Why Do Trees Get Sick and, Sometimes, Die?
Trees can live for a very long time. A Great Basin bristlecone pine that recently died in California was 4,845 years old. Clearly, though, no tree is immortal, and some die a long time before they are fully grown.
If a tree that you care about is obviously struggling to survive, here is a little help in understanding the issues from a biological perspective.
Good gardeners, and anyone interested in tree welfare, can benefit from a little science!
There are also practical tips for identifying and tackling common problems.
What Can Make a Tree Sick?
The Main Enemies of Trees
- environmental stress (water shortage, poor soil, excessive wind, too much or not enough light, for example)
- insect pests that eat or damage tissues
- mites, especially spider mites
- those fungi that can attack living trees
- invasive species and exotic diseases
- some large mammals, like bears and deer
- human beings
Sunlight, water, nutrients and shelter (in some cases) are all that a tree needs to thrive. Too much, or too little of these things can be damaging.
Sunlight is a Dangerous Power Source
Sunlight is strong stuff. It rapidly dries leaves, and UV light will damage most organic material from plastics to human skin.
Some trees, like oak in temperate climates, are adapted to grow in the full glare and these often form forest canopies. They invest costly resources in UV protection and efficient water regulation but reap the benefit of high food production.
Some trees have given up the fight to be the biggest and get all the sun. They do well in the shade, expending less energy and needing to manufacture less food.
This is a list of trees that prosper without too much sun: shade tolerance.
If a well-watered, young tree is doing badly, the first thought should be 'is it getting too much or little light?'
Soil Compaction Chokes Roots
Highly compacted soil prevents both oxygen and water reaching the roots of trees. Without oxygen, the roots are far less efficient at moving water to the leaves.
Roots secrete exudates that include protective antibacterial and antifungal chemicals as well as chemicals that encourage beneficial microorganisms. Beneficial organisms are harmed by poor soil conditions, just as root function is.
How do You Recognize Soil Compaction?
- If water pools near a tree after rainfall it can be a bad sign. Healthy roots use water fast, and aerated soil drains quickly.
- Seedlings cannot take root, and the area is bare.
- You can use a simple rod to probe soil. Often compaction is local, caused by foot or vehicle traffic. Sometimes, you will find that the topsoil is fine, but the subsoil is almost impenetrable.
A soil penetrometer will give you an accurate assessment (see picture, below).
How do you Aerate Soil Around a Tree?
Soil compaction around established trees is not easy to treat, so it is important when planting trees, to make sure the soil has a good, open structure.
Older techniques for fixing compacted soil around established trees include:
- Mulching around the trunk to preserve soil moisture and reduce the impact of vehicle and foot traffic.
- Drilling small holes around the tree and filling them with material that allows air and water to penetrate, like peat or porous ceramic.
- Radial trench treatments that involve digging a trench around the trees and back filling with a good quality soil. This works well but is time consuming.
New and faster treatments include the 'air spade' shown in the video below. Air is injected into the ground, opening and loosening soil.
Waterlogged soils are worse for roots than compacted soils. Oxygen is prevented from reaching roots, potentially dangerous fungi are encouraged and toxic chemicals can be produced by anaerobic bacteria.
If you dig holes for posts or planting, and they fill with water, this can be a sign your garden needs some drainage.
Alternatively, you can find trees that tolerate waterlogged soils. These include particular varieties of willow, ash, cedar, birch and maple, so there is plenty of choice.
Many important trees are adapted to grow in the lowest level of forests which are sheltered from strong light and strong winds. Often they cannot survive in exposed or isolated situations. This includes many palm trees and species like magnolia and rhododendron, especially in hotter regions.
Wind damage is easy to spot. Saplings and more fragile trees need the shelter of stronger trees, gullies, hills or walls, if they are going to survive. Support posts can help.
Nutrient deficiency will slow the growth of trees. Trees need nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur as macronutrients.
Other elements, like iron, are needed in smaller quantities but are very important. Sweetgum trees, for example, are prone to severe leaf chlorosis (yellowing) if starved of iron. which can lead to leaf loss.
Issues vary from region to region but this is an interesting overview of the problem: https://extension.umd.edu/hgic/topics/nutrient-deficiency
The war between insects and trees has been going on for a very long time. Trees have developed formidable defenses against insects that want to eat them or make a home in their tissues. This includes:
- tough bark that is indigestible and has little value as food
- poisonous chemicals like nicotine, pyrethrum and neem extracts
- indigestible inner tissues like lignin
But some insects have fought back and found ways around or through these defenses.
Insect Pests Include
- borers that attack trunks or roots
- browsers that can defoliate trees completely in extreme attacks
- sap suckers
A Root Borer
Root eaters are not always easy to spot, but the citrus root weevil, pictured above, is conspicuous as an adult, as it feeds on the leaves of fruit trees.
It is the larvae (young weevils) that do the real harm, though.
An adult female weevil can lay 5000 eggs in leaves that she carefully folds together to form a temporary home. When the young emerge, they fall to the ground and start boring into the tree roots.
If they eat the taproot, the tree can die.
Most caterpillar attacks are not serious. Even large numbers of individuals will not seriously harm a tree if the infestation occurs late in the growing season. The tree will already have withdrawn many of the nutrients from the leaves.
Infestations in spring can be more serious and attacks on fruit trees are bad news for farmers.
Ways to Control Caterpillars
- prune affected leaves, especially those rolled or with webs (these often contain the eggs)
- pick, brush or wash caterpillars from leaves and then kill them.
- if the above is impractical there are chemical and non-chemical insecticides
- spray with a safe, microbial-based insecticide like Btk (Bacillus thuringiensis kurstaki) and you will only kill caterpillars, not beneficial insects like bees.
Remember that if you like butterflies in a garden, you might have to live with their caterpillar stage!
A Leaf Miner
Leaf miner insects burrow inside a leaf, eating the inner tissues but leaving the cuticle (outer covering) intact. They usually leave a distinctive pattern, as shown above.
Leaf miners rarely cause serious harm, although they can cut yields from fruit trees.
Mostly, they are an annoyance that makes a tree look less attractive.
This page has some useful tips for dealing with them: leaf miner control
Sap-sucking insects like aphids, scale insects and mealybugs are relatively harmless on most trees.
They can be very unsightly though, and in large numbers are harmful.
The video below explores the damage done by leaf curl aphids to ash trees.
Spraying with soapy water is often an effective remedy for aphids.
A Trunk-boring Mega-Killer
Emerald Ash Borer
This beetle has killed tens of millions of ash trees in North America in recent years and threatens to wipe out almost every variety of ash tree.
Emerald Ash Borer larvae are large and burrow into the trees inner tissues, disrupting the flow of water and nutrients and allowing fungi to affect the vital heartwood.
Quarantining infected areas and breeding disease-resistant strains, are the main strategies to resist the onslaught.
Viruses are one of the main threats to human food crops, with examples including the potato blight virus that caused the devastating Irish potato famine and the cherry leaf-roll virus that damages fruit trees.
A common sign of viral attack is yellow or very pale-green foliage.
The best way to avoid virus problems is to buy trees from reputable suppliers. If you are taking a cutting from a neighbor's property, examine the tree carefully.
If a tree is infected, pruning and burning affected branches is the best approach. In an orchard, removing the whole tree can be the best hope of containing the disease.
Compared to viruses and fungi, bacteria are minor villains in the tree-care world.
Bacterial canker of plums, cherries, apricots and peaches is one of the few serious infections with a commercial impact.
Areas of bark die, sometimes killing whole branches and allowing fungi to take hold.
Cutting off infected branches is the standard treatment.
A few other infections of note:
- crown gall
- fire blight
Mites are close relatives of spiders but are so small they can be hard to see with the naked eye. Spider mites have more visual impact, producing distinctive webs that can coat whole trees.
There are many important pest species, mostly feeding by sucking sap from tree tissues .
Apple rust mites attack apple trees and, sometimes, pear trees in the US Northwest.
Spruce mites can destroy the appearance of spruce trees and in serious cases, kill them.
This page explores some of the commercially important mite pests: http://ipm.ucanr.edu/PMG/C107/m107bpmites.html
Clumsy pruning, root removal, soil compaction from cars and feet, can all threaten trees.
Sometimes, you are your tree's worst enemy.
Pests and diseases find it hard to gain entry to the food-rich interiors of trees. If you prune branches, or remove roots, without due care, you will be opening a door to invaders.
Take care to consult proper pruning advice for each species, including the best time of year to do the work and how to minimise damage.
Fungal Attacks on Trees
Most fungi only grow on trees that are already dead or dieing. A few, like Amarilla species (one is pictured above) can attack living trees and parasitise them.
But even Amarilla cannot easily attack a tree unless it is damaged, somehow.
Any breach in a tree's outer defences whether caused by insects, grazing animals, pruning by humans, or fire can provide an entry point.
Some fungi can attack roots, especially if they have been weakened by being water logged for a long period.
Fungi are the only living things that can digest the tougher structural components of wood, like lignin.
They produce enzymes that dissolve the wood and then absorb the nutrients for their own growth.
Mushrooms and bracket fungi can be often be seen emerging from severely infected trees. It is the mass of tube-like fungal growths inside the tree that do the harm, though, digesting the tree from the inside, out.
Dutch Elm Disease
Dutch Elm Disease was first identified in Holland in 1921. It has since killed millions of elm trees in Europe, with 25 million dying in the UK alone, Ninety percent of French elm are estimated to be gone. Around 75 percent of North American elms have died, too.
Bark boring beetles carry microfungi from elm to elm and the fungi kill the trees.
There are ways of protecting important elm trees with chemical and biological agents, but the main effort to reduce elm decline is focussed on felling infested trees and breeding disease-resistant varieties.
Invasive Species and Exotic Diseases
The Emerald Ash Borer and Dutch Elm Disease. described above, are both examples of the threat that exotic diseases pose to trees.
In the natural world, parasites and diseases are involved in a constant struggle to overcome the defences of the hosts that they feed on. The hosts evolve ways to develop resistance strategies or they die out.
When a disease or pest arrives from another part of the world armed with strategies that the tree has never encountered before, there is not time to evolve a winning response.
Both Dutch Elm Disease and the Emerald Ash Borer originated in Asia and were transported around the world by human beings in commercial timber shipments.
Chestnut Blight, another invader, has almost destroyed the once common American Chestnut tree, in large areas.
Butternut Canker is reeking havoc in northern forests.
In other instances, the invasive species might be a tree that arrives in a foreign location and, because it has no natural enemies, like insects or fungi, it is able to outcompete native species.
In the UK, Sycamore, a native of central, eastern and southern Europe has managed to take over large areas of woodland, suppressing local trees that were part of a wide web of life and reducing bird and mammal diversity.
Any animal that eats the leaves of trees can be a problem for saplings. Older trees are only be seriously affected by animals that strip bark from trunks.
- Deer rub bark from trees when they condition their antlers.
- Black bears are notorious for stripping bark from trees.
Beavers will fell trees with their sharp canine teeth in order to dam rivers and build homes for themselves and their young.
The mystery of trunk damage in, or near, wooded areas can often be solved by checking for animal tracks and droppings.