Received for publication July 9, 1980. Paper No. 6386 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh. The use of the trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of products named nor criticism of similar ones not mentioned.
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.
Present addresses: Department of Crop and Soil Sciences. Michigan State University, East Lansing, MI 48824; and Department of Horticulture. University of Wisconsin, Madison, WI 58706, respectively.