Erysiphe aquilegiae DC. 1815

California Pest Rating for
Erysiphe aquilegiae DC. 1815
Pest Rating:  C


Initiating Event:  


History & Status:

Background:  The fungal pathogen, Erysiphe aquilegiae, originally named, Ischnochaeta aquilegiae (DC.) Sawada 1959 is the cause of powdery mildew disease that infects several species of plant hosts, including Aquilegia sp. (columbine), mainly in the family Ranunculaceae.

The pathogen is also known by other scientific names, including, E. aquilegiae var. aquilegiae, E. aquilegiae var. ranunculi, and E. ranunculi.  According to Uwe Braun (Professor, Martin-Luther-Universitӓt, Institut für Biologie, Halle, Germany: personal communication to Cheryl Blomquist, CDFA plant pathologist, email dated October 29, 2015), as currently circumscribed, E. aquilegiae is a complex of species that have yet to be clearly distinguished genetically.

During October, 2013, CDFA plant pathologist, Cheryl Blomquist, detected the powdery mildew, Erysiphe aquilegiae on Ranunculus sp. nursery stock that was grown in fields in San Luis Obispo County.  Additionally, there is an earlier report of the asexual (anamorph) stage of Erysiphe, namely, Oidium sp. on Aquilegia sp. from southern coastal counties in California (A. M. French, 1987, California Plant Disease Host Index 2nd edition) that indicates that this powdery mildew pathogen is widely distributed within the State.

Hosts:  Erysiphe aquilegiae comprises numerous ornamental and weed hosts of several genera of the family Ranunculaceae: Aconitum, Actaea, Anemone, Aquilegia, Caltha, Clematis (=Atragene), Consolida, Coptis, Delphinium, Nigella, Ranunculus, Thalictrum, and Trollius (Farr & Rossman, 2015).

Other hosts in different families have been reported based only on morphological analysis of the pathogen.  These records have not been genetically proven and therefore, are doubtful (Braun-Blomquist: personal communication, 2015).  They include Alnus japonica and A. pendula (Betulaceae), Breea setosa and Cirsium spp. (Asteraceae), Magnolia liliiflora (Magnolioideae), Paeonia lactiflora (Paeoniaceae), Quercus crispula and M. mongolica var. grosseserrata (Fagaceae), Swertia spp. (Gentianaceae), Urena lobata and U. lobata var. tomentosa (Malvaceae), and Catharanthus roseus (Apocynaceae) (Bolay, 2005; Farr & Rossman, 2015; Liberato & Cunnington, 2006).

Symptoms:  White to grayish powdery, mildew grows in spots or patches on young plant tissue or covers entire leaves and other plant organs. Mildew growth is most common on upper side of leaves, but may also be found on the underside of leaves, young shoots and stems, buds, flowers and young fruit. Pinhead-sized spherical chasmothecia (completely closed fungal fruiting bodies containing spores), initially white to yellow brown later turning black in color, develop singly or in clusters on older mildew.

Damage Potential:  In general, powdery mildews seldom kill their hosts however they reduce photosynthesis, utilize plant nutrients, increase respiration and transpiration, impair plant growth and reduce crop yields up to 40% (Agrios, 2005).

Disease Cycle:  Erysiphe aquilegiae is an obligate parasite that produces mycelium on the surface of plant tissues without invading it.  The pathogen obtains nutrients from the plant by producing haustoria (specialized absorbing organs) that grow into the epidermal cells of the plant. On the plant surface, the mycelium produces short conidiophores which in turn produce numerous chains of conidia that appear as white powdery coating.  These conidia are easily dispersed by air currents to cause new infections of host plants.  When conditions are unfavorable, the pathogen may produce chasmothecia containing ascospores.  The disease is common in cool or warm humid regions, but can also be common in warm and dry climates since their spores only require high relative humidity and not free-standing water to be released, germinate and cause infections (Agrios, 2005).  Once a plant is infected, mycelium continues to spread on a leaf surface regardless of the level of atmospheric moisture.

Worldwide Distribution: Erysiphe aquilegiae is circumglobally distributed.  Africa: South Africa; Asia: Armenia, China, Iran, Taiwan, Japan, Korea, Israel, USSR; Africa: South Africa; Europe: Lithuania, Ukraine, Romania, Estonia, Belarus, Switzerland, Austria, Czechoslovakia, Denmark, Finland, France, Germany, Hungary, Italy, Norway, Poland, Sweden, USSR, Yugoslavia, United Kingdom, Bulgaria, the Netherlands, Montenegro;  North America: Canada,  Mexico, USA (including Alaska, California); South America: Argentina; Oceania: Australia, New Zealand (Bolay, 2005; Braun, 1987; Farr & Rossman, 2015).

Official Control: No official control has been reported specifically for Erysiphe aquilegiae.  However, the order Erysiphales and Erysiphe spp. are included in the ‘Harmful Organism Lists’ for Australia and Nauru (Erysiphales) and Dominica, Grenada and Saint Lucia (Erysiphe spp., specifically for Mangifera spp. which is not a reported host for E. aquilegiae) (USDA-PCIT, 2015).  In California, the current rating for E. aquilegiae is ‘Z’ (which is given to a previously unrated organism of known economic and or environmental detriment but generally distributed within the state).

California Distribution Southern coastal counties: San Benito, Monterey, San Luis Obispo, Santa Barbara, Ventura, Los Angeles, Orange, and San Diego Counties.

California Interceptions: None reported.

The risk Erysiphe aquilegiae 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): Powdery mildew disease is common in cool or warm humid regions, but can also be common in warm and dry climates since the fungal spores only require high relative humidity and not free-standing water to be released, germinate and cause infections.  Once a plant is infected, mycelium continues to spread on a leaf surface regardless of the level of atmospheric moisture.

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 High (3):  Hosts comprise ornamental and weed plants.  Most hosts of Erysiphe aquilegiae are in Ranunculaceae. Other doubtful hosts have also been reported in at least six other plant families.

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): The powdery mildew pathogen has high reproduction and, under conducive environmental conditions of high relative humidity and wind currents, has high dispersal potential.

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 Medium (2): In particular, nursery grown ornamental plants infected with the powdery mildew pathogen Erysiphe aquilegiae could result in lowered crop production and loss in sales.

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): Powdery mildew infection may significantly impact home/urban and ornamental plantings.  Endangered plant species are not affected.  Although five species of the thistle plant Cirsium spp. (Crystal Springs fountain thistle, San Luis Obispo thistle, Ashland thistle, Suisun thistle, and Surf thistle) and four species of larkspur Delphinium spp. (Baker’s larkspur, Cuyamaca larkspur, Golden larkspur, and San Clemente Island larkspur) are listed as endangered plants in California, these species are not reported hosts of E. aquilegiae.

Consequences of Introduction to California for Erysiphe aquilegiae:

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 = 12 (Medium).

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 (-3): Erysiphe aquilegiae has been detected in several southern coastal counties extending from San Benito to San Diego.  Given its high reproduction capability and suitable environmental conditions for its spread and infection, it is possible that the distribution of this powdery mildew extends further than currently reported.

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



Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Erysiphe aquilegiae is C.


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

Bolay, A.  2005.  Les Oïdiums de Suisse (Erysiphacées).  Cryptogamica Helvetica, 20:38-40.

Braun U.  1987.  A monograph of the Erysiphales (powdery mildews).  Beihefte zur Nova Hedwigia, Heft 89, J. Cramer Berlin-Stuttgart 1987. Pgs 208-209.

Farr, D. F., and A. Y. Rossman.  2015.  Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved October 22, 2015, from

Liberato, J. R., and J. H. Cunnington.  2006.  First record of Erysiphe aquilegiae on a host outside the Ranunculaceae.  Australasian Plant Pathology, 35:291-292.

USDA-PCIT.  2015.  United States Department of Agriculture, Phytosanitary Certificate Issuance & Tracking System (PCIT).

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 Friday, November 13, 2015 and closed on December 28, 2015.

Pest Rating:  C

Posted by ls