Received for publication 9 Jan. 1984. Cooperative investigation of the ARS, USDA, and the Univ. of Maryland Agr. Expt. Sta. Scientific Article No. A3444, Contribution No. 6517 of the Maryland Agr. Expt. Sta. This paper reports the results of research only. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products that also may 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.