Phytophthora: a search for answers

As Defra tries to tackle the disease, a US scientist believes he has found the cause, reports Matthew Appleby.

Phytophthora kernoviae and P. ramorum, better known as sudden oak death, have rumbled on as serious plant health problems for several years and are likely to drain more and more of Defra's slight resources in the future, if the department's latest consultation is anything to go by.

Costs are up to £8,000/ha for clearing wild rhododendron, and Swansea council is expecting £1m more in costs on top of the £800,000 it has already spent on attempted eradication.

Interested parties met with the Plant Health & Seeds Inspectorate, Central Science Laboratory (CSL), the Forestry Commission and other experts involved in trying to eradicate the problem in Cornwall last month. And Defra has proposed two strategies to rid Britain of the diseases. But are Defra's efforts even based on correct science?

No, says US scientist Dr Lee Klinger, who has witnessed the authorities' inability to cope with the devastation of oaks in California. He sees acid rain as the problem.

Klinger says: "UK forests are experiencing acidification, though not all the acidity is coming from air pollution. Here on the west coast of the US I have found significant levels of acidity in rain that appear to be derived from gaseous sulphur emissions of ocean phytoplankton. Mosses and lichens that grow on trees and soils are also significant sources of acidity. Last week I measured the pH of a solution of Usnea lichens soaking in distilled water and got readings around 3.4, which is nearly as acidic as vinegar.

"Hundreds of studies link forest decline to acidification. I have a published paper on my website linking sudden oak death (SOD) to acidic conditions.

"The science of SOD is highly politicised. That is why I remain independent. What really matters is good results. I've got those and anyone can view them on my website. The Government scientists have no such results. I seem to be way ahead of them. Hopefully they will wake up before too many trees die."

CSL's Dr Claire Sansford dismisses Klinger's suggestion that the disease is primarily caused by acidity which could be remedied by nurturing the trees back to health involving correcting the pH and introducing nutrients.

Sansford says: "We are aware of Dr Klinger's views and his website. However, we know from years of research, both in North America and Europe, including that which has been published in peer-reviewed journals, that SOD is caused by a primary pathogen, P. ramorum.

"We know that Dr Klinger has measured soil pH in areas of California where SOD occurs. In one study, published in 2005, he says soils near SOD-affected trees have a mean value of pH 5.8. This is not particularly acidic. In the same paper he says the median pH of soil for 'disease sites' is 5.7, compared to 7.27 for 'non-diseased sites'. Again, pH 5.7 is not particularly acidic. In addition, we can find no evidence in this study that he has collected data on tree symptoms and presence or absence of P. ramorum in relation to acid versus non-acid soils. For these reasons we do not agree that SOD is primarily caused by acidity."

It seems that many of California's scientists disagree with Klinger as well. Phytophthora expert Dr Dave Rizzo, a professor in the Department of Plant Pathology at University of California, Davis, says: "Dr Klinger is in a distinct minority of scientists who believe that P. ramorum is not the underlying cause of sudden oak death in California - possibly a minority of one."

He explains: "Dr Klinger's soil acidification hypothesis suggests that acid soils lead to fine-root mortality, which then leads to secondary insects and diseases that cause the decline and potential death of trees. This is a testable hypothesis, but there is no evidence this is happening in areas with sudden oak death in California. The 'hundreds of studies' Dr Klinger refers to are almost exclusively associated with forests in northern Europe or the north-eastern US and Canada. These soils often have a pH below 4; quite a bit different from the pH 5.5 to 6 that we typically find in our California forest soils. This pH range may be considered acidic for agricultural crops but not for a forest soil.

"In addition, there is no evidence that there is any fine-root mortality associated with sudden oak death. Trees typically die from the top down."

Rizzo concludes: "There is no magic-bullet treatment for sudden oak death. But despite what Dr Klinger states, there are some good protective treatment results that have been published."

- The closing date for comments on Defra's consultation is 10 October 2008. See www.defra.gov.uk/corporate/ consult/phytophthora-ram-kern/index.htm for consultation details.

- Klinger will be speaking at a conference at the National Museum in Cardiff sponsored by Treework Environmental Practice in November. See www.treeworks.co.uk/seminars/index.html. For more information on Klinger see www.suddenoaklife.org.

CASE STUDY: SWANSEA

Defra found P. ramorum and P. kernoviae in Swansea parks in 2003/04. The disease has spread from Rhododendron ponticum to Viburnum, Magnolia and Drimys in Swansea. A council representative said: "The main thrust of our removal policy was to reduce the R. ponticum, as it is an invasive non-native plant. To date, about 40ha of parks and open spaces have been subjected to this removal programme."

About £800,000 has been spent combating the disease. Swansea is seeking up to £1m from Better Woodland Wales from the Forestry Commission to continue eradication.

DEFRA'S STRATEGY

- Option 1: Meet EU minimum requirements on control of P. ramorum and remove all controls against P. kernoviae, other than maintaining a ban on the movement of infected plants to other countries.

- Option 2: Increase activity, aimed at reducing the level of inoculum to epidemiologically insignificant levels, by removal of infected sporulating hosts in woodlands and the wider environment; combined with enhanced containment and eradication measures in infected gardens and nursery sites, as well as the identification and control of any new outbreaks.


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