Additionally to @Marco's answer, if you have a kendo drop down list that you only want to enable on Add,and not on Edit, you can also use the onEdit event handler like this (typeScript code) :

public onEdit = (e) => {
  if (!e.model.isNew()) {
    var region = e.container.find("input[name='City']").data('kendoDropDownList');
    region.enable(false);
  }
}