Use this script as a workflow transition validator to allow transition only if a field in the subtask is populated.
string [] subtasks = subtasks(key);
boolean hasField = false;
string errorMsg = "You must have a value for the Fruit field in a subtask entered before this issue can be completed.";
for(string st in subtasks) {
if(isNotNull(%st%.fruit)) {
hasField = true;
}
}
if(hasField == false) {
return false, "error", errorMsg;
}