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Saturday, December 11, 2021

WEEKENDER: Bio Security Failure ? by Wiggia

It was not that long ago that I mentioned some facts on plant diseases that were plaguing the western world.

Another was announced by the Forestry Commission: a new tree disease has been found in Cornwall. How it came to light in Cornwall is interesting in its own right. This is the first time in Europe. Phytophthora pluvialis affects Douglas fir and a variety of other trees not specified.

What was not said was what the disease actually did; was it one of those that had a time frame and petered out, or was it an outright untreatable killer? Putting out a notice about a disease and nothing else is a bit like a film review where they don’t reveal the plot.

Further digging revealed in the official Government paper on the disease...

“Phytophthora pluvialis is known to cause needle cast, shoot dieback, and lesions on the stem, branches, and roots. “

Further digging failed to reveal if the disease is terminal but the area has been put into quarantine which suggests the final outcome of the disease is not known. It does however weaken the host and Honey Fungus which is terminal can take hold of the weakened plant.

None of this is good news any more than it is startling, or even interesting to regular readers, but it is one of a long line of imported diseases in plants that have been occurring on a regular basis for some time.

These diseases fall into several categories and those categories are not always that evident on early discovery. Many of them are in the ‘you will all die’ group rather like our Coronavirus experts keep preaching, but turn out not be, it is not an easy area to pre judge as many don’t quite live up! to the reputation gained elsewhere; many of course do.

The most infamous in recent times was Dutch Elm disease and that did indeed wipe out 99% of elms in this country, but not all: two were quite happily surviving 50 yards up the road from where we lived ten years ago and are still there; how is a mystery, and yet an earlier house we moved to in 1983 was called Twin Elms and there were none - the entrance trees had succumbed to the virus.

I am not going through all viruses that have alighted on these shores but a sampler of the more prominent and recent gives the picture.

The London Plane is a pretty resilient species but as with all is not immune to attacks of fungus. In the Eighties it was infected with Plane Anthracnose which causes early leaf fall; this sparked fears of losing our most famous tree in the capital responsible for lining so many stately avenues and drives. The leaves started to fall almost immediately after they had unfurled and we had autumn in May. The trees survived and the leaf fall carried on for several years getting later and later into the season before all returned to normal; scare over.

Then we had the Horse Chestnut under attack from a very nasty Bleeding Canker, which looks as nasty as it sounds: large areas of the bark turn black and bleed a tarry sap. The advice was to remove infected branches and the tree would slowly recover but the ones I saw fell into the death stage quite early on. It was a very quick change in tree and advanced with some speed. One client I had removed three chestnuts in the first year the disease appeared. For a while outbreaks threatened the species with severe depletion but again it slowly subsided and I haven’t seen an infected tree for some time.

After that it was the turn of the oak. It seemed nothing was safe; the oak has been the subject of several diseases over time but the latest is Acute Oak Decline.

An example of Acute Oak Decline

‘Acute oak decline (AOD) is a relatively new disease in the UK with an increasing number of reported cases each year, mostly in the English Midlands, with records extending into Wales. The main symptom is extensive bleeding on the tree’s stem (trunk). A dark fluid oozes from splits in the bark, often from 1 metre above ground level and upwards. It is usually found on mature trees, 50 years or older,  and both major oak species (Pedunculate and Sessile) in the UK are affected.

'Unlike chronic oak decline, a tree affected by AOD may die quickly, within five years. The exact cause(s) of AOD are not known at present but Forest Research believe that a bacteria may be the likely factor.’

The worry with this one is self-evident in the text: they have no idea at present what causes it. The good thing! is that the disease has not spread into the other regions, which gives time to find the cause and maybe a cure.

Another much more widely spread virus has attacked Ash trees; this is worrying as the Ash is such a widely-planted tree.

‘Ash dieback is a serious disease of ash trees, caused by a fungus now called Hymenoscyphus Fraxineus. The fungus was described as a new fungal species in 2006 as the cause of ash (Fraxinus Excelsior) mortality in European countries during the previous ten years.

'The disease affects trees of all ages. Young trees can be killed in one season and older trees tend to succumb after several seasons of infection. Whilst ash dieback is certainly capable of killing trees in its own right, in many cases the weakened tree is colonised by another pathogen, particularly honey fungus, which then accelerates the decline and death of the tree.

'Ash dieback has spread rapidly in continental Europe. In the UK, the disease was first confirmed in trees growing in nurseries or on recently planted ash trees. However, many cases have now been confirmed in the wider environment in the UK and the disease is widely distributed. The latest distribution maps for cases of the disease in the wider environment can be found on the Forestry Commission website.’

Again the potential to be devastating has been somewhat ameliorated by the discovery that in certain growing conditions the disease is not so prevalent. Locally I have not seen any Ash trees with the virus as yet, despite this area (Eastern England) being one of the first to report it in 2012. Much of the problem has been the popularity of Ash; it is the third most commonly planted broadleaf in the country, both in municipal and home plantings with several very popular cultivars; one or two are exempt from getting the virus and research is centred on those for an answer to the problem.

And there are economic implications in all this as well the Ash alone…

‘Research by a team from the University of Oxford, Food and Environment Research Agency Science, the Sylva Foundation and the Woodland Trust published in May 2019 has calculated the true economic cost of ash dieback to the UK is estimated to be in the region of £15bn, with half of this over the next ten years, principally related to management and replacement costs.’

There has been a significant increase in the number of non-native tree pests and diseases being introduced to the United Kingdom since the early 2000s.

Some of these diseases cross over in species. In the US the disease known as Sudden Oak Death' is misleadingly named as it attacks most trees including beech, larch, ash, horse chestnut and sweet chestnut. It also attacks garden plants including rhododendron, viburnum and camellias.

I actually found a new Rhododendron disease in a garden I had planted, on an annual inspection some 15 years later. I was not sure what it was so I sent samples of leaf to the RHS who confirmed it was a silver leaf blight similar to others in trees, but fortunately it responded to the same treatment, which was an annual spray. Plant diseases are definitely on the up across species and the government bio security measures though good on paper are not stopping a continuous feed from abroad. 

As with so many of these foreign diseases they don’t have the natural predators that are present in their homelands. For example, the new box caterpillar now devastating box topiary everywhere is prey to a killer moth that has not accompanied it from Japan, and it has no predators here. Frequent spraying in time works but the caterpillar only needs a couple of days to strip a plant and you have to spray almost endlessly through the season.

The fact we import so much nursery stock does not help, but it is not the only avenue for diseases to enter the country. People still stupidly bring back plant material from foreign lands and don’t get stopped at customs or declare it; we are nowhere near as strict in this matter as say Australia and that obviously contributes to the problem.

Naturally the government, because of climate change, have schemes to plant trees everywhere and encourage individuals to do the same ‘to save the planet.’ Has any thought gone into advising what and what not to plant? None as far as I know, but the millions of trees mentioned in these plantings will nearly all be coming from abroad. What restrictions on species being brought in have been put in place? So many tree species are suffering from disease problems that there are not many ‘clean’ species left.

An article recently gave an update on the subject in the Times. Apart from the facts above it quoted a doubling of tree imports in four years and other plants which can also bring with them diseases have increased in quantity by fourfold in 25 years; how much real scrutiny has gone on during that period?

The Woodland Trust speaks of 20 serious tree pests and diseases imported since 1990, and the loss of tens of millions of trees.

The emphasis must be on UK nurseries producing more of our own stock but this of course is not an overnight solution. Very few UK nurseries have the enormous capacity of the Dutch and Belgian outfits alone and with size comes price differentials, so we are nowhere near competitive.

The government response is one of too late and to feeble to start with. Banning species from countries with these disease problems should have been done years ago; they are here now. They claim to have some of  the highest bio security measures in Europe but the facts would counter that claim. Stricter measures were introduced in March on imports of olive, almond, lavender, rosemary and oleander plants from countries affected with Xyella, a bacterium spread by insects; why so late? The disease devastating olives in Italy and spreading elsewhere is not new, it should have been singled out on the first news of the disease. We are not at the forefront as claimed, the diseases here already prove otherwise.

Scotland in Autumn

We don’t want to lose woodland, it creates and gives so much. Here the late David Bellamy says his bit on it all.

Dr David Bellamy, botanist:

‘I don’t know how long we mortals have stood in reverence of trees but I have been under their spell for a long time – I call them the “Time Lords”.

'Having lived within the demesne of Hamsterley Forest in County Durham for 40 years I must say, my favourite autumn walk is in my own back yard. All you have to do is choose one of the many well-marked tracks that fan out from the meanders of the Bedburn Beck. They’re all there, from the oaks, providing habitats and food for hundreds of species of creepy crawlies; alder beside the rivers, alongside weeping willows; hazel, with its catkins and nuts; rowan, with fruits that make great jam; fungi popping up under the canopy of beech leaves.

'My favourites, birches, were among the first large trees to colonise our isles, as the last Ice Age began to come to an end. Hooray for climate change.’

As I finished this another tree and  similar species comes under threat: the Larch is now in deep trouble with woodland areas nationwide confirming the disease. If a tree is diagnosed with the fungus an area of trees at least a hundred metres around has to be cleared; this alone will hasten the clearing of wood stock.

‘Sudden larch death is a disease of several host trees and plants including larch (all species), beech, chestnut and woody ornamentals including rhododendron, Camellia and Viburnum.  It is caused by the fungus–like organism, Phytophthora Ramorum. In North America the pathogen mainly affects oak and tan oak, giving it the common name Sudden Oak Death. In the UK the pathogen is not a large threat to oak species, and is more damaging to the coniferous plantation species Larch where it causes a disease known as Sudden Larch Death.‘

As I said at the beginning, not all of these diseases go the full cycle but so many are now present and active it is inevitable that large concentrations of woodland will be lost. The replacement is going to be difficult: simply, what do you plant?

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