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.

 But 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.
  2. Endogenous Chalara infections (spores arising
    from the leaf litter) infect the lower branches and can easily pass into the
    main stem causing lesions. Theycan be managed by pruning all lower branches
    thus creating a gap between spores and leaves. 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.

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