Tackling larch enemies - genetic analysis of Phytophthora ramorum

Wood Focus magazine
22 Sep 2012
A forest in Humboldt County, California. © Yana Valachovic, UCCE Humboldt

British forestry scientists have identified a new lineage of a deadly tree pathogen that could devastate Japanese larch populations in the UK and oak species in the USA.

The Phytophthora ramorum (P. ramorum) pathogen causes ramorum disease, which is responsible for diseases in more than 120 plant species around the world. Genetic analysis has confirmed the fourth genetically distinct lineage of the pathogen – one that separated from the pathogen’s other strains thousands of years ago.

The California Oak Mortality Task Force in the USA and the UK Forestry Commission are looking to arrest the disease’s progress. Spokesperson for the US organisation, Katharine Palmieri, explains, ‘Each lineage is distant enough from the other that, if reintroduced to one another, unintended consequences could occur, such as mating successfully with each other and creating a hybrid that could be more virulent.’

P. ramorum is a water-loving organism. It is active when the weather is damp and dormant, and when it is hot and dry. Spread occurs when spores build up on the leaves and twigs of hospitable hosts, such as Japanese larch, and move into the surrounding environment via rain, wind, waterways and the movement of infested plant material and soil.

‘The symptoms vary from host to host,’ Palmieri says. ‘In general, though, foliar hosts tend to have a black diffuse margin separating healthy from diseased and dead tissue. Trunk (or bole) hosts tend to have an oozing coming out of undamaged bark. If the outer layer of bark is shaved away, you can see a black diffuse margin separating healthy and diseased tissue. The black line in both instances is where the pathogen is active.’

Until recently, there were three known lineages of the disease. A European type that affects larch is found in European nurseries and forests, and two US strains have been observed, one of which is commonly found in California’s forests. The fourth new type of P. ramorum has been found in southwest Scotland and Northern Ireland, although it has yet to spread to the USA.

The effects of the new strain are yet to be realised. However, P. ramorum has been extremely harmful to US forests. ‘California has lost well over a million trees in the forests where the pathogen is currently found (14 coastal counties), and at least another million are currently infected,’ Palmieri says. ‘The pathogen depends quite a bit on weather for spread, so in wet years we have a big build-up of inoculum that can then spread into the local environment.’

Some tree species are more resistant than others. Palmieri says US tanoak trees expire about one year after infection, whereas true oaks tend to die after about two to three years. ‘Once a tree is infected, there is no known way to eliminate the pathogen,’ she adds. ‘There is only one preventative treatment available (Agri- Fos® is used in the USA) for use against ramorum. It isn’t 100% effective, but it has about an 80% efficacy rate when used prior to infection.’

In the UK, ramorum disease has persisted despite phytosanitary measures. To date, more than three million larch trees have been felled to bring the disease under control.

The Forestry Commission has launched a Tree Health and Biosecurity Action Plan to combat the recent influx of exotic pests and pathogens. At the Fifth Sudden Oak Death Symposium in California, the UK Forestry Commission’s Clive Brasier said protocols needed to be developed to help affected industries deal with unknown pathogens and to increase public awareness.

A big effort has also been made in the USA to implement best management practices in nurseries. Science-based regulations are in place, and research is being conducted to improve regulations and information for nurseries. But, as Palmieri underlines, ‘Looking for what you don’t know exists is not an easy issue to resolve.’

Further information

To learn more about P. ramorum symptoms, visit www.suddenoakdeath.org and www.forestry.gov.uk/forestry/WCAS-4Z5JLL