Plasmopara constantinescui Voglmayr & Thines 2007

California Pest Rating for

Plasmopara constantinescui Voglmayr & Thines 2007

Pest Rating: B

PEST RATING PROFILE

Initiating Event:

On August 8, 2017, diseased leaves of Impatiens walleriana plants were collected, from a retail nursery in Placer County, by Placer Agricultural County officials and sent to the CDFA Plant Pathology Laboratory for diagnoses. The plants had been shipped from a different nursery in San Joaquin County. Cheryl Blomquist, CDFA plant pathologist, identified the downy mildew pathogen, Plasmopara constantinescui, as the cause for the disease. The pathogen was assigned a temporary ‘Q’ rating. Consequently, the infected plants, received at Placer County, will be destroyed by County officials (Walber, 2017). Impatiens walleriana plants related to the shipment from San Joaquin County were double-bagged and disposed at a landfill, by the nursery (Khan, 2017). The risk of introduction and establishment of this pathogen in California is assessed and a permanent rating is herein proposed.

History & Status:

Background: Plasmopara constantinescui is an obligate oomycete plant pathogen that causes downy mildew disease in its host plants. Presently, the host range for the pathogen only includes Impatiens species, belonging to the plant family Balsaminaceae.

Plasmopara constantinescui was originally described as Bremiella sphaerosperma from Impatiens in eastern Russia and northeastern North America (Constantinescu, 1991). However, after molecular phylogenetic analyses of DNA sequences, B. sphaerosperma was found to belong to the genus Plasmopara and transferred there accordingly. Furthermore, as there already existed, within Plasmopara, a species by the same epithet, the newly-transferred pathogen was given a new epithet, P. constantinescui (Voglmayr & Thines, 2007). This species was also shown to be closely related to Plasmopara obducens, which is a common, widely distributed pathogen of several species of Impatiens in the Northern Hemisphere, including California (Constantinescu, 1991; Voglmayr & Thines, 2007).

Hosts: Impatiens sp. (impatiens), I. capensis (jewel weed), I. noli-tangere (western touch-me-not), I. pallida (pale touch-me-not) (Constantinescu, 1991; Farr & Rossman, 2017). Plasmopara constantinescui was recently detected in Impatiens walleriana (buzzy lizzy) plants (see: ‘Initiating Event’.)

Symptoms: Pale yellowish to ochre, round to irregular, and scattered spots appear on the upper surface of leaves. These spots are small (1-6 mm-diam.), vein-limited, and with margins that are indistinct to reddish brown or violaceous. They rarely coalesce and cover larger areas. White to greyish or yellowish downy growth of sporangiophores of the oomycete develop in patches on the underside of the spots (Constantinescu, 1991). It is likely that, similar to other downy mildew-causing pathogens, Plasmopara constantinescui attacks and spreads rapidly in young, tender green leaf, shoot, and blossom tissue (Agrios, 2005).

Disease development: Generally, downy mildew pathogens overwinter as thick-walled resting spores called oospores in plant debris in the soil or on weed hosts, and as mycelium in infected, but not dead, twigs. Downy mildew develops and is severe under conditions that favor periods of prolonged leaf wetness and high relative humidity during cool or warm, but not hot, periods. During rainy period in spring, the oospores germinate to produce a sporangium. The sporangium or its zoospores are transmitted by wind or water to wet leaves near the ground where they infect through stomata of the lower leaf surface. Mycelium develops and spreads into intercellular spaces of leaves. When it reaches the sub-stomatal cavity, it forms a cushion from which sporangiophores arise and grow through the stoma. Sporangia are produced at the tips of the sporangiophores and are transmitted by wind or rain to nearby non-infected plants (Agrios, 2005; Daughtrey et al., 1995). In pathogenicity tests, Plasmopara constantinescui was able to cause systemic shoot infection of Impatiens walleriana (Personal communication: Suzanne Latham, CDFA plant pathologist).

Dispersal and spread: Wind, rain/water splash, infected plants and infected plant debris.

Damage Potential: While estimates of crop losses caused particularly by Plasmopara constantinescui have not been reported, generally, downy mildews can cause significant losses in short periods of time. Affected plants may result in defoliation, flower drop, and stem rot, similar to Impatiens walleriana plants infected with the closely related downy mildew species, P. obducens (Crouch et al., 2014). Nurseries, private and public gardens, and landscape plantings may be at particular risk of contracting downy mildew disease caused by P. constantinescui. Fungicidal control of the pathogen is possible, but may be difficult. Under cool wet weathers, downy mildews are often uncontrollable and checked only when the weather turns dry and hot (Agrios, 2005).

Worldwide Distribution: Asia: Eastern Russia (formerly USSR); North America: Canada, USA (Indiana, Massachusetts, Wisconsin, Iowa, Maryland, Minnesota, Virginia, South Carolina, and California) (Constantinescu, 1991; Farr & Rossman, 2017; Voglmayr & Thines, 2007; CDFA Pest and Damage Record 2017).

Official Control: Bremiella sphaerosperma (synonym of Plasmopara constantinescui) is on the ‘Harmful Organism List’ for Brazil (USDA PCIT, 2017). Presently, P. constantinescui has a Q rating in California.

California Distribution: Based on the source of diseased Impatiens, Plasmopara constantinescui is present in San Joaquin County

California Interceptions: One intrastate interception in Placer County (see: Initiating Event).

The risk Plasmopara constantinescui would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: The downy mildew oomycete, Plasmopara constantinescui requires prolonged periods of leaf wetness and high relative humidity during cool or warm, but not hot, periods. These conditions for infection and development of the pathogen is likely to limit its establishment in California, to coastal regions in particular.

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: The host range for the pathogen is limited to Impatiens

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: Spores are produced in abundance. The pathogen is transmitted via infected plant material, winds, and rain/water splash.

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: If left uncontrolled, downy mildews can cause significant losses in short periods of time. Affected plants may result in defoliation, flower drop, and stem rot, thereby lowering crop yield and value in increasing production costs largely due to administration of control measures. Fungicidal control of the pathogen is possible, but may be difficult. Under cool wet weathers, downy mildews are often uncontrollable and checked only when the weather turns dry and hot.

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

Economic Impact: A, B, C, D.

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.

5) Environmental Impact: Downy mildew disease caused by Plasmopara constantinescui could significantly impact home/urban, private and public gardens, and landscape plantings.

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

Environmental Impact: 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

Environmental Impact Score: 2

– 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 Plasmopara constantinescui:

Add up the total score and include it here. 11

-Low = 5-8 points

–Medium = 9-12 points

-High = 13-15 points

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.

Evaluation is ‘Low’. Based on the source of diseased Impatiens, Plasmopara constantinescui is only present in San Joaquin County.

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 = 10.

Uncertainty:

None.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Plasmopara constantinescui is B.

References:

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

Calflora. 2017. Information on California plants for education, research and conservation. [web application]. 2017. Berkeley, California. The Calflora Database [a non-profit organization]. http://www.calflora.org/

Constantinescu, O. 1991. Bremiella sphaerosperma sp. nov. and Plasmopara borreriae comb. nov. Mycologia 83: 473-479.

Crouch, J. A., M. P. Ko, and J. M. McKemy. 2014. First report of impatiens downy mildew outbreaks caused by Plasmopara obducens through the Hawai’ian Islands. Plant Disease, 98: 696. DOI: https://doi.org/10.1094/PDIS-10-13-1017-PDN

Daughtrey, M. L., R. L. Wick, and J. L. Peterson. 1995. Downey mildews. Part I. infectious diseases, diseases caused by fungi. Compendium of flowering potted plant diseases. APS Press, the American Phytopathological Society. 38-38 p.

Farr, D. F., and A. Y. Rossman. 2017. Fungal Databases, U. S. National Fungus Collections, ARS, USDA. Retrieved September 7, 2017, from http://nt.ars-grin.gov/fungaldatabases/

French, A. M. 1989. California Plant Disease Host Index. California Department of Food and Agriculture, Sacramento (Updated online version by T. Tidwell, May 2, 2017).

Khan, S. 2017. Email from S. Khan, CDFA Pest Exclusion, to T. Walber, CDFA Interior Pest Exclusion, and J. Chitambar, CDFA, dated 9/19/2017. 4:43 pm.

USDA PCIT. 2017. USDA Phytosanitary Certificate Issuance & Tracking System. Retrieved September 7, 2017. 4:19:24 pm CDT. https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.

Voglmayr, H., and M. Thines. 2007. Phylogenetic relationships and nomenclature of Bremiella sphaerosperma (Chromista, Peronosporales). Mycotaxon 100: 11-20.

Walber, T. 2017. Email from T. Walber, CDFA Interior Pest Exclusion, to J. Chitambar, CDFA, dated 9/8/2017, 9:44 am.

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, plant.health[@]cdfa.ca.gov.