Tree health hunt in the Howardian Hills

By Dr Charles Lane
Consultant plant pathologist, Food and Environment Research Agency, York

I recently took a great walk on a lovely sunny evening in the Howardian hills in North Yorkshire with fabulous views over Castle Howard estate, an area of dense mixed woodland with a predominance of Ash but also Horse Chestnut and Oak.

View of the Howardian Hills, Yorkshire

With the 2014 season of the OPAL Tree Health Survey well underway, I took the opportunity to examine the woodland for signs of the main pests and diseases that the survey is tracking.


There were numerous Ash trees and the area is not too far from trees infected with Ash dieback in East Yorkshire, however, luckily I didn't see any symptoms. Chalara Ash dieback is characterised by the dieback of side branches causing cankers to form on the main stem. In conditions of low light, trees grow very vigorously seeking out light and side branches lower down often die back naturally which can initially look like Ash dieback; but there are no characteristic diamond-shaped lesions on the main stem.

Ash key galls from last year – which were very easy to see earlier this year before leaves broke – are now much more hidden away and yet to form from this year’s growth. A few wilted terminal shoots typical of Ash bud moth were seen, but you need to carefully check wilted shoots to make sure they haven’t been damaged by high wind or physical damage causing stems to break.

Nectria canker, which is characterised by lesions on stems like someone has pressed a thumb into putty, can be seen all year round (my apple tree is covered in Nectria canker!). Ash decline is quite common but you probably need to wait a little longer to make sure trees are in full foliage.

A tree suffering from Ash decline

Some interesting leaf rolling was seen. You can unroll these leaves to see small pure white 'fluffy or cottony' masses of a scale insect. I could also still see the effects of late frost causing blackening and dieback of tender foliage.

Learn more about Ash pests and diseases



I’m desperately looking for – but hoping not to find in Yorkshire – damage caused by acute Oak decline (currently thought to be restricted to mainly the Midlands) but no D-shaped insect bore holes were seen.

I’ve yet to see any damage caused by Totrix leaf roller moth although spring and early summer is the prime time to see this pest in action. I have read a paper from Central Europe that says epidemics normally occur every six to eight years; bud burst timing affects the susceptibility of Oaks with pedunculate Oaks (Quercus robur, also known as English Oak) more susceptible than sessile oaks (Quercus petraea, also known as Cornish or Welsh Oak).

There were no symptoms of Oak powdery mildew in the Howardian hills, but I saw the first symptoms on a young tree in a nearby village that always suffers with mildew. Look at the new foliage on the shady side of the tree, you should see small white diffuse spots typical of mildew.

Also no signs of knopper gall but it’s too early in the season yet. Walking bare foot under affected trees is one way of finding the very sharp and knobbly galls!

Learn more about Oak pests and diseases


Conker trees (Horse Chestnut)

Early stages of Horse Chestnut leaf miner

After waiting for several weeks I am really excited to see the first signs of damage in Yorkshire caused by Horse Chestnut leaf miner. Mines are only very small at the moment (5mm) but will soon start to grow as the insect mines the leaf further. I’d love to know if it has spread north of Newcastle.

I haven't seen any samples of fungal (Guignardia) leaf blotch but it is difficult to separate this from leaf miner especially early in the season.

I also saw the first symptoms of scale insect on trees in central York, but no signs of damage were seen in the Howardian Hills (which is not surprising as it is more common on urban trees growing in stressful situations).

Learn more about Horse Chestnut pests and diseases


Want to analyse tree health in your own neighbourhood?

Take part in the OPAL Tree Health Survey this summer by downloading the free ID guides and instructions. Then just submit your results online or by freepost to help scientists like me track the spread of pests and diseases across the UK, when symptoms appear, and which areas have trees that are unaffected – even a survey which doesn’t find any symptoms is really useful.

And please don’t forget to send in pictures if you do find something – it can really help us scientists to verify what you’ve seen. Good luck and enjoy taking part in the survey!