Answer: Black margins on leaves can be caused by environmental factors, cultural practices or disease problems. Sun scorch or heat stress can show up on the leaves; over-fertilization or over-watering (especially a concern if your Champaca is planted in a lawn) can affect the leaves; fungal or bacterial diseases can cause these symptoms, as well. I think the best approach is to take samples of the affected leaves to your local nursery or Master Gardener Clinic at the Cooperative Extension office for up-close inspection. Once the cause has been identified, you can take corrective action.
Because Michelia Champaca is a semi-evergreen (it never really loses all of its leaves all at once), you'll need to expect some leaf shedding nearly all year around. It is normal for broadleaf evergreens (or semi-evergreens) to lose the oldest leaves - those on the inside of the branches, closest to the trunk - in favor of producing new foliage. There's not much you can do to stop this because it is normal and natural.
Best wishes with your landscape!
Q&A Library Searching Tips