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Pathogenesis and Halo Formation of the Tobacco Brown Spot Lesion. C. E. Main, Assistant Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27607; Phytopathology 61:1437-1443. Accepted for publication 9 July 1971. DOI: 10.1094/Phyto-61-1437.

Leaf lesions caused by Alternaria tenuis on susceptible tobacco cultivars are comprised of a fungal-invaded necrotic center, surrounded by a noninvaded chlorotic halo and biochemically detectable “prehalo” zone, beyond which is located healthy tissue. Alterations in noninvaded tissues are believed to result from the response of host tissue to diffusible fungal metabolite(s), and are associated in a cause and effect relationship gradient to the tissue zones within the lesion. Plastid pigments, starch, and reducing sugars decreased, whereas individual mono- and disaccharides increased progressively from healthy to halo tissue. Changes in total nitrogen, soluble protein, and amino acids indicate a disruption of protein metabolism. Free phenols decreased toward the lesion center, indicating polymerization and brown pigment formation. Cultivar tolerance in tobacco is related to the extent of chlorotic halo formation. The lesion is presented as a working model for the study of pathogenesis and the nature of tolerance to the brown spot disease.