Plant Doctor Archive

Sooty mould on citrus

LAST summer you helped my mother with a magnolia disease and now they are in bloom and looking healthy. But her new problem is with citrus. The leaves had a lot of black sooty mould, so she sprayed them with the same copper spray you suggested for the magnolias. However, they've now lost several leaves and those that are left seem dry and brittle, although still green. She's worried she might have mixed the spray too strong. Is there anything that can be done for the trees?

COPPER sprays can damage soft new leaves and, in strong concentrations, can also affect mature foliage. Gardeners in cold parts of the country often apply copper sprays in autumn to harden foliage which can help many shrubs better survive severe winter frosts. Too much, however, can cause leaves to drop off.

Fortunately, there should be no serious damage to the stems and with the warmer weather we've been experiencing lately I'd expect the trees to break out into new growth before long. Give them a light feed of citrus fertiliser and some water if the soil is dry.

The solution to the original sooty mould problem wasn't to apply a fungicide like copper but to use an insecticide to get rid of the scale insects that were almost certainly the source of the honeydew on which the unsightly black sooty mould was growing. One of the easiest and most environmentally friendly ways of controlling scale on citrus is to give the tree a thorough spray with an insecticidal oil, such as Eco Oil, Conqueror or Clear White Oil. But, like copper sprays, for best effect and to avoid plant damage you need to mix and apply oils according to the label recommendations.

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