Is it the lowest branch on the trunk? Often that's the first to show signs or over- or underwatering. Either for newly planted trees can result in what you describe. Check under the mulch for excessive dryness or wetness...the soil should be slightly moist but not wet the length of your index finger. If it's dry, water; if it's wet, remove the mulch for a half day and let the soil dry out some. You may have to plant it higher or provide better drainage as well if that's the case. There are many posts on this in previous threads on this forum. Do a search and you'll learn a ton!