Sooty Mould on Lemon Tree
What is wrong with my poor lemon tree and what can I do about it? It looks terrible!
From the photo you supplied, it looks like you have a fungus problem known as sooty mould which is caused by scale and/or other sucking insects and is a common problem on citrus. Carefully inspect your tree; especially the undersides of the leaves and you will see small scale insects adhered to the leaf. These insects suck the sap out of leaves, producing a sticky sweet substance called honey dew which attracts the growth of sooty mould (the black mould you see on the leaves). This mould can cover much of the leaves, reducing the plant’s ability to photosynthesise, affecting growth and fruit production.
To treat the problem; kill the insects by spraying them with a good quality horticultural oil - good coverage is important, particularly on the undersides of leaves. Once the insects are dead and there is no more “food” for sooty mould to grow on, it will begin to disappear.
Healthy trees are less prone to pests and diseases so regular watering, feeding and attention is important. Citrus are ‘gross feeders’ so apply Daltons Garden Time™ Fruit & Citrus Fertiliser just outside the tree’s ‘drip line’ (directly under the outer circumference of the tree’s branches), at six-weekly intervals from late October to early December, recommencing late February through to early April.