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Incidence of Tobacco Leaf Microflora in Relation to Brown Spot Disease and Fungicidal Treatment. Harvey W. Spurr, Jr., Research Plant Pathologist, Plant Science Research Division, ARS, USDA, Oxford, North Carolina 27565, Associate Professor of Plant Pathology, North Carolina State University; R. E. Welty, Research Plant Pathologist, Market Quality Research Division, ARS, USDA, North Carolina State University, Raleigh 27607, Associate Professor of Plant Pathology, North Carolina State University. Phytopathology 62:916-920. Accepted for publication 28 February 1972. DOI: 10.1094/Phyto-62-916.

The brown spot pathogen, Alternaria alternata (Fr.) Keissl., and 18 or more other microorganisms (fungi, yeast, and bacteria) may colonize tobacco leaves without producing visible effects. The influence of this large and variable population of leaf microflora on yield and quality is not yet understood. The principal fungal species which occurred within the leaf were Alternaria and Cladosporium, being found in 67 and 25% of the leaves examined, respectively. Alternaria was found more often in the interior of lower and middle leaves, and Cladosporium more often in the interior of middle leaves. Yeasts and bacteria were most numerous in dilution plate cultures of leaf homogenates, in which organisms from both the interior and exterior of the leaves were recovered. Colonies of Cladosporium were the most numerous of the species observed. Cis-N-[(1,1,2,2-Tetrachloroethyl)thio]-4-cyclohexene-1,2-dicarboximide (Difolatan) was most effective for the control of brown spot and leaf microflora, and effectively inhibited the mycelial growth of several tobacco fungi in vitro. Benomyl reduced the internal incidence of Cladosporium. Other fungicides controlled brown spot but not the leaf microflora. The efficacy of several chemicals for the control of brown spot and leaf microflora are tabulated and provide a basis for future chemical control studies.