Also called lemon scab, this fungal disease causes raised roughened warty lesions to form on the outside of the fruit. Rough patches also appear on the stems and leaves, and fruit drop often occurs. Although the inside of the fruit is unaffected, the disease should be controlled because it gradully reduces the vigour of the tree. Damp, cool weather encourages the disease.
Remove affected fruit, twigs and leaves. Dispose of them in the garbage. Copper spray should be applied before, after and during flowering to prevent further infection.