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Wisteria is generally a problem-free climber, with the most common complaints usually being a lack of flowers or wilting and yellowing leaves. But like all plants, there are a few pests and diseases to watch out for. Some of these can cause your Wisteria to not flower or to die back, so they cannot be ignored.
We have put together a guide to help you identify what’s wrong with your Wisteria and what you can do about it.
Scale insects were first found in Britain in a garden in London in 2001. These insects, with soft bodies beneath hard shells, suck the sap out of the leaves on a Wisteria. They leave large, brown-black scales on the stems of the plant.
Wisteria scale insects are larger than other types of scale insects. You often find mature insects in the late spring on the stems of the Wisteria. These insects often develop heavy infestations and thickly cover the stems. They deprive the Wisteria of energy and the plant dies back.
What to do
If you have only a few scale insects on your Wisteria, you can ignore them. Healthy plants can handle light infestations of these insects, the trouble arises when many of these insects move in.
Try an organic spray, such as natural pyrethrum, in May or June when the insects are newly hatched and vulnerable. You need to keep reapplying such types of pesticides as they have a short life. For a heavier infestation, use an insecticide that contains the organic compound acetamiprid, such as Bug Clear Ultra.
Always follow the instructions on the bottle of spray or insecticide.
Vine weevil grubs eat the roots
Vine weevil grubs are often found in the roots of Wisteria plants that are being grown in containers. These are insects of which the adults eat the plant leaves and cause notches around the edges; the grubs are the ones that actually cause the most damage because they snack on the roots.
The adults appear in the spring to late summer and the grubs in the summer through to the next spring. The adult vine weevils are about 9mm long and are dull black with yellow marks on the wing cases. The grubs are white and legless and grow up to 10mm long.
Plants can wilt and die in the autumn and winter because the grubs eat the roots.
What to do
Keep a look out for the notches on the leaves in the spring and summer to catch the adult weevils before they produce the grubs. Pick off the adult weevils by shaking the shrubs over a newspaper. Use sticky barriers around the plants to trap the adults. Encourage birds, frogs and hedgehogs into your garden as these are natural predators of vine weevils.
As for insecticides, use one such as Bug Clear Ultra Vine Weevil Killer as a liquid applied to the compost. This is an insecticide that contains acetamiprid, an organic compound. This gives your plant four months of protection from grubs. Treat the plant with this in mid-to-late summer to cover the autumn to spring period.
Powdery mildew is a common fungal disease that can be difficult to spot on Wisteria. It usually results in a white dust-like coating on the leaves, stems and flowers, we recommend keeping a close eye on them from spring onwards. You may also find irregular brown blotches ringed in yellow on the leaves as a symptom of this disease, or you might notice the leaves wilting.
Despite it being unsightly, luckily powdery mildew doesn’t often cause significant damage to the plant.
What to do
Remove any infected leaves from the plant and destroy any fallen, infected leaves in the autumn so that the spores won’t spread during the winter. Make sure that the plant is in its best environment so that it grows to be healthy and strong. Ensure good air circulation around the Wisteria and that the soil is well-drained.
As for chemical solutions, you can use fungicides on the leaves to target the fungus. You can also consider using a combination of insecticide and fungicide if you also have a pest problem.
Phytophthora root rot
This is a common cause of root and stem decay in many plants, including Wisteria. It occurs from microscopic fungus-like organisms that cause the root and stem bases of plants to decay. The organisms live in the soil where they may hang around for several years.
The roots of your Wisteria will become decayed way before you notice any symptoms above ground. The root problems are not exclusive to root rot and result in the plant not taking up enough water and nutrients. You may notice that the leaves are yellowing or wilting and the branches are dying back.
If you examine the roots below ground, you will see that the root system has been disrupted. Many of the smaller roots rot away and all show evidence of decay.
What to do
Expose the roots and cut away all that is decayed or dead. If this leaves you with very few roots, then you will need to pull up the plant and destroy it. If you can save enough of the Wisteria, rework the soil so that it drains well and doesn’t become waterlogged. Standing water is a prime environment for phytophthora root rot. There are no chemical treatments for this disease.
Honey fungus is a collection of different species of fungus that attacks a plant’s roots. It can be quite difficult to spot because its main symptom is a white fungal growth between the wood and the bark of the plant’s trunk mainly at ground level.
You may notice groups of honey coloured mushrooms on infected stumps in the autumn. This fungus can cause your Wisteria to die in parts, or totally. The leaves may be pale in colour, be smaller than expected and they may also wilt, on top of this, the plant may not produce any flowers either. Below the ground, the roots begin to decay and die.
What to do
Stopping the spread of this fungal virus between your plants is the main step in its control. Place a barrier of pond lining material between the plants below the soil so that it also sits 2cm to 3cm above the ground. That’s about it for what to do – there are no chemical controls available to help with this problem. You may need to remove and destroy the entire plant if it becomes heavily infected.
The coral spot fungal disease may make an appearance when your Wisteria is already weak and sickly. After a branch of the plant dies, small coral-pink spots form. This indicates that the plant has a lot of other problems.
What to do
When this happens, you probably know that your Wisteria is in trouble. Treat as many of its problems as you can. Prune out any infections and dead material as soon as possible.
I have also covered some other Wisteria problems in this guide.