The Times article is well worth reading, and points up another problem - producing an immune species by cross-breeding is hampered by the limited variety of orange stock available. Orange trees are propagated by cuttings (cloning) and grafting the cloned cuttings to stock of a different variety. Nearly all of Florida's orange juice is derived from just two varieties - the Hamlin orange and the Valencia orange. In addition, oranges are themselves a hybrid -- a cross between Pomelos and Mandarins, making the development of new varieties even more problematic. New varieties developed in recent years have often been the result of bombarding orange seeds with radiation, thus speeding up the mutation or evolutionary process.
On the positive side, the development of a transgenic orange resistent to citrus greening would stop the industry from bombarding orange groves with more and more chemicals in an effort to save their crops. None of the choices in addressing this devasting disease are good.