Species of tree-killing pathogens can have several “strains” that may vary in virulence or hosts affected.
`ohi`a tree on Hawai`i
This is a phenomenon well known to pathologists, but regulators have not adapted their programs to address it. Once a pathogenic species is determined to be established in the country, APHIS considers the entire species to be “non- actionable” and will not attempt to prevent introduction of any new strains. As the examples below illustrate, allowing introduction and spread of new strains poses risks to North America’s trees.
World-renowned British forest pathologist Clive Brasier has spoken out often on the risk posed by various strains of a pathogen. He has also written about the potential for pathogen species to hybridize and for that hybrid to threaten new hosts.
How widespread a problem is this? Some of the pathogens causing the greatest damage have several strains that vary in their virulence and host range.
- The sudden oak death pathogen, Phytophthora ramorum is known to have four strains: NA1, NA2, EU1 and EU2. The EU1 lineage has primarily been found in European nurseries and forests. It has also been recovered from several nurseries and waterways on the U.S. west coast. Last year, the EU1 lineage was detected in a forest in Oregon (see my blog posted 15August 2015). This is troubling for two reasons:
* the EU1 lineage is more aggressive than the NA1 lineage already present in the forests of California and Oregon. Some of the individual tree which now appear to be resistant to the NA1 lineage might succumb to the EU1 lineage.
* The EU1 and NA1 lineages belong to opposite mating types, so they can potentially reproduce, thereby increasing the genetic variability of the pathogen. (Sexual reproduction in P. ramorum can only occur when opposite mating types meet; in the absence of opposite mating types, all reproduction is clonal.)
• The guava rust or myrtle rust pathogen, Puccinia psidii, also has several strains which vary in their virulence. Already, a new strain introduced to Jamaica in the 1930s caused extensive damage to the allspice industry – although a different strain had been on the island for decades (Carnegie 2016).
Hawaiian conservationists worry that a more virulent strain of P. psidii might be introduced and threaten additional species of Myrtaceae on the Islands – especially the `ohi`a tree which is the major canopy tree in 80% of the Islands’ remaining native forest. These forests are key to maintaining the Islands’ watersheds and biodiversity, especially because `ohi`a nectar is the principal food source for many of the remaining native and rare bird species. (See writeup here)
Multiple strains of `ohi`a rust have been identified in the pathogen’s native range of Brazil. Using funds from the USDA Forest Service, scientists in Brazil (Costa da Silva et al. 2014) tested five of the strains; three proved to be highly virulent on most `ohi`a seedlings tested. `Ohi`a from several locations were tested; none showed significant resistance to these three strains of the P. psidii pathogen.
The tests were carried out under conditions highly conducive to infection, so the results cannot be used to predict epidemiological behavior and ecological ramifications in natural conditions. Nevertheless, the results do support the need for greater efforts to prevent introduction of new strains to the Islands.
Additional tests are under way to determine whether the Brazilian strains are more virulent than that strain currently found in Hawai`i and to learn more about possible variation in vulnerability among `ohi`a trees from a greater variety of sites.
• The pathogen that causes Port-Orford cedar root disease (Phytophthora lateralis) has now been found to have four lineages. Scientists compared isolates from the pathogen’s putative native range on Taiwan to isolates from the North American west coast (where it has been established since early in the 20th Century) and Europe (where it began killing trees in the 1990s). They found one slow- growing strain from Taiwan, one fast-growing strain from North America and Europe, and one of intermediate growth from a small area of the United Kingdom (Brasier et al. 2012).
Sometimes, pathogens behaving in unexpected ways are initially thought to be a strain or lineage, but are later classified as a novel species. Thus the Ceratocystis causing `ohi`a wilt was initially thought to be a strain of C. fimbricata, a widespread fungus that has been on the Hawaiian Islands for decades. Scientists now think it is a new species (Keith 2016).
Pathogens are difficult to manage. The vast majority of species remain undescribed. They are difficult to detect until they cause noticeable damage. For a longer discussion of the challenges posed by pathogens and other unknown organisms, read Chapter 3 of Fading Forests II, available here.
However, the great threat to our forests necessitates that APHIS and other phytosanitary agencies (in states and around the world) develop improved methods for addressing the challenge that pathogens pose. Our forests simply cannot afford introductions of more tree-killing fungi, oomycetes, and other pathogens.
At a minimum, APHIS should respond to evidence that a particular pathogen is composed of multiple strains with varying virulence by agreeing to designate such novel strains as “actionable” and applying all its authorities and powers to prevent introduction and spread of the novel strains.
As I noted in my blog of earlier this month, APHIS also needs to develop more effective strategies for addressing introduction and spread of pathogens generally. USDA should assist such efforts to improve controls over pathogens by bringing about prompt finalization of two APHIS initiatives:
1) Prohibiting temporarily plants suspected of transporting known damaging pathogens. This action is allowed under the NAPPRA (not authorized for importation pending pest risk assessment) program.
2) Requiring foreign suppliers of living plant imports to implement “hazard analysis and critical control point” programs to ensure that the plants are pest-free during production and transport. This approach is allowed under ISPM#36 and would be authorized under pending changes to APHIS’ “Q-37” regulation. (See Federal Register Vol. 78, No. 80 April 25, 2013.)
(See longer discussions of these programs in Fading Forests III, available here.)
Sources
Clive M. Brasier, C.M, S. Franceschini, A.M. Vettraino, E.M. Hansen, S. Green, C. Robin, J.F. Webber, and A.Vannini. 2012. Four phenotypically and phylogenetically distinct lineages in Phytophthora lateralisFungal Biology. Volume 116, Issue 12, December 2012, Pages 1232–1249
Carnegie, A.J., A. Kathuria, G.S. Pegg, P. Entwistle, M. Nagel, F.R. Giblin. 2016. Impact of the invasive rust Puccinia psidii (myrtle rust) on native Myrtaceae in natural ecosystems in Australia. Biol Invasions (2016) 18:127–144 DOI 10.1007/s10530-015-0996-y
Costa da Silva, A., Magno Teixeira de Andrade, P. Couto Alfenas, A., Neves Graca, R., Cannon, P., Hauff, R., Cristiano Ferreira, D., and Mori, S. 2014. Virulence and Impact of Brazilian Strains of Puccinia psidii on Hawaiian `Ohi`a (Metrosideros polymorpha). Pacific Science (2014), vol. 68, no. 1:7-56
Keith, L. 2016. Ceratocystis fimbriata, Rapid O’hi’a Death: Unraveling the mystery. 27th USDA Interagency Research Forum on Invasive Species January 12-15, 2016 Annapolis, Maryland
Posted by Faith Campbell