which cover larger regions of the state.  The pathogen is well adapted to tropical and subtropical climates and greenhouse conditions, and requires wet weather for infection.

Evaluate if the pest would have suitable hosts and climate to establish in California.

Score: 2

– 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.

2) Known Pest Host Range: Presently, the host range for Phytophthora parvispora is limited to relatively few, yet diverse hosts which are found in six monocot, dicot and coniferous families. Important hosts for California include mainly ornamentals (Arbutus unedo – strawberry tree, Mandevilla, Choisya ternata – Mexican orange blossom), conifer (Pinus pinea) and few fruit (Citrus sp.).  A low score is ascribed to this category.

Evaluate the host range of the pest.

Score: 1

– Low (1) has a very limited host range.

– Medium (2) has a moderate host range.

– High (3) has a wide host range.

3) Pest Dispersal Potential: Phytophthora parvispora is soil-borne and water-borne and therefore, primarily spreads 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.

Evaluate the natural and artificial dispersal potential of the pest.

Score: 3

– 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.

4) Economic Impact: Although losses caused solely by Phytophthora parvispora have not been quantified, the potential for the pathogen to infect mainly ornamental, conifer, and citrus plants in California could result in root and crown rot, and shoot dieback thereby decreasing healthy stands, causing yield losses, increasing production costs and causing loss of market of nursery stocks. Also, the pathogen’s potential to survive and spread in infected soils and irrigation water could require changes in normal cultivation practices of host plants.  A ‘High’ score is given for this category.

Evaluate the economic impact of the pest to California using the criteria below.

Economic Impact: A, B, C, D, G.

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.

Economic Impact Score: 3

– 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.

4) Environmental Impact: Phytophthora parvispora may impact conifers and other ornamentals grown in commercial and private gardens and landscape environments.  The pathogen has never been reported from natural environments nevertheless, infected nursery plants provide a pathway for the introduction of the pathogen to outdoor environments, and the possibility of its establishment under favorable conditions. Infestations could trigger additional treatment programs.  Infections of perennial shrub and tree hosts could disrupt natural communities or alter ecosystem processes.

Evaluate the environmental impact of the pest on California using the criteria below.

Environmental Impact:  A, D, E.

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.

Environmental Impact Score: 3

– 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.

Consequences of Introduction to California for Phytophthora parvispora: Medium (12)

Add up the total score and include it here.

-Low = 5-8 points

-Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Presently, Phytophthora parvispora has only been officially reported from one region, namely, San Francisco County. California.

Score: (-1)

-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.

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



Conclusion and Rating Justification:

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


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

Ann, P. J., and W. H. Ko.  1985.  Variants of Phytophthora cinnamomi extend the known limits of the species.  Mycologia 77: 946-950.

Farr D.F., & Rossman, A.Y.  2016.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved December 7, 2016, from

Jung, T., I. Colquhoun, G. E. St. J. Hardy.  2013.  New insights into the survival strategy of the invasive soilborne pathogen Phytophthora cinnamomi in different natural ecosystems in Western Australia. Forest Pathology 43: 266-288.  doi:10.1111/efp.12025.

Kröber, H., and R. Marwitz.  1993.  Phytophthora tentaculata sp. nov. und Phytophthora cinnamomi var. parvispora var. nov., zwei neue Pilze von Zierpflanzen in Deutschland. Zeitschrift fur Pflanzenkrankheiten und Pflanzenschutz 100, 250-258. [Original Description].

Latham, S.  2016.  Email to J. Chitambar et al., CDFA, on Monday, November 7, 2016, 10:12:21 am.

NPGS.  (Date unknown).  Taxon: Choisya ternata Kunth.  U. S. National Plant Germplasm System.

Scanu, B., Hunter, G.C., Linaldeddu, B.T., Franceschini, A., Maddau, L., Jung, T., and Denman, S. 2014. A taxonomic re-evaluation reveals that Phytophthora cinnamomi and P. cinnamomi var. parvispora are separate species. Forest Pathology 44: 1-20.

Responsible Party:

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

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Pest Rating: B

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