Plant Doctor Archive

Black mould on citrus

MY grapefruit tree isn't growing well and a lot of the leaves have black stuff on them. I don't like using toxic sprays in my garden. Can you help?

THE sample you sent has a fairly serious infestation of scale under the leaves. If you look carefully you can see small greenish-brown scaly creatures there, 1-2mm across. They suck sap from the leaves which debilitates the plant, causing poor growth and fruit drop. They also exude a sticky excretion, called honeydew, which drips on to the leaves below. The black stuff on the upper surface of the leaves is sooty mould, a fungus which grows on the honeydew. It does no real physical harm to the plant, but prevents light reaching the leaves and so reduces photosynthesis.

If you get rid of the scale the sooty mould will eventually disappear. Spraying is the only efficient way to get rid of such a serious scale infestation. The nontoxic option is to use spraying oil such as Clear White Oil, Conqueror or Super Spraying Oil. You need to make sure you actually spray it on to the scale on the undersides of the leaves. It's best to spray oils on a cloudy day to avoid the risk of leaf burn. I recommend you spray the tree at least twice between now and spring in an attempt to get good control of the scale before summer growth starts. The oil is harmless to people so you can eat the grapefruit even after they've been sprayed.

Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.