Two things. Firstly, Pittisporums don't like wet roots and damp. Secondly they can be prone to leaf spots especially if planted in areas with high rainfall and/or humid conditions and minimal air circulation. I would not use chemicals. Make sure the plant has good drainage. If you must have it in a pot add grit to the compost to help drainage and don't overwater. Also, thin it out a little, prune out the worse affected branches to get some air around the leaves. Then, if the roots are happy it should thrive. The north facing location isn't a problem. I have a vigourous Pittisporum in the ground on a north wall. It gets plenty of light but no sun.