Certified GKH Gardening Expert
This is bacterial. Generally, the most common issue with these trees will be overwatering.
This can be treated with a fungicide, but it is best to try and wait until the leaf it close to completely dead before removing, once you have treated the tree and soil.
Make sure that the container has drainage, and that all extra water is allowed to leave the container, completely.
Make sure to only water after the soil dries down to about 3 or 4 inches.
Standing water or saturated soils will be the primary cause for infections like this.
Here are some articles that will help: