lilac leaves deseases problem
There can be many reasons for leaf browning... it's a very general symptom (like a cough in a person it doesn't link to a specific disease but could be related to many things). This late in the season I wouldn't worry about it. The leaves have done their job feeding the tree for this year and will be dropping soon anyway. Be sure to clean up ALL the leaves as they fall and remove any other plant debris around the shrub that could harbour fungal spores or insects. Next season watch carefully for the first signs of any trouble and look for accompanying symptoms: the extra information may be able to help someone narrow down potential problems. But for this year don't worry about it... it's too late, but won't have serious impact on the tree.