Hi Tui, why do my rhododendrons have grey leaves and rust on the back of the leaves?
This is a sign of thrips or mites, which are sap sucking insects that cause this damage. Thrips are small black elongated insects, found on the underside of the leaf, there may also be black spots and russeting. Mites leave a fine web or dust and can be seen using a hand magnifying glass as tiny spider-like insects. The best way to control these insects is with a spraying oil or broad spectrum insecticide, you may be able to purchase these at the supermarket, depending where you shop or from your lovcal garden centre or DIY store.
To prevent further spread of these insect pests next season collect up fallen leaves and dispose of in the rubbish or burn, do not compost these leaves as the insects overwinter in the soil and leaf matter. We have had such a dry summer and plants have become stressed, make sure your rhododendrons are well watered through the summer months, and regularly feed with a fertiliser such as Tui Acid Food, and mulch around the trees in spring and late summer to keep them actively growing, this is the best way to help prevent insect pests causing this damage.
Thanks, Tui Team.