For control of Alternaria brown spot on dooryard tangelo trees, copper fungicide can be applied at these times:

(1) When the spring growth flush is about 1/4 to 1/2 fully expanded.

(2) When the spring flush is nearly fully expanded.

(3) Shortly after most of the petals have dropped from the flowers.

(4) In April and May, spray every 10 days (if rainy) or as infrequently as once a month (if the weather is dry).

(5) In June, make 2 applications.

The fruit usually becomes resistant to Alternaria about the middle of July.

For more information see this fact sheet:
http://edis.ifas.ufl.edu/CG021