Received for publication 4 June 1984. This research was supported by USDA grant number 81-CRSR-2-0732. The authors acknowledge Anne Nooteboom for sample preparation, Ann von Kugelgen for typing, and Don Durzan and Gale McGranahan for manuscript review. 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.