Pseudocercospora purpurea | Pest Rating Proposals and Final Ratings

California Pest Rating for

Pseudocercospora purpurea (Cooke) Deighton 1976

Pest Rating: B

PEST RATING PROFILE

Initiating Event:

On November 17, 2016, USDA APHIS PPQ inquired if CDFA had conducted a pest risk assessment of the fungal pathogen, Pseudocercospora purpurea on avocados in California. Subsequently, the risk of infestation of P. purpurea in California is evaluated and a permanent rating is herein proposed.

History & Status:

Background: Pseudocercospora purpurea is a fungal plant pathogen that causes Pseudocercospora (Cercospora) spot (blotch) disease exhibiting leaf and fruit spot symptoms in Persea spp., including avocado (P. americana) plants. The pathogen was originally known as Cercospora purpurea. In South Africa, the disease is known as black spot or Cercospora spot and is the most serious pre-harvest disease affecting all cultivars of avocado, particularly, cv. Fuerte (Crous et al., 2000; Pohronezny et al., 1994). The disease occurs in warm, humid and rainy climates and is found in southeastern USA, South America, northern Australia, Asia, Africa, and the Caribbean (CABI, 2016; Menge & Ploetz, 2003). The pathogen has not been reported from California.

Disease cycle: Initial inoculum of conidia (asexual spores) mostly comes from infected leaves. New shoot tissues are infected wherever this disease occurs. The pathogen penetrates host tissue either directly or through wounds. Conidia are easily detached and blown by wind often over long distances. On landing on surfaces of a plant host, conidia require water or heavy dew to germinate and penetrate the host. In South Africa, the pathogen remains latent for about 3 months after penetration. Infected plants produce conidiophores (specialized hypha) that arise from the plant surface in clusters through stomata and form conidia successively. Substomatal stroma (compact mycelial structure) may form from which conidiophores develop. Fruit are susceptible when developed to a quarter to three-quarter of their full size. Very small fruit (< 4 cm diameter) and those at or near maturity are almost immune. Disease development is severe during warm, rainy weather when fruit are about a quarter size (Agrios, 2005; Menge & Ploetz, 2003; Pohronezny et al., 1994). High relative humidity is necessary for conidial germination and plant infection. The pathogen can overwinter as mycelium (stromata) in old infected leaves (Agrios, 2005).

Dispersal and spread: Wind, rain, irrigation water, infected nursery plants, infected leaves, insects (Menge & Ploetz, 2003).

Hosts: Avocado is the main host; Persea spp. in the family Lauraceae, namely, P. americana (syn. P. gratissima, avocado), P. borbonia (redbay), P. drymifolia (Mexican avocado), P. palustris (swamp bay), and Persea sp. (Farr & Rossman, 2016).

Symptoms: Symptoms occur on leaves, stems, and fruit (Pohronezny et al., 1994). On leaves, lesions initially appear as small (1-5 mm) angular, purple to purplish brown flecks or spots near leaf margins. Over time, chlorotic halos surround older spots and are visible on both leaf surfaces. The fungus sporulates under high humid conditions, appearing as gray, felty mycelial growths in the center of lesions. Individual lesions may coalesce forming larger regions of necrotic tissue. Leaves become curled, deformed and may fall.

On fruit, lesions initiate as small flecks which later become slightly sunken, expand or coalesce becoming somewhat circular, and turn brown to brownish black in color. Fissures or cracks usually develop in fruit lesions and serve as avenues for infection by other pathogens. In certain cases, if the disease is temporarily arrested, the lesions appear as minute, raised, shiny, black specks associated with the corking of lenticels. While blotch is usually confined to the rind of fruit, in advanced cases, the flesh may be invaded. Once defoliation occurs, fruit may turn chlorotic, shrivel and drop. Dark brown to black, 2-10 mm lesions may also form on green twigs and fruit pedicels (Pohronezny et al., 1994; Menge & Ploetz, 2003).

Damage Potential: Pseudocercospora spot (blotch) is one of the most common diseases of avocado in Florida (Pohronezny et al., 1994). Losses in avocado production may be severe and have been reported to be up to 69% in non-sprayed orchards in South Africa (Pohronezny et al., 1994; Menge & Ploetz, 2003). Photosynthetic area can be reduced due to leaf spotting. In severe infections, leaf wilt and drop may be expected. In California, avocado production is a major industry producing 75% and 92% of the nation’s avocado fruit supplies (Lazicki et al., 2016). Therefore, losses due to this pathogen is of particular concern.

Worldwide Distribution: Pseudocercospora purpurea is widespread in subtropical and tropical regions. Asia: India, Japan, Philippines; Africa: Cameroon, Democratic Republic of Congo, Côte d’Ivoire, Guinea, Kenya, South Africa; North America: Bermuda, Mexico, USA; Central America and Caribbean: Dominica, El Salvador, Honduras, Jamaica, Nicaragua, Panama, Puerto Rico, Trinidad and Tobago, United States Virgin Islands; South America: Argentina, Bolivia, Brazil, Chile, Guyana, Peru, Venezuela; Oceania: Australia, Palau (CABI, 2016; Farr & Rossman, 2016).

In the USA, the pathogen has been found in the states of Florida, Georgia, and Mississippi (CABI, 2016).

Official Control: Presently, Cercospora purpurea (syn. Pseudocercospora purpurea) is on the ‘Harmful Organism Lists’ for Namibia and South Africa and P. purpurea is on the ‘Harmful Organism Lists’ for French Polynesia and New Caledonia (USDA PCIT, 2016).

California Distribution: Pseudocercospora purpurea has not been reported from California. The pathogen is not known to be established in California.

California Interceptions: None reported.

The risk Pseudocercospora purpurea 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: 2

– 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): In California, Pseudocercospora purpurea may be able to establish on avocado, under high moisture and warm climate conditions. In the State, avocados are grown mostly along the southern coast (Lazicki et al., 2016).

2) Known Pest Host Range: Evaluate the host range of the pest.

Score: 1

– 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 for Pseudocercospora purpurea is limited to Persea spp. with avocado being the main host.

3) Pest Dispersal Potential: Evaluate the natural and artificial dispersal potential of the pest.

Score: 3

– 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): Pseudocercospora purpurea has high reproductive potential resulting in the successive production of conidia which are mainly dependent on wind, rain, and infected plants for dispersal and spread.

4) Economic Impact: Evaluate the economic impact of the pest to California using the criteria below:

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.

Score: 3

– 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): Infected host plants with leaf and fruit spot symptoms caused by Pseudocercospora spot (blotch) disease could lower value and yield of commercially produced avocado plants as well as affect nursery productions resulting in loss of markets.

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: 2

– 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): The pathogen could significantly impact avocado plants grown for fruit and aesthetic value in private residential and public environments.

Consequences of Introduction to California for Pseudocercospora purpurea:

Add up the total score and include it here:

-Low = 5-8 point

–Medium = 9-12 point

-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: 0

–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): Pseudocercospora purpurea is not established in California.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score:

Final Score: Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 11

Uncertainty:

None.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Pseudocercospora purpurea is B.

References:

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

CABI. 2016. Pseudocercospora purpurea (spot blotch) (basic) datasheet. Crop Protection Compendium. http://www.cabi.org/cpc/datasheet/12266 .

Crous, P.W., A. J. L. Phillips, A. P. and Baxter. 2000. Phytopathogenic fungi from South Africa. University of Stellenbosch, Department of Plant Pathology Press, 358 pages (referenced by Farr & Rossman, 2016).

Farr, D. F., & A. Y. Rossman. 2016. Fungal Databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved August 18, 2016, from http://nt.ars-grin.gov/fungaldatabases/.

Lazicki, P., D. Geisseler, and W. R. Horwath. 2016. Avocado production in California. https://apps1.cdfa.ca.gov/FertilizerResearch/docs/Avocado_Production_CA.pdf. (Last updated April, 2016.)

Menge, J. A., and R. C. Ploetz. 2003. Disease of Avocado. In Diseases of Tropical Fruit Crops, Edited by R. C. Ploetz, CABI Publishing, CAB International, UK, USA, 527 p.

Pohronezny, K. L., G. W. Simone, and J. Kotzé. 1994. Pseudocercospora spot (blotch). In Compendium of Tropical Fruit Diseases, Edited by R. C. Ploetz, G. A. Zentmeyer, W. T. Nishijima, K. G. Rohrbach, and H. D. Ohr, APS Press, The American Phytopathological Society, 79-80 p.

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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