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1Department of Plant Pathology, University of Wisconsin, Madison, Wisconsin
1
Received for publication Nov. 8, 1961.
2
This work was supported in part by the Rockefeller Foundation, the American Cancer Society, the National Institutes of Health, and by the Research Committee of the Graduate School from funds supplied by the Wisconsin Alumni Research Foundation. Published with the approval of the director of the Wisconsin Agricultural Experiment Station. Mr. Eugene Herrling assisted with the illustrations.