I, too, am a strong proponent of getting a good diagnosis of the problem before applying any pesticides. But my other concern is your application of a half pound of Sevin on one plant. Unless this is a very low concentration product, that has to be too much. Check the pesticide label and mix and apply it according to the directions. There are a lot of negative side effects to using pesticides. The ideal objective should be to use the minimum amount of product that gives you good control.

It looks like a lack of water. Can't tell for sure from photos how large a tree, but it looks to be a good size. These need a lot of water to get established, especially in a hot climate like Texas. And how it is watered makes a big difference as well - best to use a soaker hose or just the hose (sans nozzle) at a slow trickle, allowing sufficient time for the water to soak down to the base of the rootball. And no amending planting holes, especially if working in a heavy clay or poorly draining soil. Put any desired amendments on top as a mulch if you wish. And get rid of the rubber!! It is a poor choice in terms of ability to leach heavy metals and other chemicals, providing zero nutrients source and low permeability. The only place rubber mulch could be used in good conscience is mixed with playground chips in a kids play area. Not with plants.

Dear DallasGarden; Thanks for the helpful words. I am renting this home, and should I leave, I don't want to leave my plant behind, so I leave it is its enormous pot. It seems to have lots of room to grow. Thanks for that idea. I'm happy for you with your Magnolia Jane. I started seeing browning leaves a couple of months or so ago. Do you think I can leave it in the pot on my patio, and bring it in for the winter? Thanks.

Your tree SHOULD be fine. It is likely a combination of...cold damage (from a dreadful Winter), transplant shock, and typical, Spring leaf drop. More leaves will continue to drop in May and June, but should be fine as long as you are also seeing new growth. Don't know about your area, but here we are bordering on drought conditions. Make sure you maintain regular watering throughout the growing season.