You can use copper based fungicides.

Dear Sebastian

Obviously prevention is better than cure, but the cleaning out and removal of dead wood to remove inoculum of the melanose fungus is important, especially in older trees. Protectant copper sprays are the only product registered for melanose control. Timing of spray applications is very important. With Washington navel and Valencia oranges the spray should be applied at full petal fall. With lemons where lemon scab is also a problem, the initial application should be made at half petal fall.

In wet weather, especially if melanose is serious (particularly in older trees which contain a lot of dead wood), a follow-up spray should be applied 6–8 weeks after the initial spray. If a treatment is applied at petal fall, the fruit will be too small to retain much fungicide and what little material is retained will soon be dissipated through fruit enlargement, if not by rainfall or irrigation. With the petal fall treatment, only 3–4 weeks protection against melanose can be expected in wet conditions, leaving another 6 weeks for the fruit to be attacked. The copper sprays act as protectants, preventing infection of the young developing fruit. The melanose fungus harbored in the dead wood throughout the framework of the tree is little affected by these sprays and they do not reduce the inoculum available from the dead wood.

Where melanose leaf infections are likely to be serious on foliage in late summer, a further protectant copper spray should be applied.

Dear Dr Seied Mehdi Miri,

Thank you very much for shared your information.

Best regard

Dr Sebastian Reznikov