rust on leaves
Identification of the disease is essential to treating the problem and it especially difficult without detailed photos. It might be a fungal disease (other than rust) or a bacterial canker. If you can send photos, please include the reference number of this post.
Skip down to page 7 of this article to see if you recognize the symptoms of one of these diseases:
There is also some descriptive information here:
In any case, these types of diseases are very difficult to control, especially if the plant is small or stressed. Make sure it has bright light, water it deeply and allow it to dry out slightly between waterings, and do not allow the foliage to get wet. If you can send photos, perhaps I can offer some additional suggestions for the problem you describe.