One thought on “Phytophthora kernoviae Brasier, Beales & S.A. Kirk 2005”

1. Comment on Pest Rating Proposal for Phytophthora kernoviae Brasier, Beales, & S.A. Kirk 2005
   Submitted by Faith T. Campbell, President, Center for Invasive Species Prevention (CISP)

   The Center for Invasive Species Prevention wishes to provide additional information that supports the proposed ranking of “A”.

   Hosts

   Comment from F. Campbell: The CABI reports that hosts have been confirmed in 21 genera (12 families).

   Response from H. Martin: CABI says “at least” 21 and from their list I counted 25 genera and included them in the pest rating – hosts are sourced from CABI https://www.cabidigitallibrary.org/doi/10.1079/cabicompendium.40972 and from USDA’s Fungal Database https://fungi.ars.usda.gov/

   Comment from F. Campbell:
   Transmission:

   The description in the proposal leaves out key details. The proposal cites Fichtner et al. (2012) that experience in the United Kingdom indicates bole hosts (trees) need to be within five meters of a sporulating host to become infected. The proposal also notes that P. kernoviae sporulates on three plant species native to West coast forests — Rhododendron macrophyllum, R. occidentale &, Umbellularia californica. However, the proposal does not explore the rest of their findings. Fichtner et al. report that the level of inoculum on individual plants belonging to two of these species – U. californica & R. occidentale – is equal to or greater than that on individual plants belonging to R. ponticum. Fichtner et al. suggest but do not address explicitly the implications that since the North American shrubs don’t reach the density of reached by R. ponticum in forests of southwest England, the level of inoculum the on landscape scale in the United States will probably be lower.

   Response from H. Martin: This pest rating is a summary of many researchers’ work to justify a numerical score. Unfortunately, many of the details in the literature cannot be included in the interest of succinctness. The excellent work of Fichtner et al., 2012, cited here is available online without a subscription. https://bsppjournals.onlinelibrary.wiley.com/doi/pdfdirect/10.1111/j.1365-3059.2011.02506.x

   Official Control:

   Comment from F. Campbell: The Proposal makes no mention of official efforts by the United States Department of Agriculture (USDA) to prevent introduction of P. kernoviae or to detect it should introduction occur.
   • USDA Animal and Plant Health Inspection Service (APHIS) requested adoption of a “response plan” targetting P. kernoviae under the National Plant Disease Recovery System (NPDRS). This plan was adopted in 2008 https://www.ars.usda.gov/ARSUserFiles/00000000/opmp/P.%20kernoviae%2081100.pdf and updated in 2015 https://www.ars.usda.gov/ARSUserFiles/np303/NPDRS/Documents/NPDRS%20Disease%20Recovery%20Plans_March%202022.pdf
   • APHIS has prohibited importation of plants (but not seeds) of several genera that are hosts of P. kernoviae from any country except Canada, specifically to prevent introduction of P. kernoviae. https://www.aphis.usda.gov/plant-imports/nappra There are exceptions: Fagus may also be imported from the Netherlands; Magnolia & Michelia may also be imported from China and South Africa; Rhododendron may also be imported from Japan.
   • Phytophthora kernoviae has been designated a National Priority Pest for survey in 2024 CAPS program.
   • Purdue University’s “pest tracker” website Survey Status of Phytophthora leaf blight – Phytophthora kernoviae (2023) (pesttracker.org) indicates that a few states have reported carrying out surveys for P. kernoviae in one or more years since 2016. States reporting surveys included Oregon, Tennessee, and to lesser extents Pennsylvania and Virginia. None detected P. kernoviae. Surveys in Oregon were carried out in 2018 – 2020; in 2020 the counties surveyed included Curry County, which borders California and where three strains of P. ramorum have become established. These data are not certified as accurate or complete.
   Lindsey Thiessen (USDA APHIS, pers. comm.) reports that National Plant Diagnostic Network laboratory staff are trained to use a variety of diagnostic tools (reportedly including tools effective for P. kernoviae), and many have the ability to use sequencing tools for identification. However, carrying out genetic sequencing might be limited if the entity submitting the sample is unwilling to pay for those diagnostics. Diagnosticians understand the potential for novel pathogens in trade sensitive materials and the PPCDL provides support for NPDN labs if they have concerns with a specific sample.
   On the other hand, USDA Forest Service does not try to detect Phytophthora kernoviae in its P. ramorum stream monitoring program (R. Cooksey, USDA Forest Service, pers. comm.).

   Response from H. Martin: Official control has a specific meaning in this situation – P. kernoviae is not on the list of U.S.-regulated pests https://www.aphis.usda.gov/plant-imports/regulated-pest-list. Surveys, potential response plans, risk assessments, and pest tracking do not constitute official control in this context. For this section, I cite the EPPO and USDA PCIT.

   Comment from F. Campbell: CISP notes that many aspects of reproduction remain uncertain: the role of oospores; potential dispersal of propagules from infected roots; and the role of water in propagule dispersal (Lindsey Thiessen, USDA APHIS, pers. comm.)

   Response from H. Martin: Agreed, it is difficult for Phytophthora researchers to work with a species that does not occur in the U.S.

   Damage Potential: California Distribution: none. California Interceptions: none.

   Comment from F. Campbell: CISP contends that the accurate answer is “unknown”. While it is true that no survey has detected P. kernoviae in the United States, much less California, the finding is uncertain given the extremely low level of effort and extreme difficulty of detecting this species. (These difficulties are discussed in detail by Fichtner et al. 2012.) CISP is particularly concerned because P. kernoviae is most likely to become established in the same ecosystems in which P. ramorum is already established. Distinguishing these two species requires sequencing samples taken from plants, soil, and baited traps deployed in water courses. CISP has found no evidence that agencies are conducting this expensive and challenging task on a sufficiently large proportion of samples.

   Detection in recent years of two additional strains of P. ramorum – EU1 and NA2 – demonstrates that introductions of Phytophthoras continue despite phytosanitary measures.

   Response from H. Martin: Only official reports can be cited in this section. There have been no official detections made by CDFA, USDA, any County Agricultural Commissioner, or an agency such as CalFire, or reported by anyone affiliated with a University from inside California or associated with incoming shipments. If any individual or organization were to report a suspect, we would follow up and try to collect an official sample. Our CDFA Plant Pathology diagnosticians are experts in identifying Phytophthora spp. and are always alert for new threats.

   Comment from F. Campbell: Consequences of Introduction:
   1) Climate/Host Interaction:

   CISP agrees that Phytophthora kernoviae is likely to encounter known hosts among California native species in the forest, including Pinus radiata, Sequoiadendron giganteum, Rhododendron macrophyllum, R. occidentale and Umbellularia californica. Other known hosts are likely to be encountered in cultivation. CISP is most concerned about Liriodendron tulipifera, which has a wide native range in eastern North America. While possible infection of this species might not be a major concern in California, CISP suggests that the Golden State should be a “good neighbor” and consider actions it could take to protect other states.

   Response from H. Martin: CDFA is very concerned about the risk of exporting pathogens and we strive to always meet or exceed the phytosanitary requirements of other States and our trading partners, and to be leaders in nursery cleanliness.

   CISP is also concerned that other North American native species or important horticultural plants that are in same genera as hosts infected in Great Britain might also be at risk. These include our native Fagus americana, and numerous species in the Quercus, Aesculus, Castanea, Ilex, Leucothoe, Magnolia, Pieris, Prunus and Vaccinium genera.

   Response from H. Martin: I share this concern for protecting the health of plants in the forests and landscapes.

   CISP is also concerned that the criterion for determining the geographic area at risk is too narrow. Clearly, the areas at greatest risk are those with wet climates – similar to those now invaded by P. ramorum. It is true that these constituted a “limited part of California” (Criterion 2, Medium). However, virtually no pathogen would be able to become established across the full range of climatically distinct regions in California – a large and diverse state. In sum, this criterion can almost never be met. CISP suggests it would be more appropriate to consider the level of devastation that an introduced organism would cause in significant and important ecosystems within the state, in this case, forested ecosystems.

   Response from H. Martin: Medium (2) is for pathogens that may be able to be established in a larger but limited part of California – I believe this is the correct category for P. kernoviae as it could be established in multiple regions where the temperature and moisture levels would be sufficient for disease to occur. This could include the North Coast, the Central Coast, and the Sierra Nevada. It is not likely to be found in the Sacramento Valley, San Joquin Valley, or Southern California, except possibly in a nursery situation.

   The criteria for High (3) is – likely to establish a widespread distribution in California – there are plant pathogens that meet this standard and have been found across many climatic zones, especially if associated with production agriculture. The level of potential damage is captured by the economic impact and environmental impact scores. Phytophthora kernoviae received a high (3) for each.

   Comment from F. Campbell
   3) Pest Reproductive Potential:

   The Proposal ranks P. kernoviae as “3 / High” for both high reproduction & dispersal potential.
   CISP agrees with this ranking, but asserts that the Proposal would be strengthened by mentioning the many uncertainties re: reproduction. These include
   • likelihood of additional hosts continuing to be detected;
   • fact that P. kernoviae produces zoospores on asymptomatic hosts;
   • lack of understanding of role of oospores and root infections.

   Response from H. Martin: These are good points, and I will add them to the Uncertainty section

   Comment from F. Campbell
   Uncertainty: None

   CISP agrees that there is no reason to doubt that Phytophthora kernoviae poses a severe risk to the environment and agricultural economy of California. In that sense, there is minimal uncertainty. However, as we noted under several specific points above, there is considerable uncertainty regarding the details of the species’ status in North America, its modes of reproduction, the extent of hosts that might be at risk, and even the current level of effort by American authorities to address these questions.

   Response from H. Martin: I have added to the Uncertainty section.

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