Peronospora mesembryanthemi

California Pest Rating Proposal for
Peronospora mesembryanthemi Verwoerd 1924
Pest Rating: B


Initiating Event:

On December 11, 2015, Oregon State University Plant Clinic (OSUPC) informed the CDFA that they had received an Aptenia cordifolia (red apple iceplant) sample infected with downy mildew from San Diego County, California.  A sample of the diseased plant was submitted by OSUPC to the USDA APHIS National Mycology Laboratory where the identity of the associated pathogen was confirmed as the downy mildew pathogen, Peronospora mesembryanthemi. Consequently, in California, official samples of diseased red apple iceplant were collected by Pat Nolan, plant pathologist, San Diego County, and submitted to the CDFA Plant Pathology Laboratory.   The downy mildew pathogen, P. mesembryanthemi was detected in those samples by Suzanne Latham, plant pathologist, CDFA.  The current temporary Q rating for P. mesembryanthemi is reassessed here and a permanent rating is proposed.

History & Status:

Background:  Peronospora mesembryanthemi is an oomycete in the order Peronosporales, which causes downy mildew disease of host plants in the family Aizoaceae.  The pathogen was first detected in Mesembryanthemum sp. in South Africa (Verwoerd, 1924).  Several years later, it was also reported from the United Kingdom (Wales) and New Zealand (Francis & Waterhouse, 1988; McKenzie & Dingley, 1996).  The recent detection of P. mesembryanthemi on Aptenia sp. in San Diego County, California, marked the first record of this pathogen in the USA.  Downy mildew disease is currently widespread in San Diego County, over several acres of red apple iceplant growing in mature landscapes, specifically in shaded areas and areas that receive sunlight for limited periods of the day (personal communication: Pat Nolan, plant pathologist, San Diego County).

Disease cycle:  Peronospora mesembryanthemi causes downy mildew disease in its host plants.  Generally, downy mildews overwinter as thick-walled resting spores called oospores that are produced through the fertilization of two mating types.  However, no oospores have so far been reported for this pathogen.  It is likely that the pathogen survives as mycelium and/or condia (spores) in infected plant buds, plant debris, leaf tissue and shoots.  Downy mildews are severe in cool or warm (but not hot), high humid climates and when a film of water is present on plant tissue.  They primarily cause foliar blights and rapidly spread in young green leaf, twig and fruit tissues.  Under favorable weather conditions, condia are carried by wind or water to wet leaves near the ground where they infect through stomata of the lower leaf surface.  A conidium germinates via a germ tube that grows through leaf stomata into intercellular spaces within the leaf tissue and eventually penetrates plant cells through special structures called haustoria.  Developing hypha that spreads intercellularly forms a cushion of mycelia just below the stomata.  From this cushion, conidophores arise and emerge through stomata.  At their tips, conidia (sporangia/spores) are produced simultaneously and are carried by wind and rain to new infection sites of the same or different plant.

Dispersal and spread:  The pathogen can spread through contaminated plant cuttings, transplants, fresh leaves and within seeds.  Also, it produces airborne conidia (spores) that can disperse and be carried by moist winds.  It can also be present in soil associated with host and non-host plants and therefore, can spread by any means that aids in the movement of soil and/or water from infected plants to non-infected ones.

Hosts: Presently, only few plant species within the family Aizoaceae are known to be hosts of Peronospora mesembryanthemiAptenia sp. (red apple iceplant), Mesembryanthemum sp. (iceplant), Dorotheanthus bellidiformis (syn. Mesembryanthemum criniflorum) (Livingstone daisy, iceplant) (Farr & Rossman, 2016; Francis & Waterhouse, 1988; McKenzie & Dingley, 1996; Verwoerd, 1924).

Symptoms and damage potential: Infection starts on lower leaves and progresses upwards.  Initially symptoms appear as yellowing (slightly chlorotic) of affected leaves with the veins remaining green. Following this, the central portion of a chlorotic lesion may become necrotic; slight curvature of leaves occurs followed by premature leaf fall.  In some cases entire area of the leaf surface is affected.  Grey to brown, furry or downy (conidia) growth is apparent on underside of symptomatic leaves giving the leaves a dirty appearance.  These symptoms may sometimes occur on upper surfaces of leaves.

Damage Potential: While estimates of crop losses caused in particular by Peronospora mesembryanthemi have not been reported, downy mildews can cause rapid and severe losses of young plants in seedbeds and in the field. Currently, downy mildew disease on red apple iceplant is widespread over several acres of mature landscapes in San Diego County.  Nursery stock producers and landscape growers of iceplant species may be at risk of severe damages caused by this downy mildew pathogen.

Worldwide Distribution: Africa: South Africa; Europe: United Kingdom (Wales); North America: USA (California); Oceania: New Zealand (Farr & Rossman, 2016; Francis & Waterhouse, 1988; McKenzie & Dingley, 1996; Verwoerd, 1924).

Official Control: Peronospora mesembryanthemi is on the “Harmful Organism Lists” for Namibia and South Africa (USDA PCIT, 2016).  Currently, it has a temporary “Q” rating in California.

California Distribution: San Diego County.

California Interceptions:  There are no reports of the detection of Peronospora mesembryanthemi in plant shipments imported to California.

The risk Downy mildew of iceplant 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 Medium (2):  Peronospora mesembryanthemi is likely to establish in cool to warm and very humid to wet climates where iceplant grows in California.

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): Presently, the host range is limited to a few reported species of iceplant.  In California, the pathogen has only been reported from red apple iceplant that is a common landscape cover plant grown in coastal regions of the State.

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): Conidia are easily produced simultaneously and in abundance.  The pathogen is transmitted via infected plant material; conidia are dispersed by winds, water and associated soil.   

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): While estimates of crop losses caused by Peronospora mesembryanthemi have not been reported, presence of the pathogen in open fields/landscapes and/or in nursery stock produced in greenhouse environments can cause severe damage under cool or warm and humid climates resulting in significantly lower crop value and yield.  Infected, symptomatic nursery stock plants are not marketable, resulting in total loss in recovery of production costs.  Markets for crop sale are directly affected.  Normal cultivation practices, including delivery and supply of irrigation water, would need to be altered to prevent spread of the pathogen.

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): Commercial landscape plantings of iceplant can be significantly impacted if infected by the downy mildew pathogen.  Under favorable climate conditions, disruption of natural communities and changes in ecosystem could occur with severe and widespread infestations of downy mildew. 

Consequences of Introduction to California for the downy mildew pathogen, Peronospora mesembryanthemi:

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 to California = 11

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): Presently, the downy mildew pathogen, Peronospora mesembryanthemi, has only been reported from San Diego County.

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.


The full host range of Peronospora mesembryanthemi is not yet known.  There may be other species of iceplant and weeds that may also be infected by the pathogen.  Also not known is the impact and spread of this pathogen in other areas where red apple iceplant and/or other host plants are growing naturally.  Future reports of the detection of P. mesembryanthemi in California could affect the overall score and rating that is being proposed here.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Peronospora mesembryanthemi is B.


Agrios, G. N.  2005.  Plant Pathology (Fifth Edition).  Elsevier Academic Press, USA.  922 p.

Farr, D.F., & A. Y. Rossman.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA.  Retrieved January 28, 2016, from

Francis, S., and G. Waterhouse.  1988.  List of Peronosporaceae reported from the British Isles. Trans. Brit. Mycol. Soc. 91: 1-62.

McKenzie, E.H.C., and J. M. Dingley.  1996.  New plant disease records in New Zealand: miscellaneous fungal pathogens III. New Zealand Journal of Botany 34: 263-272.

USDA PCIT.  2016.  USDA Phytosanitary Certificate Issuance & Tracking System.

Verwoerd, L.  1924.  Peronospora mesembryanthemi n. sp., die oorsaak van ‘n donsige skimmelsiekte van Mesembryanthemum-soorte. Ann. Univ. Stellenbosch 2A (1): 13-23.

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,[@]

Comment Period:  CLOSED

The 45-day comment period opened on Monday, February 29, 2016 and closed on April 14, 2016.

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Consequences of Introduction:  1. Climate/Host Interaction: [Your comment that relates to “Climate/Host Interaction” here.]

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Pest Rating: B

Posted by ls