Received for publication 2 Dec. 1986. Contribution of the Oregon Agricultural Experiment Station in cooperation with the ARS/USDA. Technical paper no. 8065 of the OAES. Mention of a trade mark or proprietary product does not contribute a guarantee or warranty of the product and does not imply its approval to the exclusion of other products that may also be suitable. 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.