Myrtle Rust: Puccinia psidii G. Winter

California Pest Rating for
Puccinia psidii G. Winter
(Myrtle Rust)
Pest Rating: C


Initiating Event:

A review of the current pest rating of Myrtle Rust has been requested by certain counties due to the increased detection of the pathogen within California.  Currently, Puccinia psidii has a B rating.

History & Status:

Background:  In 1884, Winter first described the fungal rust pathogen, Puccinia psidii from infected guava (Psidium guajava), of the plant family Myrtaceae, in Brazil.  Since then, several rust species described from infected members of Myrtaceae and named after the host they were detected on, are now considered synonyms of P. psidii. This pathogen is also commonly known as guava rust, eucalyptus rust (in Florida, the Caribbean, and Central and South America), and ohi’a rust (in Hawaii) after its respective host.  In 2010, P. psidii was detected in Australia (as Uredo rangelii) on common myrtle and subsequently known as myrtle rust – a name which best fits P. psidii as it “captures the occurrence of the pathogen on a very wide host range including numerous genera and species of Myrtales” (Roux et al., 2013).

There are several biological biotypes or races of Puccinia psidii known to exhibit varying virulence on different hosts, environmental tolerances, sporulation characteristics, and spore survival (Glen et al., 2007).

Puccinia psidii is native to South and Central America, but since its original report, it has spread to several countries worldwide, including California, Florida and Hawaii in the USA.  As a result of its spread, the pathogen is considered an important quarantine threat to many countries (Glen et al., 2007). Puccinia psidii was first reported in the USA in Florida in 1977, then in Hawaii in 2005 and in California in 2006.  It is likely that the pathogen was present in California prior to 2006 and introduced from Florida through imported, diseased plants (Killgore & Heu, 2007; Marlatt & Kimbrough, 1979; Mellano, 2006).  In 2011, the rust pathogen was first reported on Melaleuca quinquenervia (paperbark) in a nursery in San Diego, California (Zambino & Nolan, 2011) but has now spread to outside environments and landscape (personal communications: Pat Nolan, plant pathologist, Department of Agriculture, Weights and Measures, County of San Diego, CA.) Although the pathogen currently has a B rating that allows implementation of eradicative action in nurseries, this may not have always been implemented, thereby, providing a pathway of introduction and spread to outside environments.  Furthermore, this rust – as well as many other rust pathogens – is very difficult to eradicate due to its ease of long distance dispersal and wide host range.  Once established in a region it can and has spread rapidly to amenity plantings, commercial and native environments (CABI, 2015).

Disease cycle:  Puccinia psidii is considered to complete its life cycle on the same host (autoecious), having an incomplete lifecycle. The suggestion that the pathogen is heteroecious with an unknown aecial host is considered doubtful (Glen et al., 2007).     With the exception of spermagonia, all other spore states are produced on the same host.  Urediniospores are produced abundantly under natural conditions, but the production of teliospores and basidiospores are comparatively rare and spermagonia and aeciospores are unknown.    Under favorable conditions of humidity and temperature, urediniospores present on host germinate to penetrate the tissue through stomata.  As the fungus grows, uredinia (fruiting structures) are formed and urediniospores are produced in abundance.   Disease development is favored by low temperatures of about 20°C, high relative humidity (80%) at night and high levels of airborne urediniospores.  On rose apple (Syzygium jambos) in Brazil, rust epidemics incidence and severity were dependent on the duration of leaf wetness over 90% or leaf wetness periods greater than 6 hours and nocturnal temperature between 18 and 22°C (Blum and Dianese, 2001; CABI, 2015; Tessman et al., 2001).

Dispersal and spread: Rust spores are wind-dispersed over long distances.  Long and short distance spread is through rust-contaminated planting material, nursery stock, cuttings, flowers, timber, wood packaging, equipment, tools, and human clothing/contact.  Foraging honey bees, bats, and birds in contact with rust spores have vectored the pathogen to uninfected host plants (Carnegie et al., 2010).  Under sub-optimal conditions, the pathogen can remain in non-symptomatic plants for 4-6 weeks before symptoms are visible, thereby enabling undetected spread to non-infected regions (Carnegie & Lidbetter, 2012)

Hosts:  Several genera and species belonging to the order Myrtales, in the family Myrtaceae. The natural and experimental global host list for Puccinia psidii includes 445 species in 73 genera and 16 tribes of the family Myrtaceae.  However, a large proportion of these hosts are known only as experimental hosts (CABI, 2015).  Primary hosts include genera and species of: Agonis flexuosa (willow myrtle), Eucalyptus spp., E. dunnii, E. globulus, E. gracilis, Eugenia spp., Eugenia uniflora (Surinam cherry), Melaleuca spp., Melaleuca quinquenervia (paperbark), Metrosideros polymorpha (ohi’a), Myrtus communis (myrtle), Pimenta spp., Pimenta dioica (allspice), Psidium spp., Psidium guajava (guava), Syzygium jambos (Malabar plum), S. samarangense (wax apple), Rhodonyrtus tomentosa (rose myrtle) (CABI, 2015; EPPO, 2015).  Other hosts are also included in the genera of Myrtaceae: Actinostemon, Asclepias, Callistemon, Calycorectes, Campomanesia, Jambosa, Marlierea, Metrosideros, Myrcia, Myrciaria, Myrceugenia, Pseudomyrcianthes, Psidiopsis, and Syzygium (Farr & Rossman, 2015).

Symptoms and damage potential:  Symptoms vary depending on the host species, level of susceptibility within a host species and age of host leaf.  Actively growing, young leaves, stems, and shoot tips, also fruit, sepals, and flowers are susceptible to P. psidii infection.  Initial symptoms of rust infections appear two to four days after infection as tiny chlorotic specks that develop into 0.1-0.5 mm-diameter spots or pustules (uredinia).  Uredinia occur in groups on leaves (commonly on the underside, but not always), on stems, flowers and fruit.  After a few days, these pustules erupt due to the production of bright yellow to orange-yellow urediniospores.  The infected area or spots expand and multiple pustules merge over time.  Older lesions have purple margins on leaves and shoots of many Eucalyptus, Melaleuca and Callistemon hosts.  Infected plant tissue becomes necrotic. If left untreated, disease-affected plants result in deformed leaves, heavy defoliation of branches, stunted growth, dieback, and eventually, death (CABI, 2015; Glen et al., 2007).

Puccinia psidii can cause significant losses to economically important young tree crops, such as, Eucalyptus spp., and other environmental trees in Myrtaceae – including myrtle and paperbark.  Severe disease infections on foliage, inflorescences, and young succulent fruits can seriously impact guava production causing defoliation, death of shoot tips, and mummified fruit (Glen, et al., 2007; Hernandez, 2006).  80-100% loss of guava production was been reported in Brazil (CABI, 2015).

Worldwide Distribution: Asia: China, Japan; Africa: South Africa; North America: Mexico, USA; Central America and Caribbean: Costa Rica, Cuba, Dominica, Dominican Republic, Guatemala, Jamaica, Panama, Puerto Rico, Trinidad and Tobago, United States Virgin Islands and British Virgin Islands; South America: Argentina, Brazil, Colombia, Ecuador, Paraguay, Uruguay, Venezuela; Oceania: Australia, New Caledonia.   Records of the absence of Puccinia psidii from India and Taiwan are considered unreliable (CABI, 2015; EPPO, 2015).

In the USA, Puccinia psidii is considered an invasive species that has been reported from Florida, Hawaii, and California (CABI, 2015).

Official Control: Puccinia psidii has recently been made federally actionable for imports destined for Hawaii, however, domestic shipments are not affected (CABI, 2015).  Puccinia psidii is on the Harmful Organism List for the following countries: Mexico, Morocco, Nambia, New Zealand, South Africa, and Vietnam (USDA-PCIT, 2015).  In 2007, it was declared a Quarantine pest in Jordan (EPPO, 2015).  Currently, P. psidii is a B-rated pathogen in California.

California DistributionPuccinia psidii has been detected in nursery stock in Contra Costa, Orange, San Diego, San Francisco, Santa Barbara, Santa Clara, and Santa Cruz Counties.

California Interceptions: Puccinia psidii has been detected in several interceptions of infected nursery stock shipped from Hawaii and Florida.

The risk Myrtle Rust 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)The Myrtle rust pathogen is already established in California and is suspected to be present wherever Myrtaceae host plants are grown in nurseries and natural environments.  These environments include mainly coastal counties.

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)Puccinia psidii has a wide host range of over 400 species within the plant family Myrtaceae.  However, a large proportion of these hosts are experimental ones.  Main hosts included in several genera listed under “hosts” in the above text.  While several strains of the pathogen are known to exist, exhibiting different virulence and host specificities, at least one strain attacking Melaleuca quinquenervia (paperbark) is known to occur in outdoor environments in California.  Planted Melaleuca is common in coastal southern California.  Other Myrtaceae hosts present in the State, such as Eucalyptus, may also be at risk although the rust pathogen has not been reported on Eucalyptus in California.

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): Puccinia psidii has high reproduction and dispersal potential via its windblown spores that are capable of being transmitted by strong winds over distances of several hundred kilometers.  Also they may be spread over long distances via infected nursery stock, cuttings, flowers, timber, wood packaging, equipment, tools, and human clothing/contact.  Other agents of spread include foraging honey bees, bats, and birds in contact with rust spores.  Under sub-optimal conditions, the pathogen can remain in non-symptomatic plants for 4-6 weeks before symptoms are visible, thereby enabling undetected spread to non-infected regions.  The pathogen has already spread from nursery sites to outdoor natural environments within California.

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): The presence of Puccinia psidii in California is already a quarantine issue for exports of Myrtaceae hosts destined for Hawaii, as well as an actionable concern to some other non Myrtle rust-infected countries. Furthermore, rust infections could result in lowered crop value, yield and altered cultural practices in nursery grown plants.  The fungus can be vectored to non-infected hosts by insects and larger animals contaminated with spores.

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:

– 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):  Puccinia psidii infections could affect growth of hosts within Myrtaceae. There are several species within Myrtaceae which are listed as endangered and may potentially by threatened by the fungus.  However, reports of Myrtle rust affecting these other host plants established in outdoor environments in California is currently not known. 

Consequences of Introduction to California for Myrtle Rust:

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 = 13 (High).

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 High (-3)Puccinia psidii has been detected in nursery stock in Contra Costa, Orange, San Diego, San Francisco, Santa Barbara, Santa Clara, and Santa Cruz Counties.  It has also been detected in outdoor environments in San Diego County as well as suspected to have spread to outdoor environments of the above listed 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 = 10 (Medium).


Puccinia psidii is a quickly spreading rust pathogen that is very difficult to eradicate manly due to its ease of long distance dispersal and broad host range.  It is already established in outdoor environments within the State.  More information on its parasitism of different plant species in California will increase current knowledge of host range and further strengthen its proposed rating.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Puccinia psidii is C.


Blum L. E. B., and J. C. Dianese .  2001.  Patterns of urediniospores release and development of rose apple rust. Pesquisa Agropecuaria Brasileira 36: 845–850.

Carnegie, A. J. and J. R. Lidbetter.  2012.  Rapidly expanding host range for Puccinia psidii sensu lato in Australia. Australasian Plant Pathology, 41(1):13-29. mu25rh72870/fulltext.html.

Carnegie, A. J., J. R. Lidbetter, J. Walker, M. A. Horwood, L. Tesoriero, M. Glen, and M. J. Priest.  2010.  Uredo rangelii, a taxon in the guava rust complex, newly recorded on Myrtaceae in Australia. Australasian Plant Pathology, 39(5):463-466.

EPPO.   2015.  Puccinia psidii (PUCCPS).  PQR database.  Paris, France: European and Mediterranean Plant Protection Organization.

Farr, D. F. and A. Y. Rossman, Fungal databases, Systematic Mycology and Microbiology Laboratory, ARS, USDA.  Retrieved August 24, 2015 from

Glen, M., A. C. Alfenas, E. A. V. Zauza, M. J. Wingfield and C. Mohammed.  2007.  Puccinia psidii: a threat to the Australian environment and economy – a review.  Australasian Plant Pathology, 36: 1-16.

Hernandez, J. R.   2015.  Invasive fungi, Puccinia psidii.  Systematic Mycology and Microbiology Laboratory, ARS. USDA.  27 February 2006. Retrieved July 30, 2015, from /sbmlweb/fungi/index.cfm .

Killgore, E. M. and R. A. Heu.  2007.  A rust disease on ‘Ohi’a, Puccinia psidii Winter. New Pest Advisory 05-04 (Updated December 2007). Honolulu, Hawaii, USA: Hawaii Department of Agriculture.

Marlatt, R. B. and J. W. Kimbrough.  1979.  Puccinia psidii on Pimenta dioica in south Florida. Plant Disease Reporter, 63(6):510-512.

Mellano, V.  2006.  Rust on myrtle found in San Diego County. Healthy Garden-Healthy Home. University of California Cooperative Extension Retail Nursery Newsletter, 1:3

Roux,  J., I. Greyling, T. A. Coutinho, M. Verleur, M. J. Wingfield.  2013. The Myrtle rust pathogen, Puccinia psidii, discovered in Africa. IMA Fungus, 4(1):155-159.

Tessmann, D. J., J. C. Dianese, A. C. Miranda, and L. H. R. Castro.  2001.  Epidemiology of a Neotropical rust (Puccinia psidii): periodical analysis of the temporal progress in a perennial host (Syzygium jambos). Plant Pathology 50, 725–731. doi: 10.1046/j.1365-3059.2001.00646.x

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

Zambino, P. J., and P. A. Nolan.  2011.  First report of rust caused by Puccinia psidii on paperbark, Melaleuca quinquenervia, in California.  Plant Disease, 95:1314.

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 Monday, September 28, 2015 and closed on November 12, 2015.

Final Pest Rating:  C

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