The first thing I would check is the watering system, since alders aren't very drought tolerant. Another possibility is root damage by grubs or gophers. If you are in Sunset zone 13, or some of the hotter parts of zone 11, Italian Alder isn't heat tolerant enough to go through summer unscathed--it may be better to replace it with an India Rosewood (Dahlbergia sissoo). If the leaf loss seems to be proceeding one branch at a time, there is a possibility of it being some form of borer.

This happened to my Rowan tree last year by July all its leaves were gone. I watered it and this year it has recovered. I suspect the roots on mine may be going round and round and not outwards into the soil? This tree is over 10 years old but last year was the first year its done this.