On 04/10/2020 10:22, Nick Maclaren wrote:
In article ,
Jon Nicoll wrote:
I'm trying to determine why our viburnam has died, in somewhat
spectacular fashion

The symptoms are wildly unlike either honey fungus or phytophthora, and the
Web and media do us no service by banging on about those.

I am afraid a comment the OP made - and I missed at first reading -
/does/ suggest Phytophthora. He stated "There are deep cracks running up
one or two the trunks".

In June last year I had a Grevillea rosmarinifolia die within two weeks.
It had been in the ground for six years and had been growing very well,
being about 150 cm high and across. It appeared to die by drying out, as
the flowers and even some of the leaves were shrivelled. Remember that
this is a plant from SE Australia, well used to very hot and dry
conditions. (One I had previously growing at another garden shrugged off
the extreme heat of the "noughties" and grew to 350 x 350 cm. It was
still growing well when we moved.)

The plant I had which died showed one particular condition which puzzled
me, and that was deep longitudinal cracks along its trunks:
https://ibb.co/TH9Nfnh

When I asked about this with someone I know at the Royal Botanic
Gardens, Sydney, he replied:
"... in an Australian context I would put this down to attack by a root
pathogen, and in this country the most likely culprit would be
Phytophthora cinnamomi... In the UK context it could be a different
pathogen of course, but the suddenness of attack and the stem split is
reminiscent of ‘Pc’ as it is often called here. Basically the
pathogen, the spores of which are borne in water and soil, infects the
roots (often at times when the plant is under stress from other causes)
and blocks the water-conducting tissues of the roots, leading to sudden
dehydration, sudden wilting/browning of foliage, and (sometimes) stem
splits as the tissues are denied water"

--

Jeff