Tibouchina leaves are yellowing

What is wrong with my tibouchina? The leaves turn yellow and brown and drop off. It is in a south-facing conservatory but shielded from full sun.

Q: What is wrong with my tibouchina? The leaves turn yellow and brown and drop off. It is in a south-facing conservatory but shielded from full sun.

A: Your leaves arrived rather decomposed, but I suspect the main problem is red spider mite. If you turn affected leaves over and study their undersides, preferably with a hand lens, you’ll probably see the web-like film over the leaf and you might even spot the pest moving about on the surface.
The most effective cure is Provado Ultimate Bug Killer, used either as a spray or watered into the compost. As well as killing red spider, it will control aphids, scale and whitefly, so is a useful all-round treatment. If you prefer biological control, go for the predator mite Phytoseiulus persimilis which is available online from Defenders at www.defenders.co.uk