mould on citrus
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?
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.
Gardener, Issue 152, 2004, Page 26
Reproduced with permission from the former Weekend Gardener magazine. The views expressed here are not necessarily those of the RNZIH.