2019 Update on Ash Trees in Nursery Wood

Ash die back disease (hereafter called Chalara) was first noted in Nursery Wood (planted Spring 2008) in summer 2014. Infected stems were cut out, new leaders encouraged, and a program of pruning all ash trees up to 2 metres was established (see details in an earlier blog). This management seemed to be working well, previously infected trees were growing away well and new infections were minimal. The exception was an isolated control tree that had never been pruned. This tree had repeated infections and die back every year but despite this it continues to put out strong shoots of 1 metre or more each summer.

In late summer 2017 I began to see new infections in the crowns of many trees. By August 2017 these infections had blackened the leaves and led to premature leaf fall on several trees. I feared that they would not survive. I tagged 6 of the ash trees that showed a gradient of symptoms from light infection to nearly defoliated and waited to see how they would survive in 2018.

One of the tagged trees had shown the characteristic basal lesions for 2 years, however it only had light new infections in summer 2017 and over the winter it showed a strong terminal shoot with a strong bud. New tissue growth around the basal lesions also gave me hope that it might recover. But in 2018 this tree never came into leaf, the basal infection had spread deep into the stem and had prevented water from moving up the tree stem thus killing the tree completely. All the other tagged trees came into leaf and sent out strong new shoots. The trees that were most badly infected the previous year had extensive dieback in the crown but the new growth was healthy. Trees that were lightly infected the previous summer showed some die back in a few shoots in the crown, but this was quickly masked by the new season’s growth.

Every year in May woodpigeons land on the fresh ash shoots which then bend with their weight and snap or become distorted. In late summer 2018 I noticed rather more distorted shoots on certain trees than usual, and they appeared all over the tree. The effect was similar to a light dose of hormone weed killer, but perhaps it was a result of the Chalara infections in summer 2017. I shall watch these trees closely in summer 2019.

Unlike 2017, there are minimal new Chalara infections in summer 2018. As I write this at the end of August all the ash trees have healthy green leaves despite the summer drought. 2018 is also a good seed year for ash in Nursery Wood and elsewhere. I shall collect ash keys from trees that have never shown any signs of infection. The seed will be grown in the nursery and hopefully have improved tolerance to ash dieback.

Some of the points I have drawn from this are:

  1. Basal lesions are generally fatal for young trees as they penetrate to the middle of the stem and prevent water flow. But are these lesions caused by Chalara or another fungal infection such as Honey Fungus?
  2. Endogenous Chalara infections (spores arising from the leaf litter) infect the lower branches and can easily pass into the main stem causing lesions. They can be managed by pruning all lower branches thus creating a gap between the woodland floor and the lowest leaves of the ash trees. Unmanaged trees are reinfected annually from the leaf litter.
  3. Exogenous infections (infections from the wider environment) occur every few years when environmental conditions are suitable (2013 and 2017 in Nursery Wood). These are characterised by multiple infections (thousands of separate infections on a single tree) which lead to blackening of the leaves in the crown and early leaf fall.
  4. Neighbouring trees show large differences in the number of these infections, implying that there is a strong genetic element to disease tolerance/resistance.
  5. Each infection only lasts for one year allowing the trees to recover the following year. Young trees may become badly forked, but older trees that have developed a crown are largely unaffected.