Thompson & Morgan

Important delivery notice
The products on this site are only delivered to UK addresses. If you require delivery to another country please visit one of our other sites below.

diseases - powdery mildew


Powdery Mildew - Diseases


Commonly asked question(s):

- How should I look after an oak sapling which looks like it has mildew? How should I look after a walnut sapling in a pot?
- Is it worth keeping hardy geraniums after they had grey powdery mildew?
- My large box topiary plants have white powder on them?
- What do I do about gooseberry mildew?
- What do I do when my clematis has white powdery mildew on the leaves and stems?
- What is the best course of action for getting rid of powdery mildew?
- Why have my syringa got white residue on the leaves? What should I do to get rid of the white residue on my syringa leaves?


How should I look after an oak sapling which looks like it has mildew? How should I look after a walnut sapling in a pot?

Powdery Mildew is quite prevalent at this time of year (August) and affects a wide range of plants. It is often associated with humid conditions and the plant becoming dry at the roots. It can be controlled by spraying with a fungicide. We would not suggest pruning your tree as this will promote lateral shoots to form and would ruin the shape of your young oak. It will probably defoliate when it is ready to so just make sure you collect up the infected leaves as they drop. We would also repot it in a deep container using soil based compost such as John Innes No.3, and make sure that it is kept moist at the roots. Container grown plants are particularly susceptible to powdery mildew because they are reliant on us for their water source. Try not to position it in full sun all day long. A semi shaded site will slow water loss and reduce plant stress. Look after it, and it should be fine next year.

Repot the Walnut at the same time. Use the same John Innes No. 3 compost and a deep pot. Ideally you should start thinking about where you eventually want to plant it as large trees like this do not do very well in containers in the long term and would much rather be planted in the ground.



Is it worth keeping hardy geraniums after they had grey powdery mildew?

It is definitely worth keeping your geraniums! It sounds as though they were suffering from a fungal infection called mildew. Remove and destroy any old foliage to prevent the spores overwintering and re-infecting your plants next spring. If you notice any powdery mildew on the leaves when they have re-emerged in the spring then spray with a fungicide from your local garden centre (it will say on the bottle whether it is suitable for powdery mildew on ornamental plants).

Powdery mildew thrives in poorly ventilated, humid, damp conditions. Try to keep the plants well ventilated next year and water them from beneath the foliage where possible to reduce the risk of infection.



My large box topiary plants have white powder on them?

It sounds like they are suffering from powdery mildew. The coating of powdery white fungal spores tends to appear under humid conditions at this time of the year, and is particularly seen when the plants are dry at their roots. Although it is rarely fatal on box plants, it may cause them to drop some leaves and can certainly look unsightly.

To treat it, spray the plants with a systemic fungicide on a dry, overcast day. There are lots of fungicides available in garden centres, but you should always check the manufacturer’s instructions first.

To prevent further outbreaks I would suggest keeping your plants well watered (try to water them at the roots and not from above as this is likely to spread the spores). Cut out any heavily infected shoots and burn them. In autumn, remember to clear any neighbouring plant debris away from the area as this is likely to harbour over-wintering mildew spores. Finally, try to maintain good air circulation around the plants by keeping them spaced well apart.



What do I do about gooseberry mildew?

It sounds like your gooseberries are suffering from powdery mildew. The coating of powdery white fungal spores tends to appear under humid conditions at this time of the year, and is particularly seen when the plants are dry at their roots. To treat it, spray the plants with a systemic fungicide on a dry, overcast day. There are lots of fungicides available in garden centres, but you should always check the manufacturer’s instructions first. Obviously if you spray them then there will be a delay of several weeks before it is safe to harvest and eat any fruit. You will need to check the manufacturer’s instructions for the recommended safety measures.

If you are thinking of replacing your gooseberries then try to go for a mildew resistant variety such as Gooseberry ‘Invicta’ or Gooseberry ‘Xenia’.



What do I do when my clematis has white powdery mildew on the leaves and stems?

Powdery Mildew is quite prevalent at this time of year and affects a wide range of plants. It is often associated with humid conditions and the plant becoming dry at the roots. It can be controlled by spraying with a fungicide. There are lots available, just check the manufacturers label to make sure you choose one that is suitable for clearing powdery mildew on ornamentals.

Prune out and burn any infected plant material to reduce the spread of spores, and try to prevent the plants becoming dry at the roots by ensuring that they are adequately watered. You could also apply a mulch in spring to help conserve moisture in the soil throughout next summer.



What is the best course of action for getting rid of powdery mildew?

To prevent powdery mildew it may be helpful to mulch your plants in spring and water them regularly to reduce stress and make them less prone to infection. Removing and destroying the infected parts will control the spread of the mildew but if the infection is over a wider area you may have to use a chemical control. Fungicidal sprays are widely available and will normally specify if they’re effective against powdery mildew. To prevent infection next year make sure you clear away any fallen leaves as the fungus may over-winter on these.



Why have my syringa got white residue on the leaves? What should I do to get rid of the white residue on my syringa leaves?

Powdery mildew can be controlled by spraying with a fungicide. There are lots available, just check the manufacturers label to make sure you choose one that is suitable for clearing powdery mildew on ornamentals. Collect up and burn any infected plant material as the plants defoliate to reduce the spread of spores. Also, try to prevent the plants becoming dry at the roots by ensuring that they are adequately watered. Container grown plants are particularly susceptible to powdery mildew because they are reliant on us for their water source. Once they are large enough, you can plant them outdoors where they will be less susceptible.