Detecting and treating hedge diseases
Targeted action is required to combat hedge problems. Our guide will show you how to nurse your plants back to health.
12.07.2023
Diagnosing hedge diseases correctly
Healthy, dense hedges are every garden owner’s dream. But like many other plants, hedge plants are not safe from fungal diseases and pest infestations. If discolouration, bare patches or holes develop, it should be possible for you to identify the cause in good time, and to promptly remove and treat possible diseases. In this overview, we will talk about the most common hedge problems and how to combat them.
How to prevent hedge blight and diseases in hedges
- Only work with clean garden power tools. Otherwise, fungal spores and dirt may get into vulnerable, freshly cut plant shoots. Our guide will show you how to clean your hedge trimmer.
- Do not dispose of affected plants in your composter, as this could spread fungi and pests further around your garden.
- To keep your hedges healthy and vital, you should only use hedging plants suitable for your location, and you should regularly trim and fertilise the hedge to help healthy growth.
- Humidity and dampness promote the spread of fungi and pests. Always follow the recommended spacing when planting. This helps the hedge form a dense, but well-ventilated wall. If planted more closely together, the plants will interfere with each other. Water mainly at the root area, keeping stems and leaves dry.
- Fertilise hedges well, but not too much. Use organic fertiliser in spring and potassium-rich homemade nettle or horsetail brew in late summer. This will strengthen the plants and enrich them with sufficient potassium.
- You can help avoid hedge diseases by selecting high-quality plants when purchasing your hedging.
Causes of hedge problems and pests
In general, all hedge plants can be affected by fungi and pests, but weakened plants are particularly susceptible. With the right care and under the right conditions, you can strengthen their defence system and protect it from problems.
Hedge diseases are usually due to the following causes:
Unfavourable location: too much or too little sun; waterlogging; dryness; wrong soil pH value.
- Incorrect care and trimming: too little or too much fertiliser; trimming in direct sunlight; cutting back too far in the case of coniferous wood.
If these causes can be ruled out, you are most likely dealing with a fungal disease or a pest infestation.
Fungal diseases in hedges
Even with proper care, fungal diseases cannot be completely avoided and can damage even robust hedge types. The following types of fungi are particularly common and occur in many plants.
Powdery mildew on hedges
| Symptoms | Dusty white and later, brown, coating on the leaf surface, which wipes off easily. The infected leaves dry out and entire parts of the plant can die. If the infection is advanced, this hedge fungus is transferred to the shoots, flowers and buds. |
| Cause | Powdery mildew spreads quickly in dry, warm weather. |
| Remedy | Remove affected leaves and branches. To prevent further contamination, you should also dispose of any fallen leaves and always keep the ground around the hedges clear. |
Leaf spot
This is an umbrella term for a number of fungal diseases that can only be distinguished microscopically. Fortunately, you can combat fungal diseases even without a detailed analysis.
| Symptoms | Brown, black, red, grey or yellow spots on the surface of the leaf, that vary depending on the infection type. The spots are round, 3-6 mm in size and you can sometimes see a different coloured dot – the actual fungus – in the middle. In severe cases, the spots spread, merge together, and the leaf dries out and dies. |
| Cause | The main fungi responsible for leaf spot are alternaria, ascochyta and septoria. Fungal diseases can occur in dry and warm (alternaria, septoria) or cool and humid (ascochyta) conditions. Spores are cold-resistant and are transferred to host plants by infected seeds or by wind and water. |
| Remedy | If your hedge has a fungal disease, you should cut off affected leaves and branches. |
Leaf spot caused by pests
Leaf spot can also be a sign of pests, such as spider mites. Examine the plants, looking out for sticky substances or bugs on the undersides of leaves and on branches.
Hedge pests
Changes in appearance and decreased vitality may be a sign that your hedge has an insect infestation. Pests, which prefer weakened plants, suck from or nibble at leaves, stems or roots. Insects are usually visible to the naked eye: they sit on young shoots and under leaves or fly up if you touch the plant. The impact of the infestation depends on the type of insect and on how far it has advanced.
Aphids on hedges
| Symptoms | Green, black, brownish or yellow bugs that suck out the sugary sap from the leaves and fresh shoots. This causes the leaves of the plant to curl up and warp. Honeydew is another sign of aphid infestation in hedges. This is a sticky substance excreted by the insects and is also a particularly popular food source for ants and certain types of fungi, e.g. sooty moulds. |
| Cause | Leaf aphids usually attack weakened plants or plants that have been over-fertilised with nitrogen. They are active from spring to autumn. |
| Remedy | It is often enough to spray affected plants several times with a powerful water jet. Useful insects such as ladybirds and ichneumon wasps, birds or plants with intense scents such as lavender and thyme are also helpful. A nettle broth is a natural pest control tool: cover 1 kilogram of fresh nettles with 10 litres of cold water and let it ferment for about 24 hours. Spray the hedges with the undiluted solution. In case of extreme infestation, chemical remedies will help. For the sake of the environment and the beneficial insects in your garden, only use gentle agents such as neem or rapeseed oil. |
Spider mites
Spider mites are tiny but can increase in number rapidly and cause significant damage. These pests can easily be removed or combated with neem oil or nettle brew for severe infestations.
| Symptoms | Tiny insects that only grow up to 0.8 mm. Depending on the season, they will turn greenish-yellow, red or brown. Spider mites live on the underside of leaves and feed on the cell sap, creating light-coloured spots on the surface of the leaf. Some types of spider mite cover the leaves and shoots with a fine white web. |
| Cause | These pests are mainly active during dry, warm spells. |
| Remedy | Plants that are regularly watered, ventilated and fertilised in a balanced manner are less susceptible to spider mites. To combat these pests, spray leaves and shoots from all sides with rapeseed oil or neem oil. This forms a fine film of oil under which the spider mites suffocate. Alternatively, the following mixture may help: mix 50 millilitres of rapeseed or olive oil with 5 litres of warm water. Pour 200 grams of tansy herb (available for infusion as a tea) into the oil and water solution. Spray the plant with the solution. For advanced pest infestations, remove the affected plant parts. |
Black vine weevils
| Symptoms | The weevils leave irregular-shaped notches by eating the edges of leaves, shoots and buds. They are nocturnal and rest under stones or leaves during the day. Black vine weevils grow to about 10-12 mm, with brown, black and sometimes yellow colouring. The cream-coloured larvae cause much worse damage to plants and can even kill them: they live underground and nibble at the roots of the host plant. In particular, they bite through fine roots and gnaw the bark of the main roots down to the wood. The plant can die as a result. |
| Cause | The larvae of the weevil are often carried into the garden with new plants. |
| Remedy | To combat larvae, pour nematodes into the ground; these are available from specialist retailers. Nematodes are tiny, fine worms that poison the larvae of the black vine weevil with a special bacterium but are harmless to humans. You can tackle the beetles themselves by spraying the hedge with neem oil. However, you should not use the two methods at the same time, as the oil is also fatal to the nematodes. To prevent infestation, examine the root balls of new plants for larvae when purchasing them. |
Hedge-specific diseases
Some hedge diseases only occur in certain plant species.
Cherry laurel hedge disease
Shot hole disease (or Coryneum blight)
| Symptoms | From May, red to brown spots will appear on the leaves. Affected areas dry out, and the leaves appear as though they have been punched through with a small shot. |
| Cause | The disease is caused by a fungus called stigmina carpophila, which is particularly active in humid seasons and in rainy areas. |
| Remedy | Remove affected leaves and shoots with a clean, disinfected hedge trimmer. |
Hedge diseases in thuja hedges
In winter, the tips of many lush, green coniferous plants such as thuja discolour and take on a bronze hue – this is normal, and they should change back in summer. If the hedge stays brown, this is a sign of thuja blight.
Leaf spot and discolouration on thuja hedges
Dryness
| Symptoms | The needles gradually turn yellow and brown. The hedge’s growth is inhibited. |
| Cause | Inappropriate location or dryness. |
| Remedy | Adjust the water supply as soon as you notice any changes. |
Waterlogging
| Symptoms | Green shoots that are soft and often mouldy near the root. The ground around the plant is wet. |
| Cause | Waterlogging. |
| Remedy | Cut off affected parts and refresh the ground by adding dry soil. |
Corrosion with de-icing salt or over-fertilisation
| Symptoms | The shoots near the ground turn brown. |
| Cause | Corrosion with de-icing salt or over-fertilisation. |
| Remedy | If the salt-sensitive hedge has been damaged or over-fertilised by road salt, only thorough rinsing and extensive watering of the plant will help. |
Fungal disease in thuja hedges
Cedar leaf blight
| Symptoms | Some older leaf scales turn brown in spring. Once the infected shoots have died, round or oval brown fruits appear in their place. These in turn infect new, younger shoots. |
| Cause | Cedar leaf blight is caused by the didymascella thujina fungus. |
| Remedy | Cut off affected shoots immediately, without damaging the old wood. If necessary, refresh the soil so that it is free-draining and rich in nutrients. |
Branch and shoot dieback
| Symptoms | In spring, individual shoot tips and even young leaf scales turn greyish-brown to light brown. The distinction between healthy and diseased tissue is particularly striking. The bark becomes mostly sunken in at the base of affected shoots. Tiny black fruits can be detected with a magnifying glass. The scales and tips die as a result of fungal disease. |
| Cause | If shoots are dying off, the plant is infested with the kabatina thujae fungus. Acidic soil and a lack of calcium and magnesium encourage the disease. |
| Remedy | Thoroughly remove all affected shoots. Add lime to increase the pH value of the soil. |
Root rot
| Symptoms | Shoots and leaves turn dull yellow within a short period of time before turning uniform brown. The usually cream-coloured bark tissue at the root crown is brownish in colour. The plant is no longer stable and can easily be removed from the ground. |
| Cause | The harmful hedge fungus phytophhora cinnamomi attacks plants from below and destroys the bark tissue of the roots. |
| Remedy | Plants damaged by root rot can no longer be saved and should be removed. You can plant new hedges after completely replacing the soil. |
Pests in conifers
Thuja mining moth
| Symptoms | Small, white-grey moths with brown or black stripes. They are usually visible from June onwards. The tips of the branches often turn greyish in winter and then brown before they die in May. The moth eats into the scales of the thuja, where it lays its eggs. To distinguish the infestation from fungal disease, hold the damaged shoots against the light – in the case of pest infestation, you will be able to see the distinctive tunnels. |
| Cause | Infrequently trimmed hedges are commonly affected. Several mild winters in a row will also promote the multiplication of the moths. |
| Remedy | Cut the hedge before the beginning of June, before new moths can hatch. Don't forget to check for nesting birds before trimming any hedges. |
Bark beetle
| Symptoms | Individual side shoots of the plant dry and die but remain attached to the plant. You will see small holes on affected branches and trunk sections, through which the beetle burrows and builds corridors throughout the plant. The beetle lays eggs in these corridors; the larvae later eat through the shoots and weaken the plant. |
| Cause | The beetle is rare and primarily chooses thuja plants that have been weakened by dryness. |
| Remedy | To combat bark beetles, cut off affected shoots as quickly as possible. |
Summary: Hedge diseases
Weakened plants, in particular, are susceptible to hedge diseases.
The wrong location and incorrect care are the most common causes for diseases in hedges. If these can be ruled out, you are dealing with fungal or pest infestation.You can help to reduce the risk of hedge diseases by only working with clean equipment and choosing high-quality plants when buying.
Do not dispose of affected plant parts in the compost, but instead throw them into your organic waste or residual waste bin. The regulations in this regard differ from place to place, so please consult your local authority for details. Correct disposal will help you prevent the spread of fungi and pests in your garden.
Mildew, leaf spot and root rot are common in many hedge types. Aphids, mealy bugs, cicadas, spider mites and black vine weevils also infest several types of plant.
There are also many hedge-specific diseases: shot hole disease typically affects cherry laurels. Thuja often suffers from cedar leaf blight, branch and shoot dieback and thuja mining moths, and occasionally bark beetles.
