Received for publication 6 Nov. 1986. We thank Darwin Atkin, Gray and Barbara Martin, and Lisa Ciano for collecting bark samples; J.W. Cameron, N.C. Ellstrand, H.K. Wutscher, and W.S. Castle for comments on the manuscript; and the Citrus Research Board for support of this research. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.