Answer: Michelia alba could brown and defoliate for a variety of reasons, but here are some guidelines to help you begin to troubleshoot.
This is a tropical plant that likes heat and humidity, however it will not take too much direct sun. Full morning sun or bright dappled light all day should be sufficient; yellowing would be an early sign of too much direct sun.
This is a shallow rooted plant that needs evenly moist yet well drained soil, this means damp like a wrung out sponge, not sopping wet or saturated and not bone dry. Over or underwatering could both cause root damage which would then cause foliage symptoms and eventually leaf drop.
Excessive wind can dry it out and cause the foliage to discolor as a result.
Overfertilizing (using amounts in excess of the label rates) can cause root damage if applied to the soil (root damage in turn causes foliage problems), or can cause foliage damage if applied to the leaves in water soluble form. This would be the so-called fertilizer burn.
I hope this helps.
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