PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
Hysterographium fraxini, disease of Ash in New Zealand
HYSTEROGRAPHIUM
Forest Health News 149, March 2005
In December last year, as a result of a public enquiry regarding
dieback of Fraxinus excelsior in Timaru, a Northern Hemisphere
fungus was recorded in New Zealand for the first time. It is
Hysterographium fraxini and its occurrence here was reported in
the February issue of “Biosecurity” (Issue 57) with the comment
that Biosecurity New Zealand is carrying out an initial
investigation to assess whether further actions are required against
the fungus.
The fruit bodies of H. fraxini (see below), which are found on
dead branches and twigs, are very distinctive and have been likened
to “smiley lips”. When the sample was being examined in the
laboratory there were so many oohs and aahs from the pathologists
that a normally staid entomologist had to go over and duly admire
the specimen under the stereo microscope.
In the Northern Hemisphere H. fraxini is generally recorded as a
non-specialised, facultative saprophyte. In Switzerland in the
1940s it was reported that H. fraxini might be responsible for
extensive damage to ash ( Fraxinus excelsior and F. ornus ) under
conditions adverse to the host, e.g., humid sheltered situations or
water-logged soils. The fungus enters branches of any age through
fresh wounds in the spring and at first develops saprophytically,
but later it may encroach on the surrounding living tissue. Towards
the end of the summer the leaves above the site of infection begin
to wither, since by this time the branch is almost completely
girdled. In vigorous trees the formation of callus between the upper
and lower parts of the branch arrests further growth. Re-infection
can occur only through a fresh injury in healthy trees, but in trees
which are seriously weakened and where no callus is formed the
fungus may pass from the already invaded branches to sound ones
and finally to the trunk. More recent references from Switzerland
and England describe the fungus as “rare”.
Since the initial find in South Canterbury, H. fraxini has been
found in mid-Canterbury, Marlborough, Marlborough Sounds,
Nelson, Wellington, and Wairarapa on Fraxinus excelsior,
F.angustifolia, F. ornus, and Fraxinus sp. Its behaviour in New
Zealand is that of a saprophtye. The dieback of the tree in Timaru
was probably caused by high numbers of Acizza sp. (Psyllidae).
(John Bain, Editor, based on information supplied by
Margaret Dick. Forest Research)
