Wednesday 7 November 2012

Ash dieback, back to the future

Whilst we are reeling from the shock of finding ash dieback in the UK, European foresters have been fighting an ongoing battle to find practical solutions to this relatively new ash tree killer.

With new cases of ash dieback disease detected  on a daily basis in the UK, the more likely scenario is that the disease has been bubbling away for a couple of seasons – unnoticed until you deliberately look for symptoms.

Judging by the progress of the ash dieback in Europe, where it was first identified in the early nineties, the next three years will see a dramatic decline in our ash population. Whether this is by the disease itself or a frenzy of pre-emptive logging is another question.

How does it all start?

A microscopic fungal spore lands on an ash leaf and infects it. Fine fungal threads, called hyphae, grow through the leaf and into the stem. Blocking the plant vessels carrying water in the plant, they kill off the leaves, petioles and wood. They create the symptoms so aptly described in the ashtag app you can download here. This is the fungal disease known as Chalara fraxinea or ash dieback!

The infected dead leaves fall to the ground. Here, small white cup shaped fungal fruiting bodies appear on the leaf stems (petioles) - known as the fungus Hymenoscyphus pseudoalbidus. These produce the spores that infect the leaves of both diseased and still healthy ash trees nearby.

Octobertest in Bavaria

Can we save our ash trees? I found an interesting 2012 paper by a Bavarian group of forestry researchers who have been battling with this questions for the past four years. They looked at over 1000 ash trees in a spread of experimental plots throughout Bavaria. Read the paper by Heike Lenz and colleagues, here. (For those unfamiliar with German, here's my quick translation). Some key points below.

Finding supertrees?

Even after three years, exposed to heavily infected neighbours, about 6% of the Bavarian ash trees were healthy or only lightly infected. This phenomenon has been observed elsewhere too. Are the healthy trees just lucky or are they partially or even fully resistant? Unfortunately, we will be experiencing natural selection in ash in real time, in the woods of Bavaria and Britain.

Far above the fungal cloud

However, the level of healthy trees was lower by half in young stands. Could this be because the leaves on the older trees were higher above the ground, away from where the spores were being produced. Tantalisingly, the data for the effect of altitude was still unavailable, presumably in progress. Measurements did show that rain spread more spores in the air.

Stressed out or seeing the light

Early results from ground shading experiments in the field mirrored those in the lab. Spore formation appears to be promoted by light. So could we reduce the infection pressure due to spore formation by other factors too? Faster leaf decomposition, liming the soil, changes in nutrients and other stresses all need to be tested for their effects on spore formation and germination.


We can use existing research from EU foresters who are ahead of us in the disease curve. We are still going to see a dramatic population crash in the number of healthy ash trees over the next few years. However, unlike the Elms, which were all genetic clones of a Roman import, ash trees have a natural diversity. It looks as if tolerant or even resistant individuals will survive and, in the long run, we may get our ash stands back.

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