I think the nurseryman is right. I have never heard of any other remedy. You can't hurt the aucuba by cutting back,it's a tough plant, so I would think you are not cutting back far enough. I would go back at least three nodes below the affected area.
LindaGail

I agree with Linda re cutting back hard, and be sure you sterilize your pruners between each cut and afterwards with alcohol. josh

I don't think that fireblight affects Aucuba. Fire blight is a bacterial disease that is fairly specific as to the plant families it can infest and Aucuba just isn't one. The fungicides you mentioned would not be affective against fireblight anyway. I may be wrong about this, but I don't think so.

However, there are several other diseases (fungus diseases) that DO affect this plant. A very important step in controlling your southern blight or whatever you might have, is to do some housekeeping around your plants. Remove all fallen leaves and old mulch and replace with new. Avoid over watering and over-head watering. If you have one of the soil borne diseases, you will have a far greater difficulty in controlling the problem. Try to find someone who can ID the problem or problems accurately.