|Hi Nancy -
If the plant died unnaturally, then yes the urushiol oil remains within the plant leaves, stems, roots, drupes (fruit). Those dried up, dead-looking leaves are dead but the oil is quite stable and remains. You'll want to treat any plant remnants as you would if it were still alive. Depending on various environmental factors, the oil can remain potent for years. Apparently, drier conditions tends to mean the oil remains for some time.