During the summer, tiny rust mites feeding on the fruit will cause a russeting of the peel. They prefer to feed in the shade which often causes uneven discoloration of the peel. Unless the infestation is severe, the interior quality of the fruit is generally not affected. Frequent spraying with horticultural oil while the fruit is maturing can help control this problem.

Additional Resources

http://edis.ifas.ufl.edu/HS122