The UK’s new plan that will deal with ash disease might not come in time, as most of the country’s 90 million ash trees are likely to be wiped out. Ash dieback is caused by the fungus Chalara fraxinea and it will probably kill 99% of the ash trees in the UK.
Diseased trees will have to be identified and destroyed. This includes trees in nurseries and newly planted trees. Mature trees will be left standing, since they take longer to die and are valuable to wildlife. There is an import ban on ash trees that has been in place since October.
These measures won’t eradicate this disease from the UK. Ash trees are the third most common in the UK, and there are as many as 90 million ash trees at risk. This comes in the wake of the largest tree survey undertaken in the UK. 500 staff and volunteers combed through 2,500 square kilometers of British countryside looking for sites of infection. It’s possible that the disease reached the UK from imported timber, but it’s more likely that the spores arrived naturally. The sites of infection are scattered all over the country, meaning that the spores were blown in the wind from continental Europe, where the fungus has ravaged ash trees from Poland to France for more than 10 years.
Ash trees grow quickly, reproduce quickly, and have a high capacity for self-seeding, so reforestation might not be too difficult. The resistant strains of ash need to be identified so that they can be bred and repopulate ash trees.
One of the reasons why it’s taken so long to tackle the disease was that it was unclear what exactly was causing the dieback. Initially, mycologists attributed the dieback to Hymenoscyphus albidus, a species endemic to Europe. It was thought to have developed a more virulent strain. In 2011, mycologists determined that it was down to another species altogether.
The latest biologists think that the fungus is native to Japan. It’s estimated that 90% fatality is over-optimistic. It will be worse than that. Even if biologists can halt the spread of C. fraxinea in the United Kingdom, the worldwide spread of plant pathogens shows little sign of abating in a globalized economy.