Distinguishing Isolates of Peronospora tabacina from Geographic Regions Utilizing Tobacco Leaf Disks and Fluorescence Microscopy. M. D. WIGLESWORTH, CIBA Plant Protection, Lansing, Michigan 48917. W. C. NESMITH and M. R. SIEGEL, Department of Plant Pathology, University of Kentucky, Lexington 40546-0091; M. R. BONDE, USDA-Foreign Disease-Weed Science Research, Ft. Detrick, Frederick, MD; and C. E. MAIN, Department of Plant Pathology, North Carolina State University, Raleigh. Plant Dis. 78:456-460. Accepted for publication 29 November 1993. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1994. DOI: 10.1094/PD-78-0456.

A laboratory bioassay to distinguish isolates of Peronospora tabacina D.B. Adam was developed utilizing 7-mm acetone-dipped disks cut from tobacco leaf panels from several cultivars of Nicotiana tabacum, sporangiospores of P. tabacina, a germination indicator (Calcofluor), and fluorescence microscopy. Sporangiospore germination percentage of each isolate x cultivar pair was determined. Mean germination of sporangiospores differed between domestic and international collection locations (except between the two Mexican isolates). These results suggest the presence of different pathotypes of P. tabacina.