By Dr Charles Lane
Consultant plant pathologist, Food and Environment Research Agency (Fera), York
Earlier this month I was joined by entomology colleagues from Fera, Neil Audsley and Howard Bell, to train a group of undergraduate students from Leeds University as part of their week-long field studies course at Scarborough with Prof Elwyn Isaac.
After induction training on the OPAL Tree Health Survey, we headed out on a hot sunny day to Weaponess Park, which was a short walk from Hull University’s Scarborough campus.
Great fun was had trying to measure the height of trees – at one point we had a new world record measured at 1,180 metres! However, after some recalculation we soon came to something a little bit more sensible at 11.8 metres. The area had some very fine ash, oak and horse chestnut which made surveying for pests and diseases very easy indeed.
The ash trees were generally in quite good health, but due to the quite stressful growing conditions, some ash decline was seen as typified by thin crowns and die back in the lower branches. We saw quite a lot of damage due to ash key gall, but so far we have only seen the galls from last year and keys have yet to be formed. No Nectria canker was seen, but this can be quite difficult to observe when the trees are so tall (perhaps we should bring a pair of binoculars next time!).
There was early excitement from the students when looking at the oak trees after we thought we had found a Tortrix larva as it was suspended on a single silken thread. However, this was soon scotched as it was merely a sawfly larva. In my time carrying out OPAL Tree Health Surveys I have yet to see these elusive caterpillars dangling on silken threads from the branches of oak trees. However, last weekend, I was lucky enough to see a Tortrix caterpillar hanging from an ash on a thread about one metre long swaying in the wind.
The horse chestnut trees were in quite a poor state of health, due to the presence of several large weeping cankers (see picture, right). It can often be quite difficult to decide the cause of these cankers, as both the fungus Phytophthora and the bacterium Pseudomonas syringae pathovar aesculi can cause very similar damage.
The trees are also susceptible to another bacterium, commonly called slime flux or wet wood, which occurs when a tree has been wounded and saprophytic bacteria colonises the wound resulting in production of large amounts of liquid fluid. This runs down the trunk of the tree to cause a large wet patch which, when it dries out, commonly takes on an off-white to yellow colouration.
There are lots of scale insects on the branches (see picture, left) and I am now seeing them regularly on limes and sycamores. This insect pest does not cause any major damage to mature trees but young trees in stressful urban environments may not be able withstand these sap-sucking pests. The scale insect is thought to be native to Asia but was introduced into the London area in the 1960s (as well as France at a similar time) and has subsequently spread north. The pest affects several species of tree and is commonly found on lime and sycamore.
Surprisingly we saw very few horse chestnut leaf mines which in other parts of Yorkshire are causing a huge amount of damage. Driving back through Scarborough there seemed be much lower incidence of this pest on roadside trees, perhaps it's all that fresh sea air? I would be interested if anyone has seen similar differences at seaside resorts.
Everyone had a great day out and the students gave some great feedback, but best of all we now have 14 new reports for the health of oak, ash, and chestnut for Scarborough. I’m looking forward to running the survey next year with a new group of students but also surveying the same trees a year on.
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!