Why Do Trees Get Sick and Sometimes Die?

What Can Make a Tree Sick?

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.

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
• Bacteria
• Viruses
• Invasive species and exotic diseases
• Some large mammals, like bears and deer
• Human beings

Growing Conditions

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).

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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

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.

Wind Damage

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 Deficiencies

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.

Insect Pests

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

• leafminers
• borers that attack trunks or roots
• browsers that can defoliate trees completely in extreme attacks
• sap suckers

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.

Caterpillars

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!

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 Suckers

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.