‘Ohi’a rust or Puccinia rust (Puccinia psidii Winter) is a rust apparently native to parts of the American tropics. It was first described on Psidium pomiferum in Brazil in 1884 (Loope and La Rosa 2008). Puccinia psidii is unusual among rusts in having a wide host range, which is believed, so far, to include all the species in the family Myrtaceae. The full host range of P. psidii is unknown (Loope 2009).
Puccinia psidii was discovered in Hawaii during spring 2005, when authorities were alerted to an infected native ‘ohi’a (Metrosideros polymorpha) tree. The rust spread rapidly — by August 2005 it had been found throughout the main Hawaiian Islands (Loope and La Rosa 2008) . The rust has infected three native plant species and at least eight non-native species. Effects have been substantial on the endangered endemic plant Eugenia koolauensis (Loope and LaRosa 2008) and the non-endangered indigenous species Eugenia reinwardtiana (Loope 2009).
As of late 2010, the strain introduced to Hawai`i is causing little damage to ‘ohi’a trees – despite the presence of multitudes of spores released by the rust’s infestation of the invasive non-native shrub rose apple (Syzygium jambos) (Loope and LaRosa 2008). This is fortunate because ‘ohi’a trees overwhelmingly dominate approximately 80% of Hawai`i’s remaining native forest. A persistent, severe infestation of Puccinia rust that destroyed new growth on ‘ohi’a trees would cause crown dieback, low reproduction (few mature flower buds, young fruit), reduced seedling numbers, and, eventually, death of the mature trees. Loss of ‘ohi’a could result in significant changes to the structure, composition, and potentially, the function, of forests on a landscape level. ‘Ohi’a forests are home to the Islands’ one native terrestrial mammal (Hawaiian hoary bat) and 30 species of forest birds – especially the unique honeycreeper endemic subfamily. Eighteen of 19 extant Hawaiian honeycreepers in the main Hawaiian islands, including 12 of 13 species listed as endangered by the U.S. Fish and Wildlife Service, depend on ‘ohi’a for critical habitat (Loope and LaRosa 2008). Increased light reaching the forest floor following canopy dieback would increase the likelihood of invasion by light-loving non-native species, of which Hawai`i has dozens. Loss of ‘ohi’a would thus also damage habitat for one-third to one-half of Hawai`i’s approximately 300 endangered plant species (Loope and LaRosa 2008) through encouraging non-native competitors and changing understory environmental conditions.
‘Ohi’a also has significant cultural values to the Hawaiian people through its connection to the deities Ku, Pele (volcanoes) and Laka (hula) (Loope and LaRosa 2008).
Conservationists are pressing for implementation of a strategy aimed at preventing the introduction of other strains that might be either more virulent or more cold-tolerant and thus able to damage forests at higher elevations.
The most likely pathway by which Puccinia rust was introduced to the Hawaiian Islands was myrtle (Myrtus communis) foliage used in floral arrangements. Maui inspectors have intercepted the rust on myrtle from southern California several times in 2006 and 2007. (Loope 2009). The rust’s presence in California was reported in late 2005 (Mellano 2006, cited by Loope and LaRosa 2008). The identity of the strain or strains in California is unclear, but one of them is known to infect myrtle (Loope and LaRosa 2008).
Florida is known to have multiple strains of Puccinia rust. Florida has eight native species of Myrtaceae that are also native further south in the Neotropics. Only one of those species (Myrcianthes fragrans) has been recorded as a host of the rust, and infection has apparently been minimal. (Loope and LaRosa 2008)
There is strong evidence of host specialization among the various strains of this pathogen, since the strain of the pathogen associated with one host plant species often does not infect other plant species known to be hosts (Loope 2009). For example, the strain (genotype) of the pathogen now in Hawai`i does not utilize many of the species known to be infected by the rust elsewhere, including common guava (a widespread invasive on the Islands) (Loope 2009).
The presence of Puccinia psidii in Florida for at least 30 years (Loope 2009) greatly complicates the regulatory situation, since an organism that is already in the country cannot be treated as a “quarantine pest” unless there is an “official control program” targeting the pest. In August 2007, the Hawaiian Department of Agriculture adopted an emergency rule banning importation of plants in the myrtle family from “infested areas,” specified as South America, Florida, and California. This rule expired in August 2008 and has not been made permanent (Loope 2009). Meanwhile, USDA APHIS official have reiterated that the agency considers P. psidii a non-actionable, non-reportable pest, and therefore it would not be kept out of Hawai`i if intercepted at the border coming from a foreign country. To change that APHIS verdict, HDOA needs to demonstrate that multiple strains or races of P. psidii exist and that there is no practical way of distinguishing new, potentially more virulent strains, from the one already present in Hawai`i (Loope 2009). A USDA Forest Service research project is examining rust strains in Brazil and their impact on ‘ohi’a seedlings.
Currently, Hawai`i relies on visual inspection – a less than ideal approach since both limited inspection capacity and the high likelihood of some infections being latent (asymptomatic) at the time material is shipped constrain the ability to detect the rust. New molecular tests could improve detection efficiency, but cost and the time required to process samples preclude routine use (Loope 2009).
Other possible pathways for movement of Puccinia rust are trade in live plants or wood products with bark. The source material could originate from South America, Central America, or Florida.
Imports of wood packaging, logs, and lumber involving tropical hardwood species (including Eucalyptus) into Hawaii must be debarked or fumigated (Code of Federal Regulations – 7CFR 319.40-5). Imports of most living plants are subject only to inspection (Code of Federal Regulations – 7CFR319.37). The tiny size of the rust spores makes detection during inspection unlikely unless the plant is displaying symptoms of the disease.
Code of Federal Regulations. January 1, 2005 (Title 7, Volume 5). 7CFR319.40-5: Logs, lumber, and other unmanufactured wood articles – importation and entry requirements for specified articles. (available by using search engines/retrieval services at http://www.gpoaccess.gov/fr/index.html).
Code of Federal Regulations. January 1, 2005 (Title 7, Volume 5). 7CFR319.37: Nursery stock, plants, roots, bulbs, seeds, and other plant products – prohibitions and restrictions on importation: disposal of articles refused importation. (available by using search engines/retrieval services at http://www.gpoaccess.gov/fr/index.html).
Loope, L. and A.M. LaRosa. 2008 ‘Ohi’a Rust (Eucalyptus Rust) (Puccinia psidii Winter) Risk Assessment for Hawai`i
Loope, L. 2009. A summary of Information Related to Regulatory Options for Prevneting Introduction of Additional Strains of the Rust Puccinia psidii Winter (Guava Rust) To Hawaii