Peronospora digitalidis Gäum, 1923

California Pest Rating for
Foxglove, Downy Mildew Plant Symptoms
Downy mildew does occur in the landscape in Washington. Note lesions with angular margins. ~ Photo Credit: Jenny Glass, 2011.
Peronospora digitalidis Gäum, 1923
Pest Rating: C

 


PEST RATING PROFILE
Initiating Event:

On February 9, 2016, USDA’s Animal and Plant Health Inspection Service (APHIS) notified the CDFA that the downy mildew pathogen, Peronospora digitalidis, was added on February 2, 2016, to their ‘List of Pests no Longer Regulated at U.S. Ports of Entry’ under the Federally Recognized State Managed Phytosanitary (FRSMP) program (USDA APHIS 2016).  Consequently, USDA APHIS will no longer take regulatory action this pathogen at ports of entry.  Therefore, and at the request of Stephen Brown, Assistant Director, CDFA, the risk of infestation and the current rating for P. digitalidis is re-assessed here.

History & Status:

BackgroundPeronospora digitalidis is an oomycete in the order Peronosporales, which causes downy mildew disease of foxglove (Digitalis spp.).  The pathogen has been reported in Europe, Asia, and New Zealand, and was first reported from the United States, in 2002, on potted common foxglove plants in commercial nurseries in Santa Cruz County, California (Tjosvold & Koike, 2002).  It is likely that all affected nursery plants were destroyed, however, following its initial detection, in 2003, the pathogen continued to be found in several nurseries in California coastal counties and most recently in 2015 (CDFA Pest and Damage Records).

Disease cycle:  Peronospora digitalidis is an oomycete belonging to the family Peronosporaceae.  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) 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: Digitalis spp. (foxglove): D. purpurea, D. grandiflora, D. lutea, and Digitalis sp. (Farr & Rossman, 2016). Foxy hybrids are very susceptible and D. grandiflora was symptomless when grown in a heavily diseased region (Tjosvold & Koike, 2002).

Symptoms and damage potential: On leaves, initial symptoms consist of light green, rectangular areas that are delimited by veins.  Later, these spots become chlorotic, coalesce and turn necrotic and purplish-gray sporulation of the pathogen develops primarily on the underside of leaves and sometimes on upper surfaces of leaves (Pscheidt & Ocamb, 2016).

Damage Potential: While estimates of crop losses caused in particular by Peronospora digitalidis have not been reported, downy mildews can cause rapid and severe losses of young plants in seedbeds and in the field. Nursery stock producers and landscape growers of foxglove species may be at risk of damages caused by this downy mildew pathogen.

Worldwide Distribution: Europe: Czech Republic, Poland, Germany, Italy, United Kingdom, Scotland, Wales; North America: USA (California, Oregon, Washington) (Farr & Rossman, 2016; Garibaldi et al., 2013; Tjosvold & Koike, 2002; Pscheidt & Ocamb, 2016).

Official Control: Since 2002, Peronospora digitalidis has been on the North American Plant Protection Organization (NAPPO) “Alert List” (EPPO, 2016).  Since February 2, 2016, it has not been regulated by the USDA (see ‘Initiating event’). Currently, it has a “C” rating in California.

California Distribution: Orange, San Diego, San Luis Obispo, Santa Barbara, and Santa Cruz (CDFA Pest and Damage Records).

California InterceptionsPeronospora digitalidis was detected in two nursery foxglove shipments imported to California in 2003.

The risk Downy mildew of foxglove 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 digitalidis is likely to establish in cool to warm and very humid to wet climates where foxglove grows in California.  According to the California Invasive Plant Council (2006-2016), foxglove is found along the coast northward from Santa Barbara, infesting moist meadows and roadsides and also in the northern Sierra Nevada foothills.

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 few reported species of Digitalis

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 digitalidis have not been reported, presence of the pathogen in open fields/landscapes and/or in nursery stock produced in greenhouse environments are expected to 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 and home garden plantings of foxglove 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 Downy mildew of foxglove:

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 Medium (-2): Presently, the downy mildew pathogen, Peronospora digitalidis, has been reported from at least five coastal counties.

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 = 9.

Uncertainty:   

The impact and spread of this pathogen to other intrastate regions where Digitalis spp. is grown, is not known.  Future reports of the detection of P. digitalidis in California could lower the overall score for the pathogen although it is unlikely to affect its final rating.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Peronospora digitalidis is C.

References:

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

California Invasive Plant Council.  2006-2016. Invasive Plants of California’s Wildland Digitalis purpurea: http://www.cal-ipc.org/ip/management/ipcw/pages/detailreport.cfm@usernumber=42&surveynumber=182.php?print=y .

EPPO.  2016.  Peronospora digitalidis (PERODG).  New PQR database.  Paris, France: European and Mediterranean Plant Protection Organization.  http://newpqr.eppo.int.

Farr, D.F., & A. Y. Rossman.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA.  Retrieved January 28, 2016, from http://nt.ars-grin.gov/fungaldatabases/.

Garibaldi, A., D. Bertetti, A. Poli, and M. L. Gullino.  2013.  Outbreak of downy mildew caused by Peronospora digitalidis on common foxglove (Digitalis purpurea) in Italy.  Journal of Plant Pathology 95:659-668. doi: 10.4454/JPP.V95I3.021.

Pscheidt, J. W., and C. M. Ocamb (Senior Eds.).  2016.  Foxglove (Digitalis spp.) downy mildew.  PNW Plant Disease Management Handbook: http://pnwhandbooks.org/plantdisease/foxglove-digitalis-spp-downy-mildew.

USDA APHIS.  2016.  FRSMP: Pests no longer regulation at U. S. ports of entry.  United States Department of Agriculture Animal and Plant Health Inspection Service.  https://www.aphis.usda.gov/aphis/ourfocus/planthealth/plant-pest-and-disease-programs/frsmp/ct_non-reg-pests.


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.


Comment Period: CLOSED

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


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Pest Rating: C


Posted by ls