The symptoms you describe are consistent with a fungal root rot. However, without a sample that can be examined by the UD Plant Diagnostic Clinic there is no certainty that this is the case. If you are confident that the plant was not subjected to environmental stressors such as drought, wet conditions, physical injury, excessive salt, etc. and you did not see symptoms of disease on the leaves (brown spots, misshapen, etc.) then a root problem is the most likely culprit. Information about the services of the UD Plant Diagnostic Clinic is at http://extension.udel.edu/ag/plant-diseases/ud-plant-diagnostic-clinic/