Received for publication 22 May 1987. Scientific Article no. A-4682, Contribution no. 7678 of the Maryland Agricultural Experiment Station. Mention of a tradename, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the author or the Univ. of Maryland and does not imply its approval to the exclusion of other products or vendors 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.