not much info to go on..

how and when do you water???

how were the plants planted...

were they potted or ball and burlap???

what was your summer like.. e.g. i had 10 weeks of drought

all you provide is the end result.. tell us about what happened from the time you brought them home..

total guesses ..

lack of PROPER WATER ...

or ...

bad plants to start with ..

any chance at a picture???

ken

It is impossible to give you a firm diagnosis based just on the info provided. Pittosporum are susceptible to an assortment of diseases, generally fungal in nature. From what you describe of your planting site (heavy, clay soil at the base of a slope), less than ideal drainage could certainly play a factor and might account for a root rot, rather common for these plants if not in a well draining soil.

Really the only way to determine if something like this is occurring is to inspect the roots -- if they are black, soft or mushy, then root rot is probably the cause. Unfortunately, root rots are typically fatal. If this IS the issue, then you will need to relocate the healthy plants at least temporarily, improve drainage and replant.

Pitts are pretty drought tolerant once established and most SoCaL gardens rely on regular irrigation to survive so while lack of water could be an issue, I'd tend to think routine irrigation combined with a potentially compromised drainage situation is more likely the case. It could also account for the leaf spot, especially if the irrigation is provided by daily sprinkling overhead, rather than infrequent, deep watering at the root zone.