Sclerophthora rayssiae var. zeae Payak & Renfro 1967

California Pest Rating for

Sclerophthora rayssiae var. zeae Payak & Renfro 1967

Pest Rating: C

PEST RATING PROFILE

Initiating Event:

Sclerophthora rayssiae var. zeae 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: Sclerophthora rayssiae var. zeae is an oomycete pathogen that causes brown stripe downy mildew disease of maize. The disease was first observed in several maize-growing regions of India in 1962, and the pathogen was described by Payak and Renfro in 1967. Since its initial discovery, the disease has spread through India has also been reported from Mynmar, Nepal, Pakistan, and Thailand (CABI, 2016; Putnam, 2007; EPPO, 2016).

Sclerophthora rayssiae var. zeae has not been reported within the USA (CABI, 2016; USDA, 2013). In 2002, the USDA designated the pathogen 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 S. rayssiae 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: Zea mays (maize), Z. mays var. indurata; Digitaria sanguinalis (large crabgrass), D. bicornis (CABI, 2016; EPPO, 2016; Farr & Rossman, 2016; Putnam, 2007).

Symptoms: Characteristic symptoms of Sclerophthora rayssiae var. zeae infection are expressed only in leaves as vein-delimited narrow chlorotic or reddish to purple stripes, 3 to 7 mm wide, depending on maize genotype, extending parallel with the leaf veins. Other parts of the plant including leaf sheaths, husk leaves, ears or tassels do not exhibit symptoms even though all the leaves may be symptomatic. Early infections appear as vein-delimited chlorotic flecks which enlarge and coalesce to form stripes. At first the stripes are chlorotic or yellowish but later turn yellowish-tan to reddish-brown and necrotic. The disease initially appears on the lowermost leaves which have a high level of striping, and appear pale-brown and burnt. Severely affected leaves may be shed prematurely. In comparison, leaves around the ear shoot show a lesser amount of striping and the leaves above show the least. The veins are not affected, but in severe infections, leaves tear apart near the apices and become tattered. Greyish-white downy growth develops on the upper and lower surfaces of the stripes on leaves. The downy growth disappears as the stripes become necrotic. Sporangia disappear as the lesions become necrotic and oospores are produced only in necrotic tissue. Unlike other downy mildews of maize, brown stripe downy mildew does not result in malformation of floral and vegetative tissues (CABI, 2016; Putnam, 2007).

Disease cycle: The disease cycle involves both sexual and asexual reproduction. Oospores (sexual spores) germinate to produce sporangia (sac-like structures containing spores), which then release zoospores (motile spores) that penetrate leaf tissue. Moisture is critical for infection. A twelve-hour wetting period of a free film of moisture on a leaf surface is essential for infection to occur. Longer wetting periods increase the amount of infection (Singh et al., 1970). Once primary infection occurs, the disease becomes established and lesions are formed in leaves. Sporangia are produced and a cyclic chain of secondary infections occur that eventually result in the spread of the disease throughout an entire crop (CABI, 2016). High levels of moisture and warm temperatures are required for disease development and spread. Asexual reproduction, resulting in the production of sporangia is most abundant at 22 to 25°C. Sporangia production, germination, and infection require a film of water (Putnam, 2007; CABI, 2016). Oospores are produced in necrotic leaf tissue and form the survival stage of the pathogen in soil or in plant debris.

Dispersal and spread: The pathogen survives as oospores in soil or plant debris. Oospores serve as primary inoculum for infections of plants, where the lower leaves show greater disease intensity than the upper leaves, and can survive in soil or plant debris for several years (CABI, 2016; Fry & Grűnwald, 2010). Experimentally, oospores were shown to be viable for up to 3 years when powdered infected leaf debris was placed around each seed at the time of sowing, resulting in heavy infection of the emerging seedlings (Singh et al., 1970). Seed transmission may also occur, although the initiation of new infections is less likely through seed transmission than infected leaf debris (Putnam, 2007). The pathogen may be found on the seed surface and within the embryo (CABI, 2016; Putnam, 2007). The pathogen is dispersed short distances by wind and rain splash or physical contact with infected plants. Long distance transmission by wind is unlikely (Singh & Renfro, 1971).

Damage Potential: Brown stripe downy mildew of maize causes severe infections in areas of high rainfall. In India, 20 to 90% in crop losses are reported. Losses above 70% occur in highly susceptible maize cultivars grown under disease-favorable conditions (CABI, 2016). 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.

Worldwide Distribution: Asia: India, Mynmar, Nepal, Pakistan, Thailand (CABI, 2016).

Official Control: Presently, Sclerophthora rayssiae var. zeae is on the ‘Harmful Organism Lists’ of the following countries: Honduras, Namibia, New Zealand, Peru, and South Africa (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: Sclerophthora rayssiae var. zeae is not present in California.

California Interceptions: There are no reports of the detection of Sclerophthora rayssiae var. zeae in plant and soil shipments imported to California.

The risk Brown stripe downy mildew of maize 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): Sclerophthora rayssiae var. zeae 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): Maize and crabgrass are the only reported hosts of the pathogen.

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, Sclerophthora rayssiae var. zeae 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. 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

Uncertainty:

None.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Sclerophthora rayssiae var. zeae is C.

References:

CABI. 2016. Sclerophthora rayssiae var. zeae (brown strip downy mildew of maize) full datasheet. http://www.cabi.org/cpc/datasheet/49244

EPPO. 2016. Sclerophthora rayssiae var. zeae (). PQR database. Paris, France: European and Mediterranean Plant Protection Organization. http://www.newpqr.eppo.int.

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

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.

Putnam, M. L. 2007. Brown stripe downy mildew (Sclerophthora rayssiae var. zeae) of maize. Plant Management Network International Plant Health Progress, published 8 November 2007. http://www.plantmanagementnetwork.org/pub/php/diagnosticguide/2007/stripe/

Singh, J. P., and B. L. Renfro. 1971. Studies on spore dispersal in Sclerophthora rayssiae var. zeae. Indian Phytopathology, 24:457-461.

Singh, J. P., B. L. Renfro, and M. M. Payak. 1970. Studies on the epidemiology and control of brown stripe downy mildew of maize (Sclerophthora rayssiae var. zeae). Indian Phytopathology, 23:194-208.

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. http://www.ars.usda.gov/SP2UserFiles/Place/00000000/opmp/Corn%20Downy%20Mildews%20Recovery%20Plan%20Revised%202013.pdf

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. https://pcit.aphis.usda.gov/PExD/faces/ReportHarmOrgs.jsp.

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.

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