Received for publication January 30, 1981. Authorized for publication as The Pennsylvania Agricultural Experiment Station Journal Series Paper No. 5137. Contribution No. 910, Department of Plant Pathology and No. 443-76, Center for Air Environment Studies. Funds provided by Environmental Protection Agency grants no. R800865 and T900011.
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Former graduate student (current address: Department of Plant Pathology, University of Georgia, Athens), Associate Professors, and Professor.