General gardening topics

Common Mahonia pests and diseases and how to treat them

Last updated on April 27th, 2022

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Mahonia plants are very popular for a variety of reasons, most commonly because of the rich, green foliage that serves well as a barrier against intruders as well as the bright, lemon yellow flowers that are produced in the winter. Thankfully Mahonia shrubs are generally free from any pests so you don’t have to head out to your garden with a hose to spray away the aphids or treat your plant against bugs trying to make it their home with pesticides. Occasionally you might find a greenfly and if it becomes a problem you can spray a pesticide specifically on the affected area but it’s not usually an issue, if you do, make sure you read the label for instructions during application.

Unfortunately, they do suffer from one or two diseases, and these are mainly rust and powdery mildew. These are not too serious, however, they are unsightly and can quickly spread if not treated.

Powdery Mildew on Mahonias

If your Mahonia has powdery mildew you will notice spots along the leaves, powdery spots that are white like those in the picture below.

The symptoms are irregular marks with the fungal growth and the lower leaves are more likely to have this issue than the leaves at the top. Cold soil, damp conditions or bad air circulation can result in this fungus. The only way to rectify this problem is to avoid these conditions if possible, which is not always easy with the weather we have in the UK.

Prevention is key with mildew. You will need to make sure your plant gets enough air, has good circulation, and has mulch around the base of the plant to keep the soil warm. If you don’t treat this issue immediately and change the conditions that are causing it can cause your plant to die. It is very rare that this happens though.

Powdery mildew usually starts in the summer and latches onto leaves that are stressed and often dehydrated. The leaves in the buds typically become infected and the signs aren’t initially visible. In the autumn and winter, the infected leaves will fall to the ground but the disease is still present, they will stay there all winter and when the weather gets warmer (in spring) those sleeping diseases or spores will come back again so make sure you remove any fallen leaves.

We recommend spraying affects plants at the first signs of mildew with a fungicide such as Provanto Fungus Fighter Plus.


Rust

If your Mahonia has rust you will notice black spots on the leaves and a yellow colour around the edges.

All varieties of Mahonia can be impacted by rust but the Mahonia aquifolium seems to be affected the worst. The symptoms include dark red marks along the top of the leaves with spots in between. If you turn the leaves over they will likely have the rust coloured powder on the bottom, hence the name.

Rust spreads when conditions are humid and there is very little ventilation. It’s important that you remove any of the leaves that are impacted, especially those that have already fallen to the ground.

If you see that the middle of your plant is very congested and no airflow is getting through it might be in your best interest to prune the plant to allow for better air circulation. You can prune any time of the year without damaging it but it is better to wait until after it has finished flowering in spring. Once you have tackled the root of the problem you can spray any remaining leaves that are affected with a fungicide.

Thankfully both of these can be treated with a garden fungicide that has copper in it.

Remember, prevention is important, providing good air circulation for your plant, and treatment as soon as you see the issue manifest.

Last update on 2022-03-17 / Affiliate links / Images from Amazon Product Advertising API

Welcome to my site, my name is John and I have been lucky enough to work in horticultural nurseries for over 15 years in the UK. As the founder and editor as well as researcher, I have a City & Guilds Horticultural Qualifications which I proudly display on our About us page. I now work full time on this website where I review the very best gardening products and tools and write reliable gardening guides. Behind this site is an actual real person who has worked and has experience with the types of products we review as well as years of knowledge on the topics we cover from actual experience. You can reach out to me at john@pyracantha.co.uk

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