Trees are under pressure from a variety of stressors. And although a single issue may cause the poor health of a tree, there are often several causes, from drought and flooding to pests and diseases.

Here are some of the most common symptoms to look out for on trees in your garden or local area, and the pests and diseases that may be causing them. Read our tips on helping trees to stay healthy and flourish.

Leaf damage

Leaves form the natural diet for many woodland creatures and, for the most part, a little damage does the tree no harm at all, particularly if the tree is healthy and vigorous. But several pests can cause significant damage.

If your tree is being damaged by insects you need to establish which species of tree it is, as some pests are specific to particular types. You also need to consider where the damage is – within the leaf, at the edges, or the complete loss of leaves.

Credit: Ian Francis stock / Alamy Stock Photo

Horse chestnut leaf miner

One of the most noticeable pests is horse chestnut leaf miner. This tiny insect eats the inside of horse chestnut (Aesculus hippocastanum) leaves which leads to brown colouring with see-through patches and can leave the whole tree looking severely damaged. But, despite the damage, horse chestnuts often seem to cope well with this pest. If you own a horse chestnut with this problem then it will help to remove the fallen leaves in autumn as the insect overwinters in the leaves and re-infests each spring. Removing and burning leaves will lower the numbers of the pest but it won’t remove it entirely.

Credit: Andrew D. Liston / Senckenberg German Entomological Institute

Sawfly larvae

Sawfly larvae can cause shocking leaf damage. These look like caterpillars but are slightly different because they develop into flies instead of butterflies. They can completely defoliate (strip) small trees in the spring, often just leaving the leaf veins in place. There are over 400 species of sawfly in the UK so most tree species are susceptible to defoliation at some point in their lives. Fortunately, the damage often happens early in the season and trees put on a second flush of leaves without any long-term effects.

Credit: FAPDCN Botany Vision / Alamy Stock Photo

Alder leaf beetle

Another commonly reported leaf-feeder is alder leaf beetle (Agelastica alni). This is a very noticeable beetle because it has a beautiful metallic blue body. It can emerge in large numbers in the spring and start to feed on common alder (Alnus glutinosa) and occasionally on other deciduous trees such as beech (Fagus sylvatica), hazel (Corylus avellana) and hornbeam (Carpinus betulus). Healthy and vigorous trees can usually withstand alder leaf beetle infestation.

Yellow and wilting leaves

Yellow and wilting leaves are a sign of tree stress and so it's important to ensure that the correct tree species is selected for the site in the first place.

If a suitable species has been selected, and the trees are not waterlogged or dried out, then another agent may be at work. There are many disease-causing pathogens of trees. Most are fungi but there are also bacterial and viral diseases.

If this is the case it is important to know which tree species is affected because a lot of diseases are specific to certain trees. The most widespread diseases in the UK right now are ash dieback, caused by the fungus Hymenoscyphus fraxineus, and Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi. If you have an ash tree of any age that is dying back then chances are it's been infected by ash dieback. Dutch elm disease goes for slightly older trees because the beetles responsible for spreading the disease need trunks of a certain size to enter the tree. So if you have a semi-mature elm tree that has dieback then it could be Dutch elm disease.

Another tree species which can have obvious signs of disease is cherry (Prunus species). Their leaves can look brown and wilted in the summer and often remain on the tree in winter. This can be caused by two fungi known as cherry leaf scorch and cherry leaf spot (Apiognomonia erythrostoma and Blumeriella jaapii). These diseases vary in severity year on year but one way to reduce infection is to remove and burn all fallen and hanging leaves in autumn.

Bleeding cankers

Trees can express stress through 'bleeds' on their trunks. These can be induced through excessive freezing, pollution, drought and waterlogging, but can also be a sign of disease.

Probably the most common pathogen to cause bleeds are Phytophthora species. This is a group of fungal-like pathogens which are very well adapted to invading and killing trees. Perhaps the most deadly species currently known is P. ramorum. It infects many trees but most severely larch (Larix species). There are also Phytophthora diseases of juniper, oak and alder. This pathogen blocks the water transport systems of trees leading to black, bleeding cankers on tree trunks and wilting or dying leaves or needles.

Bacteria can also cause bleeding cankers on tree trunks. Horse chestnut trees can be prone to horse chestnut bleeding canker, caused by the bacterium Pseudomonas syringae pv. aesculi. Cherry trees (Prunus species) can also get bleeding trunk cankers caused by similar bacteria leading to sunken, dead patches of bark and small holes in leaves. It is very important to only prune Prunus species in July and August because this is when they are at their most resistant. Pruning at any other time of the year leaves the wounds open to infection.

If a tree in your care has severe symptoms of disease then there are online resources to help in the identification of the issue. A professional arborist may also be able to provide identification and management advice.

Research the tree species that you are planning to plant so that you can match it to the soil and climate in your area.

Check the location of the tree

It is extremely important to plant the right tree in the right place. Native species that can tolerate slightly drier soil include oak, Scots pine, yew, juniper, whitebeam and goat willow. Species that tolerate wetter conditions include alder, alder buckthorn and willow.

Watering tips

A general rule of thumb is that a newly planted tree should be watered regularly while it becomes established, particularly if it was planted in the growing season (spring, summer or autumn) and if you live in a drought-prone area. A good soaking once a week should be enough.

Check moisture levels

If you have the right species but the tree is still struggling, check how dry the soil is.

To do this, push a long screwdriver into the ground to get an idea of how wet the soil is below the surface. If it is hard to push in then it is probably dry and should be watered but if the screwdriver comes back out damp then don’t. Overwatering (waterlogging) can be as harmful as drought so resist the urge to constantly water yellowing trees. Test the soil and act accordingly.

Mature trees can also suffer from water stress. This can be a result of drought, waterlogging or a combination of both. For example, flooding in the spring could lead to a tree sitting in water for several weeks followed by a period of drought through the summer. The tree doesn’t get the chance to build up the reserves it requires to get it through the winter, particularly if these conditions occur a few years in a row. In this situation, mature trees may suffer and decline over time.

Avoid soil compaction

Soil compaction is another issue that can cause mature trees to become stressed. In this case, water acts as a lubricant that increases the density of the soil when pressure is added, like high footfall in a popular park. As the soil particles become tightly packed, the volume of air is reduced. This stresses the tree because trees need air in soil for healthy root function.

Compacted soil also has a reduced rate of both water infiltration and drainage. Soil compaction can be managed by a professional arborist using equipment such as an air spade which pushes air deep into the ground through pipes. A good layer of mulch, such as bark chippings, has also been shown to improve the situation by encouraging worms.

Buy UK sourced and grown plants

Prevention of tree pests and diseases is by far the best approach because managing them can be tricky and often results in the removal of the tree.

One of the best ways to prevent pests and diseases is by careful sourcing of plants in the first place. Most of the more serious tree pests and diseases have been inadvertently imported from overseas.

Try to buy seeds and plants that have been collected and grown in the UK. If you're unsure, ask the nursery about the origins of the trees. This reduces the risk of importing a new pest or disease. The trees we sell are all sourced and grown in the UK and Ireland. Our trees have spent their whole lives in the UK, from seed to sapling.

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