Plant Doctor Archive

Black spots on citrus

I HAVE citrus fruit in pots at our back door. Every year the lime tree seems to get a black spot on it which, when I pick it off, seems to be an insect. They appear in clusters and can completely cover some branches and the undersides of the leaves. Sometimes the leaves have a black, sooty look. What is this bug and how can I get rid of it? Also the mandarin has lost all its leaves and just has spiky looking branches left. Is this caused by lack of water?

THE problem is scale, which suck the sap and excrete honeydew, which drops on to the leaves below causing the growth of sooty mould. Picking them off won't control them as the young scales, which you can hardly see, usually greatly outnumber the hard-shelled adults. Spraying is the only efficient way to get rid of a serious scale infestation.

For a non-toxic option, use spraying oil such as Clear White Oil, Conqueror or Super Spraying Oil. You need to make sure you spray it on to the scale underneath the leaves and on the stems. It's best to spray oils on a cloudy day to avoid the risk of leaf burn. Spray the tree before it breaks into new spring growth. The oil is harmless to people, so you can eat fruit even after it's been sprayed.

During summer, if there are still signs of scale, apply another couple of sprays, this time using a product like Confidor or Rogor 100. Once you control the scale, the sooty mould problem will disappear.

It does sound as if the mandarin problem is lack of water, though severe cold damage could also cause it to lose its leaves.

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