Thuja: Detect and fight diseases and pests


The brown color is the most common sign that something is wrong with the botanical thuja. You can find out which diseases are behind it here.

© Mariusz Blach -

The botanical thuja is a very robust plant, so it is a very popular hedge plant in home gardens. You can tell whether the thuja is healthy from its leaf color. If the leaves glow in a lush green and even the small leaves that grow back have a green color, you can assume that the plant is healthy. If the thuja has a problem, whether it is disease or pest infestation, its leaves turn brown. The brown color is not to be confused with the natural brown color of some species in winter. However, if the botanical thuja gets brown leaves in summer, it can be assumed that the plant is not healthy. There are several pests that can harm the plant and the thuja is also not safe from diseases.

What diseases and pests are there?

Although the botanical thuja is a very robust plant and is rarely affected by diseases and pests, there are some negative influences. Basically, the pests can be divided into the following categories:

  • fungal diseases
  • Pest diseases
  • Root rot and stem rot
  • external influences

Fungal diseases of the botanical thuja

Fungal diseases are the most common diseases of botanical thuja. Since most types of thuja turn brown in winter, the fungal attack is often not immediately apparent. There are four main types of mushrooms:

  1. Pestalotiopsis funerea
  2. Didymascella thujina
  1. Kabatina thujae
  2. Armillaria mellea

❶ Pestalotiopsis funerea

This mushroom triggers the well-known Pestalotia shoot death. This is the most common disease of botanical thuja, but is very easy to spot. The characteristic clinical picture shows a brown color, which starts from the outer tips of the leaves. The color continues to spread until it reaches the shoots. Black-brown spots form on the shoots, which are typically round and about the size of a pin head. This fungus prefers soft, already impaired tissue, because the fungus is a weak parasite that can damage the plant considerably.

❷ Didymascella thujina

This mushroom is known to trigger the needle and dandruff tan. Mainly older leaf scales turn brown. At first only a few brown spots are visible. These continue to spread until spore deposits can be seen later. These show a black color and the shoot quickly dies.

❸ Kabatina thujae

This fungus mainly affects the young leaves and the soft shoot tips. Here you can see black spore beds, which are on brown leaves. This mushroom can be distinguished from the Didymascella thujina by its sharp, black demarcation from the healthy leaves.

❹ Armillaria mellea

This fungus is also known as Hallimasch infestation. Here the plants die very quickly and can hardly be saved. The mushroom triggers a typical white mushroom network that is very easy to recognize. The braid lies between the bark and wood and is therefore so dangerous for the botanical thuja. It literally destroys the plant at the core.

What to do in case of fungal attack?

The first measure should be a radical cut back. The affected shoots should be cut generously, whereby the old wood is not cut. If the plant is affected by Armillaria mellea, the use of fungicides can be helpful. However, this is only the case if the disease is recognized early. Most often, a fungal attack is noticed very late and the plants, despite fungicides, no longer recover. Then the whole thuja has to be dug up and new bushes have to be buried. Fungus often occurs when the soil is too acidic and the plant suffers from a lack of magnesium and calcium. Therefore, when setting the thuja, you should make sure that it is a loose, nutrient-rich soil. This is how you can efficiently prevent fungal diseases.

Pest diseases

Although the botanical thuja is very robust, some pests can have a major impact on your health. Although there are only a few pests that attack the botanical thuja, these animals can do great damage. The enemies of the thuja hedge are the thujamin moth, the bark beetle and the spider mite. They trigger different symptoms.

➀ The Thujamin moth

The thujamin moth is the most common pest that can cause massive damage to the thuja. The moth was brought in from North America as early as 1970 and can often be found in the home gardens. The clinical picture that causes it is quite easy to identify. The moth primarily affects the spike. These turn brown. If the spike is not treated, complete calcification can occur. The Thujamin moth strikes mainly in spring. It eats in the thuja scales. If you look at the tips of the shoots, you can see the typical bores of the moth. The moth eats through the shoots and builds its passages where the approximately 4 mm small larvae are.

In order to effectively get rid of the thujamin moth, the affected shoot tips must be cut radically. There is no other method of treatment. Please dispose of the trimmed shoots outside the garden, because the moth starts to fly in July and can spread very quickly.

➁ The bark beetle

The bark beetle creates small holes through which it builds its passages. The adult beetle, which is about 2-3 mm in size, penetrates the plant and lays its eggs there. The bark beetle's seat can be recognized by small thickenings on the branches. The larvae hatch and eat their way through the shoots. This leads to the final death of the drive. The bark beetle primarily attacks weak plants and usually occurs after an extremely dry period.

Even if the bark beetle is infected, only a radical pruning can lead to success. The affected branches and shoots must be removed. The branches should not be disposed of in your own compost, because the bark beetle can spread very quickly.

➂ The Thuja spider mite

The spider mite rarely attacks the botanical thuja. The infestation takes place mainly in dry and warm weather. The spider mite sucks on the leaf scales. This creates speckles in yellow or silver color. Since the spider mite is only a few millimeters in size, it is often overlooked and the clinical picture is mistaken for a lack of nutrients. Webs of thread can often be recognized on the affected shoot tips.

If the botanical thuja is infested with spider mites and has been recognized, treatment is very easy. The small animals are very sensitive to all fertilizers. You can therefore treat the thuja with a pest-free spray. This is healthy for the plant itself, but kills all adult animals and also the larvae.

1 Celaflor Pest Free Careo Spray - 400 ml 58 reviews10,99 € 9,61 €To the shop
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Useful information about prophylaxis against pests

If you already had the problem that the thuja was attacked by pests, you can use a pest-free spray for prophylaxis at the end of June. The advantage of these sprays is that they work throughout the system. This means that the active ingredient penetrates inside the plant and is distributed throughout the plant. Stems, shoots and leaves absorb the active ingredient and are therefore robust against pests. Most pests suck or eat from the plant. They absorb the active ingredient and die.

Diseases caused by external influences

Botanical thuja also often results from external influences. Dryness, but also moisture and salt can damage the thuja.

■ wet

If the botanical thuja is constantly in the wet earth, stem and root rot occurs. You can recognize this by the white coating. Thuja likes it moist, but the stem and roots should be dry. If the rot has affected the stem and roots, the plant can no longer be saved. Before a new plant is planted, the soil must be replaced generously.

■ drought

If it is over 30 ° C in summer, the botanical thuja must be watered regularly. If the plant suffers from a lack of water, the leaves turn brown and the plant dies. Even in winter the water evaporates and the thuja has to be poured. This should only be done on frost-free days, because the water should never freeze on the shoots.

■ salt

The road salt in winter is an extreme stress factor for the botanical thuja. Therefore, make sure that the road salt never gets to the stem and roots of the plant. If this is the case, the plant must be well watered to dilute the salt.

The botanical thuja is a very robust plant that has few enemies. However, if the plant is infected by a pest or fungus, quick action must be taken. This ensures that the diseases cannot spread and other plants are spared. A radical cut can have the desired effect in any case and the thuja can quickly recover. If you observe your plants, changes in the shoots can be recognized very quickly. This is the prerequisite for a long life of botanical thuja.