Apple tree pests & diseases: the most common apple tree problems

Just like any other plant, apple trees are also not safe from diseases. We compiled a list of common apple tree problems which should help you identify the type of pest or disease plaguing your apple tree.

It is a harsh gardening reality but every plant in the garden can develop some sort of issue be it pests or diseases that harm the plant. Not even the apple tree (Malus domestica) is spared. Under certain circumstances, however, pest infestation or plant disease can easily be prevented with precautionary measures. Unfortunately, there are still cases, when even prevention is not enough. However, if you detect the problem early on, you may be able to stop further spreading by acting quickly. In the following, we will show you how to counteract pest and disease infestation and how to identify the most common apple tree problems before it is too late.

Apple tree problems: what makes apple trees vulnerable to pests and diseases?

Some factors can increase an apple tree’s vulnerability to diseases and pests. If the crown of the tree is too dense, for example, it can increase the likelihood of an infestation with harmful insects and fungal pathogens. Dense crowns do not dry well. The warm and humid environment is a breeding ground for apple pests and diseases. Excessive fertilising promotes growth and thus crown density. Additionally, the fruit juices of the apples can become overly enriched with nutrients if the fertilisation is overdone which attracts insects. Metabolic diseases are also promoted by disproportionate fertilisation. For example, calcium deficiency is caused by an excess of nitrogen fertilising.

We recommend finding out beforehand from your neighbours or from a tree nursery you trust which apple diseases (such as apple scab or fire blight) are common in your area. The most effective protection against crop failure due to such diseases is to select resistant apple tree varieties.

You can protect your apple tree from increased susceptibility to disease by taking the following simple measures:

• Select resistant apple varieties
• Do not over-fertilise
• Cut back and thin out the crown of the tree regularly

Apple tree problems: the most common insect pests

There are many types of insects that are quite helpful to the tree as well as the gardener. Bees pollinate the trees, ladybirds eat the aphids. Besides aphids, there are also other species of insects that can damage the apple tree. Here is a brief overview on harmful insects:

Apple ermine moth

The apple ermine (Yponomeuta malinellus) is a small white moth with black spots. However, it is not the flying moths that cause harm to the apple tree but the caterpillars. In spring, the infestation is recognised by the white webs in the apple tree. The caterpillars of the apple ermine moth hatch in summer. Before that, they live under the protective webbing layer. As soon as temperatures consistently climb above 12 °C in spring, the caterpillars awaken and satisfy their hunger on the tree’s tender buds and young leaves. The tree can be completely defoliated by these voracious insects. Thankfully, this does not cause permanent damage to the tree.

The white webs are formed after the caterpillars shed their skin for the first time. Mild winters can encourage stronger infestations, because the caterpillars are more likely to get through the cold season if the temperatures are warmer. It is not absolutely necessary to combat the apple ermine moth, as it does not cause lasting damage to the apple tree. Even a completely defoliated tree will very likely sprout new leaves again in the same year – although fruit is not to be expected at harvest time.

What to do against the apple ermine moth?

• Simply remove the caterpillar webs from the tree
• Natural control: predatory chalcid wasps
• Chemical control: available in gardening shops; read the instruction manual well before application
• Prevention: female apple ermine moths cannot fly; a glue ring trap will prevent them from crawling up the trunk

Codling moth

The codling moth (Cydia pomonella) is by far the most common pest on apple trees. They are brown-grey moths whose caterpillars are only two millimetres long and white or yellowish with a dark head. The infestation can be detected by the tiny holes the larvae drill into unripe apples. Infested apples are no longer storable and often fall off the tree early.

What can you do against the apple moth?

• Remove infested apples as early as possible
• Pheromone traps followed by granulovirus spraying or natural control with granulovirus preparations (application three times at eight-day intervals)
• Prevention: check tree trunks for pupated codling moth larvae already in winter and spring and collect larvae if necessary

Aphids

Not all aphids are the same. The apple tree is particularly targeted by apple grass aphids (Rhapalosiphum insertum) and various other apple aphids (Dysaphis spp.). An infestation with aphids shows through leaf curl and malformations of the leaves. Aphids are often to be found on younger shoots where there is more of the sweet plant sap. Excessive fertilisation leads to increased number of shoots with a spongy tissue and in turn enhances the chances of an aphid infestation.

What helps against aphids on apple trees?

• Support beneficial organisms in your garden: natural predators of aphids include ladybirds and earwigs
• Avoid the use of chemical pesticides inside the home and in the garden if possible
• Prevention: remove infested branches during summer pruning; avoid mono-cultures and do not over-fertilise

Tip: Place upside down clay pots in the apple tree. Fill them with some dry leaves or wood chippings – this will attract earwigs, natural aphid predators.

Apple tree diseases: the most common fungi and bacteria

Apart from the insects that can cause apple tree problems, there are also fungi and bacteria that can inflict damage to fruit trees. In order to prevent the spread of the pathogens, it is vital to identify them as soon as possible.

Apple scab

Apple scab (Venturia inaequalis) can already be recognised in spring in the case of susceptible apple varieties. It can be detected through olive green to brown spots on the leaves of the tree. These dry out from the middle and turn dark brown. The leaves become wavy or bumpy because the diseased leaf tissue stops growing. The infected leaves fall to the ground early. Particularly severely infested apple trees are therefore almost bare as early as August. The apples also show brown, often torn wounds. They are still edible, but less easy to store. Rotting fungi can penetrate more quickly through the cracked skin of the fruits. Apple scab can be caused by mild and rainy springs.

Apple scab treatment

• Remove infected leaves immediately
• Spray treatment with approved fungicides (seek advice from specialists)
• Prevention: removal of leaves in autumn, pruning to thin out the crown; using siliceous horsetail broth as a preventive tonic (forms a protective film); sulphur preparations
• Scab resistant varieties

Powdery mildew

Powdery mildew is yet another one of the best-known diseases in apple trees. The fungus spreads at breakneck speed and can sometimes lead to a total failure of the apple harvest. Symptoms of powdery mildew can include mealy coating on the branches, white and fluffy spots on the leaves and net-like cracks in the colour of the fruit.

What can be done about powdery mildew?

• Thorough pruning to keep the crown well aerated
• Cut out infected areas generously and preferably burn the infected plant material
• In case of a heavy infestation, use of approved plant protection products after expert advice

Fire blight

Fire blight is caused by the bacterium Erwinia amylovora. Blossoms and young shoots are usually most affected. They turn dark brown or black and look as if burnt by fire. Sticky bacterial slime can then appear on the infested parts of the apple tree. Fire blight can easily be transmitted by insects but can also be spread to other plants by humans. Since the course of the disease is so tragic, the transmission so simple and the spread so rapid, the infestation with fire blight must be reported to proper authorities in your area. If you discover a plant that clearly shows the symptoms of fire blight, do not hesitate to inform the responsible plant protection office after consulting with experts.

What can be done against fire blight?

• Infested plants cannot be treated
• Report the infestation to the proper authorities in your area
• Infested plant material should be burned
• Select of fire blight resistant apple varieties

How susceptible to diseases an apple tree is, depends on the variety. Find out what apple varieties are best for your home garden here.