Fight the sweet blight

Wood Focus magazine
20 May 2012
sweet chestnut blight

The smell of burning sweet chestnut wafted through orchards in Warwickshire and East Sussex, but no one was eating roasted fare. The sweet chestnut trees burned to stem the spread of a fungus called Cryophonectria parasitica. If left unfettered, this fungal blight could eradicate the UK’s sweet chestnut tree population.

Sweet chestnut blight was discovered in Italy in 1938. It has moved slowly through the decades. Spores passed by bird beak, wind and other media until it reached northern France. From there, it infected timber and boarded a boat to Britain.

Despite strict plant passporting, the disease can be hard to detect, lying dormant for many months. Dr John Morgan, of the UK Forestry Commission, explains, ‘Once it moves out of its latent phase, then you see it affecting the chestnut tree. You get your classic died bark and cankering of the stem wood.’

The disease does not need much time to snuff life from the tree. ‘It works by creating lesions (dead bark) in the chestnut plant,’ he adds. ‘As a result, the dead matter can no longer transfer material (water and minerals) through to the upper parts of the plant.’

Like Messrs Poirot, Matlock and Columbo, the Forestry Commission and The Food and Environment Research Agency (Fera), must trace the murderous trail. ‘It’s linked to a planting stock that came in in 2007,’ Morgan notes.

‘We go back to any consignments of plants that came in from that date, and we trace them all. We go to the nurseries that supplied the stock. We ask them for the supplier list. We’re talking well over 100 customers for these plants, and we follow them all forward.’ When the disease is found, each sweet chestnut tree is uprooted and burned lest the blight spread to other orchards.

If the problem persists, there are other solutions. In a process known as hypovirulence, C. parasitica can be infected with a virus (dsRNA hypovirus CHV1) that limits the pathogen’s spreadability. This has been quite an effective way of reducing the fungus’ impact in Europe. However, applying hypovirulent strains of C. parasitica to the growing cankers does not always work, as it is dependent on sweet chestnut species with limited genetic diversity.

In the unlikely event of drastic damage, species engineering could be an option. Having seen the entire North American native sweet chestnut population destroyed in the early 20th Century, a more resilient Asian sweet chestnut species was successfully crossbred with native trees to repopulate the forests with sweet chestnut trees. Morgan says that while this could be done in the UK, it would be a very expensive process.

For now though, the Forestry Commission and Fera will adopt a more targeted approach, which will include inspections of sweet chestnut plantings and woodlands in the vicinity of previous outbreaks. ‘If we can’t get reassurance that the material in France is disease free, then we’ll have to look at ways of controlling that trade,’ he says. ‘One possibility is quarantining stock and keeping it for a number of years before moving it on to a planting site.’

What you didn’t know about sweet chestnut blight

  • it killed an estimated 3.5bln trees in the USA after being introduced to North America from Asia
  • the horse chestnut is not afflicted by the fungal blight
  • the disease is said to have originated in eastern Asia. Asian chestnuts have co-evolved with C. parasitica and are therefore more resilient to the pathogen
  • the UK outbreaks were in two sites: Warwickshire and East Sussex
  • timber is not allowed into the UK unless it is accompanied with an official statement showing it has been kiln dried or fumigated

Facts compiled by the UK Forestry Commission