There may be a problem in the roots. Too dry or even more deadly too wet conditions in the soil can kill roots and cause the top to die back. Mites on the foliage (a hot weather problem) can also cause the foliage to lose color and turn brown in time. Finally, foliar diseases can kill branches turning them from green to brown quite quickly.
Watch the soil moisture and try to keep it just moist, not soggy. Mites will be declining very soon without treatments. The diseases require pruning out the dead areas and removing all dead material from around the plant. Then spray with a fungicide. Your County Extension Office can suggest an appropriate product for combatting the fungus.
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