- Outbreaks intensified in western North America and Western Europe (UK, France).
- Outbreaks are increasingly genetically diverse – raising the possibility of sexual reproduction and evolution.
- Evidence accumulated that eradicating Phytophthora ramorum from the environment once it is present is extremely difficult, if not impossible.
Meanwhile, APHIS proposed revisions that would weaken its regulation of nursery stock. See my earlier blog. Copies of all comments can be viewed here.
1) Intensifying Outbreaks
North America
According to the California Oak Mortality Task Force’s (COMTF) November 2018 newsletter, about 50 million trees have been killed by P. ramorum in California and Oregon. This breaks down to:
- 29 – 44 million tanoaks (Notholithocarpus densiflorus) (1.6 – 2.5% of the species’ total population in California and Oregon);
- 1.9 – 3.3 million coast live oaks (Quercus agrifolia) and Shreve oaks (Q. parvula var. shrevei), combined (0.4 – 0.7% of their populations); and
- up to 1.1 million California black oaks (Q. kelloggii) (less than 0.17% of their population).
Of course, the oaks face additional threats from goldspotted oak borer and polyphagous and Kuroshio shot hole borers hin more southern parts of California.
California bay laurel (Umbellularia californica) is not killed by P. ramorum but instead drives the spread of the outbreak in California. The state has an estimated 91.4 million infected California bay laurel trees.
These estimates are considered to be conservative. They are based only on trees that have been confirmed to be infected by direct, cultural isolation during the period up to 2014 — more than four years ago! And before a sharp intensification of infection (see below).
Data from a USDA Forest Service aerial detection survey – reported in COMTF’s September 2018 newsletter — detected a large increase in tanoak mortality in counties California counties reaching from Mendocino south to Monterey. This intensification in tree mortality was expected because the pattern is already well established: two seasons after a wet winter seasons, trees die. Such a wet and extended winter occurred in 2016-2017.
United Kingdom
Outbreaks of the EU1 strain of P. ramorum on larch (Larix kaempferi) in Scotland have also intensified. The infection is now found throughout much of Scotland, not just in the heavily infested zone in the the southwest part of the country. See updated map of outbreaks on Larch sites in woodland settings at https://scotland.forestry.gov.uk/supporting/forest-industries/tree-health/phytophthora- ramorum?highlight=WyJyYW1vcnVtIiwiJ3JhbW9ydW0iLCIncmFtb3J1bSciXQ
There is more on the status of P. ramorum in the the UK (England, Wales, Scotland and Northern Ireland) in a situation report posted by Forestry Commission England in 2018. Find it here: https://www.forestry.gov.uk/pdf/PRamorumSituationReport30June2018.pdf/$FILE/PRamorumSituationReport30June2018.pdfh
As in North America, the large number of outbreaks is attributed to favorable, wet conditions in the summer and fall of 2017. (This situation was summarized in COMTF’s September 2018 newsletter.
France
The outbreak on larch in France, first reported in 2015, is also spreading. This is particularly significant because, first, it is the first report of P. ramorum outside of nurseries and ornamental settings in mainland Europe and, second, because it is a new genotype not tied to any other outbreak. By May 2018, about 80% of the trees in the Saint-Cadou larch plantations in Brittany (Northwest France) were symptomatic or dead in the more infected plots. A second outbreak has been detected a few kilometers away in a mixed forest stand of larch, oak, and sweet chestnut (Castanea sativa). There, disease prevalence was much lower. Both stands have been removed.
(This was also summarized in COMTF’s September newsletter.
2) Increasing Genetic Diversity
EU1 Strain in Oregon
As I have reported in the past, Oregon now has a second strain of Phytophthora ramorum – the “EU1” strain. This opens the possibility of sexual reproduction between it and the NA1 strain already established in forests in Oregon’s Curry County.
According to a presentation by Chris Benemann of the Oregon Department of Agriculture to the Continental Dialogue on Non-Native Forest Insects and Diseases, in 2018 – three years after the initial detection of one tree in 2015 – the number of trees infected by the EU1 strain has risen to 73. Oregon has prioritized removing these trees and treating (burning) the immediate area – now more than 355 acres. The legislature has provided $2.3 million for SOD treatments for 2017-2019. ODA believes that eradication of the EU1 outbreak is still possible.
3) But Is Eradication Possible?
According to the COMTF September newsletter, P. ramorum was detected by a water bait in a small pond downstream from a previously-infected botanical garden in Kitsap County, Washington. The garden undertook extensive mitigation efforts – including soil steaming – and the pathogen had not been detected in this managed landscape for about 2 ½ years. Hundreds of samples of host plants were collected in September, with only one warranting further analysis to determine whether it was positive. Surveys will continue in 2019.
In the East, USDA has baited streams to detect P. ramorum for several years. Seven states participated in the 2018 Spring National P. ramorum Early Detection Survey of Forests: Alabama, Georgia, Mississippi, North Carolina, Pennsylvania, South Carolina, and Texas. As reported in the COMTF’s September newsletter, h292 samples were collected from 48 sites. As in past years, positive samples were collected from streams associated with previously positive nurseries. These included three samples from two locations in Alabama; two samples from one location in Mississippi; and one sample from North Carolina. The Alabama and Mississippi sites have tested positive for approximately a decade.
So, the pathogen is persisting in water – but how? I have been told that P. ramorum requires plant material on which to survive – so how is it persisting without detectable infested plants? Also, does the presence of zoospores pose a threat of infesting streamside plant material? What studies are examining this issue?
Awareness through Art
Artists have transformed a SOD-infected tanoak tree into 7,000 pencils as part of their thoughtful “7,000 Marks” project. They explore issues around global industrial trade, quarantine boundaries as a conservation tools, and the opposing concern that restricting trade can echo a rising tide of xenophobia. You can learn more (and buy pencils) here.
SOURCES
Cobb, R.; Ross, N.; Hayden, K.J.; Eyre, C.A.; Dodd, R.S.; Frankel, S.; Garbelotto, M. and Rizzo, D.M. 2018. Promise and pitfalls of endemic resistance for cultural resources threatened by Phytophthora ramorum . Phytopathology. Early view.
Harris, A.R.; Mullett, M.S.; Webber, J.F. 2018. Changes in the population structure and sporulation behaviour of Phytophthora ramorum associated with the epidemic on Larix (larch) in Britain. Biological Invasions. 20(9): 2313–2328.
Posted by Faith Campbell
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