Fungal disease is one of the most common problems that orange tree growers encounter. And while most fungal diseases will not kill the tree, they can be unsightly. Melanose can scab the fruit rind. Greasy spot gives leaves unsightly blisters and sooty mold turns leaves black. Luckily, most orange tree fungi can be easily treated without the need of chemical treatment. However, it is important to take in affected foliage to your local garden center to be sure that you are dealing with and treating the right disease.
Remove any affected leaves, fruit or wood immediately. Remove any affected leaves or limbs that are still attached to the tree. Then, collect any fallen leaves or dead wood from around the tree. Dead limbs that are unaffected should also be pruned from your fruit tree to prevent new fungal growth. Set some of the affected foliage aside to take to your local garden center. Discard (do not compost) the rest.
Treat an infestation. Sooty mold fungus grows on the secretions left behind by scales, whiteflies and aphids. If the infestation is not controlled, the fungus will simply crop up again. Once you remove the affected leaves, treat the tree with an organic insecticide.
Treat the tree with liquid copper fungicide if necessary. Removal of the infected foliage should be your primary defense and fungicide should only be used as a last resort. One application of liquid copper fungicide is enough to control most fungi. However, if the infestation persists, follow up with subsequent treatments every two weeks until the fungi disappear. For amount and application methods, consult the manufacturer's instructions.