TREES IN TROUBLE

TREES IN TROUBLE

During this difficult time of the Covid 19 pandemic, talk of other rampant diseases seems an unnecessary burden, or is it a counter-irritant?

Dutch Elm Disease almost seems like history now, but it is still very much with us. There is probably a generation now who have never seen a statuesque English Elm in all its glory. At the height of the latest epidemic in the 1970’s we went back to our old home in Somerset where the Elm was a major part of the landscape, and were shocked at the extent of the devastation. 

Here in Cornwall the commonest Elm is a suckering species and the course of the disease can be seen clearly as dead and dying trees can be traced down the hedges. Fortunately the disease hasn’t caused an extinction as the vector, the Elm Bark Beetle which carries the spores of the fungus that causes the disease, seems to need developed bark to burrow under to lay its eggs. So seedlings and young suckering growth survive for some years. However it is one of the most serious tree diseases as it kills elms throughout the world.

There is no known cure. The fungus blocks the water-carrying tissue within the tree, causing branches to yellow and die. The fungus moves down the tree, successive branches dying in anything from a few months to 2 or 3 years. It will move along the roots from tree to tree as well as developing further away because of spores carried by the beetle. The disease didn’t arise in Holland. It is named for the Dutch scientists who were the first to investigate it.

 There are several different species and forms of Elms in Britain, some seemingly less favoured by the beetle, but a full-grown Elm now seems to be a rarity.

For many years I had thought that Wych Elm was immune. I now read that it isn’t, so I  have been watching our fine garden specimen anxiously; particularly as it is a major feature, popular with our resident birds and home to an uncommon moth.

 

 The unusual Blomer's Rivulet. 

The caterpillars of this pretty moth feeds exclusively on  Wych Elm, and so its fortunes are closely tied to those of this tree.

 We have caught this moth in the light trap placed near our Wych Elm in the past.

 Also seen nectaring persistently on mint flowers growing very close to our Wych Elm, most lately for several days in July 2019, was the even less common White-letter Hairstreak.

 

 The caterpillar feeds on the flowers, then developing seeds then the leaves of Elms, each skin change coloured to imitate the colour of the food stage it is currently feeding on.

 The Wych Elm in our garden.

To our dismay, just over the past few weeks we see that some branches high up in the canopy are dead and another yellowing, a very ominous sign. It will be a sad loss if indeed this is DED and not 'just' a symptom of distress following the drought  a bit earlier on.

 

In 2002 an aggressive fungus called Phytophthera ramorum appeared in Britain. First seen in  Viburnums and Rhododendron ponticum, infected material spread to Larches, causing Larch Die-back. This pathogen blocks the root system and so in effect causes the tree to die of thirst.

Patches of dead or dying larches in the Alpes Maritime.

             Again, there is no known cure and unlike the species-specific Dutch Elm Disease,  this disease will affect many other tree and shrub species, both commercial and ornamental so there is a serious effect on commercial and social interests. The fungus is spread within soil and mud, therefore by vehicle movements and feet from infected sites. It needs living tissue to produce spores therefore felling is seen to be a measure of control and  Ramorum is a notifiable disease.

Other species are affected by this fungus, including Viburnums and some American oaks, although thankfully neither Sessile or Pedunculate Oak (Quercus petraea and robur) in Britain.

Other Phytophthera species cause disease in trees, one causes Alder Die-back which is widespread in Europe and particularly in SE England. 20% of native Alder is said to be affected in Britain, but growth will regenerate if the tree is coppiced.

 

Another Phytophthera  species is causing immense damage in Australia. This picture shows so-far unaffected Jarrah woodland in SW Western Australia. 

Bull Banksia, one of many Banksia species.

Not only the  indigenous Jarrah forests in SW Australia are affected but also Banksia species and many other plants growing in association with the Jarrah on the sandy soils. Jarrah (Eucalyptus marginata) is a valuable and widely used timber. An attractive richly coloured hardwood, it is used for joinery , furniture and flooring. Because such a wide range of other species in the south west of Western Australia can be infected  by this fungus there is considerable degradation both to the landscape and the ecosystem. It is another soil-borne disease, affecting the root systems and water and nutrient transmission, with no known cure.

 

Ash Die-back is the latest tree scourge, originating in Eastern Asia, it was first recognised in 2006 and first seen in the UK in 2012. It is a chronic disease in Europe, as mature trees can take several years to die, and there are some signs of resistance, although there is no known cure. It is most lethal in seedlings and young trees which will rapidly die as their leaves blacken and wilt. Newly-planted stands of Ash will succumb much more readily than isolated trees in hedges. 

 

Typical signs are this blackened wilting of the leaves especially at the growing tips of twigs, and black lesions on the bark (not to be confused with frequently seen black  canker.) As one of our most abundant trees its loss could be devastating.

The spores of this fungus Chalara fraxinea,  (now named as Hymenoscyphus fraxineus) are wind-borne, and infected wood will also spread the disease. 

 

Difficult to identify in the winter, the incidence of dieback and death shows up now in midsummer when healthy trees are in full leaf. This tree in the upper Inny valley appears to have its current year's new growth killed very recently (early May this year.)

 In the autumn and spring, because of the widely variable time of leaf fall and leaf unfurling, it’s hard to know whether the tree is infected or not, but this summer it is increasingly evident that the disease is widespread though scattered here in Cornwall.

Ash is a widespread hedgerow tree here in East Cornwall, and in our parish there are quite a few really old specimens, some still surviving as one-time coppice trees like this venerable specimen near Luckett.

Old Ash ,once coppiced, in hedge near Luckett.

 Every species has its own ecological web and the considerable loss of major components of woodland worldwide inevitably has a knock-on effect on habitats and biodiversity.

It seems as if the world is suddenly being assailed by huge outbreaks of new diseases. It’s more likely that they are being spread more widely, rapidly and on a bigger scale by the hugely increased worldwide trade in plants of all kinds, commercial and ornamental, and that the diseases are going to places where there is no natural immunity.

I PROMISE MY NEXT POST WILL BE MORE CHEERFUL!