I love my lemon tree but always seem to get this black blight over the lemons. How can I best deal with this?
It sounds like you have a fungus problem known as sooty mould which is actually caused by scale 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. This mould can cover much of the leaves, reducing the plants ability to photosynthesise and affecting growth and fruit production. To treat the problem; kill the insects by spraying them with an oil-based spray such as horticultural oil - good coverage is important, particularly on the undersides of leaves. Once the insects are dead there is no more ‘food’ for sooty mould to grow on so it will begin to disappear. Any fruit that has mould on it is perfectly fine to eat; simply remove it by washing the lemons with soapy water.
Healthy trees are less prone to pests and diseases so regular watering, feeding and attention is important. Citrus are ‘heavy feeders’ so apply fertiliser just outside the branch ‘drip line’ at six-weekly intervals from late October to early December, recommencing late-February/early-March through to early April.
DALTONS TOP TIP: Citrus can be quite frost tender so protect them with a frost cloth or alternatively spray with copper oxychloride which will help harden leaves and make them more robust in case of early frost.