Black, or sooty, mould on citrus trees is the result of insects attacking the branches and leaves. As the insects feed on the tree, leavings drop onto the leaves below them and begin secreting a waxy coating as they dry. Black mould also occurs in trees with excess foliage, especially trees in shade. While this mould is not inherently harmful to the tree, mould thickly coating the leaves prevents sun absorption and can cause the plant to sicken. If the tree does not die from lack of sun, it will produce inferior fruit, or no fruit at all. Additionally, mould on the fruit can trap moisture on the peel, causing fungi that will eat away at the fruit. A number of sprays will eliminate sooty mould on your tree. Organic solutions are the best for getting rid of mould. These have no noxious chemicals that may damage fruit or the tree itself.
Mix only one of the following: 2 tbsp of tea tree oil to 2 cups of water, 20 drops of grapefruit seed oil to 2 cups of water, 1 cup of apple cider vinegar to 2 cups of water, or 2 tsp of hydrogen peroxide to 3 cups of water.
Fill a garden sprayer with the chosen mixture. A garden sprayer is a nozzle for your garden hose with a plastic bottle attached to its underside. As you spray, the bottle's contents mix with the water. Turn the nozzle to its widest spraying range to coat the foliage evenly.
Spray mixture up into the branches, dampening all the foliage you can reach. Redirect the spray over the top of the tree so the mixture dampens the upper foliage as well.
Allow the foliage to soak under the mixture for about two hours. Spray the tree thoroughly twice more. Allow the tree to soak overnight.
Rinse the tree thoroughly with water. Most of the mould should come off as the tree is being rinsed. Any of these mixtures will also repel or kill the pests causing the mould.
Prune your tree after treating if it is in shade or has an excess of foliage. This will help prevent the mould from returning.