Received for publication Sept. 8, 1981. Colorado State University Scientific Series No. 2684. Research supported in part by Contract EPA-IAG-D60168 awarded to USDA-ARS with Colorado State University Experiment Station cooperating.
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.
The authors acknowledge assistance from H. R. Cams and K. H. Norris (USDA, Beltsville, Md.) and thank the Aspen Ski Corporation for use of the high elevation site.
Present address: Department of Botany and Plant Pathology, Purdue University, West Lafayette, IN 47907.
Present address: Fruit Crops Department, University of Florida, Gainesville, FL 32611.