Peronosclerospora philippinensis (W. Weston) C. G. Shaw, 1978

peronosclerospora philippinensis | photo by Bob Kemerait, Univ of Georgia,
California Pest Rating for
Peronosclerospora philippinensis (W. Weston) C. G. Shaw, 1978
Pest Rating: C

Initiating Event:

Peronosclerospora philippinensis was recently proposed by the USDA to be removed as a select agent from the 2016 updated Select Agents Registration List and Select Agent Regulations.  Currently, the pathogen is not rated in California.  Therefore, the risk of introduction and establishment of this pathogen in California is assessed and a permanent rating is proposed herein.

History & Status:

Background: Peronosclerospora philippinensis is an oomycete pathogen that causes Philippine downy mildew disease.  The disease is destructive mainly to corn in tropical Asia, endemic to the Philippines, and has also been reported from China, India, Indonesia, Nepal, Pakistan, Taiwan, and Congo, Mauritius, and South Africa (CABI, 2016; EPPO, 2016; Farr & Rossman, 2016).  The pathogen has not been reported within the USA (Farr & Rossman, 2016; USDA, 2013).

In 2002, the USDA designated Peronosclerospora philippinensis a ‘select agent’ deemed to be very damaging to susceptible maize and sources of resistance, if any, had not been established for U. S. maize varieties (USDA, 2013).  However, on January 14, 2016, after its fourth biannual review, the USDA proposed to remove P. philippinensis from the updated Select Agents Registration List and Select Agent Regulations.  Removal of select agents, by the USDA, was based on either the absence of viable samples present in the U.S., no climate conducive to growth, or the availability of adequate treatments for the agents (USDA, 2016).

Hosts: Hosts include species within the family Poaceae.  Zea mays (maize) is the main host.  Other hosts include, Andropogon sorghum (Sorghum bicolor, sorghum), Avenae sativa (oats), Euchlaena luxurians, Saccharum officinarum (sugarcane), S. spontaneum (wild sugarcane), Sorghum halepense (Johnson grass), Zea mays subsp. mexicana (teosinte) (CABI, 2016; EPPO, 2016; Farr & Rossman, 2016).

Symptoms: Systemic symptoms are expressed in the first true leaf stage as stripes or overall yellowing of an entire leaf.  Local symptoms are expressed as long chlorotic streaks with downy growth of conidia (spores) and conidiophores.  This downy growth is the site of spore production and may be present on both upper and lower leaf surfaces, but is more common on the lower surface.  Tassels may be malformed and produce less pollen, and ears may be aborted.  Early infected plants are stunted and may die.  Infected stems do not show external symptoms, but may be stunted (CABI, 2016).  The pathogen invades the stem, and becomes established in the shoot apex producing chlorotic areas, which are initially confined to the base of the lower leaf but later increase in size in succeeding leaves.  The youngest leaf emerging from the whorl becomes completely chlorotic.  The pathogen becomes established within seed, as mycelium in the pericarp layer, and also within the embryo and endosperm.  However, no external symptoms on seed are expressed and seed quality is not affected (CABI, 2016).

Disease cycle:  Although the Philippine downy mildew pathogen was reported to produce oospores (overwintering sexual spores) on corn leaf, there have been no subsequent reports.  Even though the Philippine downy mildew pathogen was reported in 1967 to produce overwintering sexual spores (oospores), on corn leaf, there have not been any subsequent reports since then and therefore, the role of oospores has not been established in the life cycle or disease caused by this pathogen (USDA, 2013). Airborne conidia (spores) released from infected crops or weeds form primary source of inoculum for infection. Germinating conidia produce germ tubes which penetrate stomata of leaves.  The optimum temperature for germination and germ tube growth is 18-30°C.  Penetration is followed by invasion of the mesophyll.  Soon the disease becomes established and lesions are formed in leaves.  Conidia are produced under night temperatures ranging from 21 to 26°C and free moisture.  Moisture is critical for infection.  Secondary infections occur that eventually result in the spread of the disease throughout an entire crop.  Seed transmission can occur at low rates from seeds harvested with higher moisture content (CABI, 2016).

Dispersal and spread Peronosclerospora philippinensis is commonly spread by wind and rain.  Production of conidia requires night temperatures ranging from 21 to 26°C and free moisture.  Disease severity is highest in areas that receive 39-78 inches of rain annually and in tropical climates.  The pathogen is dispersed short distances by wind.  Although the pathogen is present within infected seed, it has been demonstrated that once the seed or grain is dried to below 14% it will not produce an infected plant (Adenle & Cardwell, 2000; USDA 2013).

Damage Potential: Before resistant varieties became widely available in the Philippines, annual yield losses of maize were often 40 to 60%.  Yield losses of sweet corn were 100%. Disease severity is highest in areas that receive 39-78 inches of rain annually and in tropical climates (USDA, 2013).  In California, the required warm temperature and long wet periods (heavy rain durations) for disease development and spread are not present.  Therefore, the potential for damage caused by the pathogen to California’s maize production, can be considered to be minimal, if at all.  Furthermore, the pathogen is seed transmissible, but transmission will not occur once the seed has been dried to the moisture content required for storage.  Seed treatments are available to eradicate the pathogen (CABI, 2016).

Worldwide Distribution: Asia: China, India, Indonesia, Japan, Nepal, Pakistan, Philippines, Taiwan, Thailand; Africa: Mauritius, Congo, South Africa (CABI, 2016; Farr & Rossman, 2016).

Official Control: Presently, Peronosclerospora philippinensis is on the ‘Harmful Organism Lists’ of the following countries: Colombia, French Polynesia, Guatemala, Honduras, Indonesia, Japan, Republic of Korea, Morocco, Namibia, New Caledonia, New Zealand, Peru, South Africa, and Timor-Leste  (USDA-PCIT, 2016).  The USDA designated S. rayssiae var. zeae a select agent in 2002, however, on January 14, 2016, the USDA proposed to remove S. rayssiae from the updated Select Agent Registration List and Select Agents Regulations (see ‘Background’).

California Distribution: Peronosclerospora philippinensis is not present in California.

California Interceptions:  There are no reports of the detection of Peronosclerospora philippinensis in plant and soil shipments imported to California.

The risk Philippine downy mildew disease 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 Low (1):   Peronosclerospora philippinensis is not likely to establish in California as the required warm temperature and long wet periods (12 hours or more) for disease development and spread are not present.

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): The host range is limited to include species within the family Poaceae.  Maize is the main host, cultivated sugarcane, oats, sorghum cultivars, and weedy grass species including Euchlaena luxurians, wild sugarcane, Johnson grass, and Zea mays subsp. mexicana (teosinte).

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):  Under favorable wet conditions, Peronosclerospora philippinensis has high reproductive potential.  The pathogen is dispersed primarily through infected soil, plant debris, and maize seeds.  Short distance spread is by wind and rain splash or physical contact with infected plants.  Long distance transmission by wind is reported to be unlikely.

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 Low (1):  In California, the required warm temperature and long wet periods (heavy rain durations) for disease development and spread of the pathogen, are not present, thereby, making it most unlikely for the pathogen to establish and cause infections to the State’s maize cultivation. However, within contained and artificially controlled conditions as in greenhouses, it is possible for pathogen infections to occur. Seed transmission of the pathogen will not occur once the seed has been dried to the moisture content required for storage and the pathogen can be eradicated from seed through seed treatments. The economic impact is therefore, regarded low.

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 Low (1):  No environmental impacts due to the pathogen are expected to occur in California.

Consequences of Introduction to California for Brown stripe downy mildew of maize:

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 = 7

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 ‘Not established’ (0):

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 = 7



Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Peronosclerospora philippinensis is C.


Adenle, V. O., and K. F. Cardwell. 2000.  Seed transmission of maize downy mildew (Peronosclerospora sorghi) in Nigeria. Plant Pathology 49:628-634.

CABI.  2016.  Peronosclerospora philippinensis (Philippine downy mildew) full datasheet.

EPPO.  2016.  Peronosclerospora philippinensis (PRSCPH).  PQR database.  Paris, France: European and Mediterranean Plant Protection Organization.

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

Fry, W. E. and N. J. Grűnwald.  2010.  Introduction to Oomycetes.  The Plant Health Instructor.  DOI:10.1094/PHI-I-2010-1207-01

Payak, M. M., and B. L. Renfro.  1967.  A new downy mildew disease of maize.  Phytopathology, 57:394-397.

USDA.  2013.  Recovery plan for Philippine downy mildew and brown stripe downy mildew of corn caused by Peronosclerospora philippinensis and Sclerophthora rayssiae var. zeae, respectively.

USDA, 2016.  Stakeholder announcement: USDA proposes updates to select agents registration list and select agent regulations.  USDA APHIS. Published January 14, 2016.

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

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: C

Posted by ls