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Hello. Our 7 yo Viburnums are thinning out with the leaves turning yellow and then red before falling off. I have noticed white growth on the roots as well. Welcome your advice on treatment. Please see also photos.
Hello @dcsk
Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your Viburnum.
Thanks for sharing the photo and details. From what you’ve described—yellowing leaves that turn red before falling off, thinning foliage, and white growth on the roots—it sounds like your 7-year-old Viburnums are experiencing a combination of stress, likely caused by root issues, possibly a fungal infection, and maybe some nutrient deficiency or minor pest damage.
The white growth on the roots is a key concern. If it looks fluffy, stringy, or powdery, it’s likely fungal mycelium, which often accompanies root rot, especially in overly damp or compacted soil. If the area around the plants doesn’t drain well, or if they’ve been getting too much water (including from lawn irrigation), the roots can begin to suffocate and rot. This weakens the plant, which explains the leaf drop and thinning.
You might also be seeing signs of leaf spot fungus—the speckled patterns and minor blemishes on the leaves in the photo suggest a fungal issue on the foliage as well. This usually isn’t fatal by itself, but paired with root stress, it can push the plant into decline. Some pest damage (like mites or thrips) may also be contributing, though that appears to be minor.
To treat this, start by gently checking the soil moisture. If it feels constantly wet or compacted, it’s a good idea to aerate the area by forking around the base to a shallow depth (around 10–15cm), taking care not to damage the roots. If any roots are visibly mushy or covered in white growth, trim them back to healthy tissue using clean secateurs and discard the infected parts in the bin, not your compost.
Next, treat the plant with a systemic fungicide like Yates Anti Rot (available at Bunnings). This is a phosphorous acid product that gets absorbed by the plant and helps it fight root and crown rot from within. If you’d prefer a gentler option, you could use an eco-fungicide (potassium bicarbonate-based), but note that those work mostly on leaf issues and won’t help much at the roots.
Once the soil is under control, give your Viburnums a boost with a slow-release fertiliser, such as PowerFeed 1kg All Purpose Controlled Release With Troforte. Follow that up with a tonic like Seasol or PowerFeed every couple of weeks—these are mild, seaweed-based and help reduce transplant or stress shock without burning the roots.
Don’t forget to top up the mulch—about 5–10cm is ideal—but be sure to keep it a few centimetres away from the base of each plant. Mulch helps retain moisture and reduce temperature fluctuations, but if it sits right against the stems, it can trap moisture and worsen fungal problems. Also, try to water deeply but less frequently, allowing the top few centimetres of soil to dry out in between.
Lastly, prune away any dead or badly affected leaves to help the plant focus its energy on new growth. Clean up fallen leaves from the ground to avoid reinfection. If your soil is clay-heavy, applying gypsum can also help improve drainage over time. My best advice is to spray your cutting tools with DIGGERS 125ml Isopropyl Alcohol - 125ml to disinfect them.
Please remember to wear personal protection such as gloves, goggles and a mask when spraying your plant.
If you have any other questions we can help with, please let us know.
Eric
