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Hi. My Emperor Mandarin plant is looking very sick. It has a black coating on leaves and stem and is no longer growing. I am worried about cutting it back this time of year (far South Coast NSW) as we are now getting frosts, but sunny days.
Hello @KM22
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your mandarin tree.
Your Emperor Mandarin is showing clear signs of sooty mould, which is the black coating on the leaves and stems. This mould forms on the honeydew excreted by sap-sucking insects like aphids, scale, or whiteflies, which means the underlying problem is likely an insect infestation. The mould itself doesn’t directly harm the plant but can block sunlight, reducing photosynthesis and overall plant health.
Given the frosty conditions on the far South Coast of NSW, you’ll need to be cautious about pruning, as it can stress the plant further. Instead, focus on controlling the pest problem first and cleaning up the existing mould.
Start by gently washing the leaves with a hose or a damp cloth to remove as much mould as possible. For the insect control, you can use an eco-friendly, frost-safe product like OCP 750ml Ready To Use Eco-Oil or OCP 750ml Ready To Use Eco-Fungicide, both available at Bunnings. These are low-toxicity options that won’t harm beneficial insects like bees but will target the sap-sucking pests effectively. Apply them in the morning on a dry day to avoid frost damage overnight. Make sure to spray under leaves as well for full coverage.
Once the pests are under control, the sooty mould should stop spreading, and you can trim back any heavily infested branches in late winter or early spring, just before the new growth starts. If the plant is in a pot, consider moving it to a slightly more protected spot for the winter to reduce frost exposure.
Please remember to wear personal protection such as gloves, goggles and mask when spraying your plants.
Here is a handy guide that should help: How To Plant, Grow And Care For A Mandarin Tree
If you need further assistance, please let us know.
Eric
