William M. ProebstingDepartment of Horticulture, Oregon State University, Corvallis, OR 97331
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Received for publication 15 Dec. 1986. Technical paper no. 7564 of the Oregon Agricultural Experiment Station. This work was supported in part by a grant from the Northwest Christmas Tree Assn. 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.