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Relation of Leaf Age to the Reaction of Tobacco to Alternaria alternata. J. R. Stavely, Research Plant Pathologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705; L. J. Slana, Research Assistant, Plant Pathology, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705. Phytopathology 61:73-78. Accepted for publication 10 August 1970. DOI: 10.1094/Phyto-61-73.

The reaction of tobacco leaves to infection by Alternaria alternata was governed by age. The fungus directly penetrated all inoculated leaves within about 48 hours after inoculation, caused necrosis of epidermal cells, and began intercellular invasion of the mesophyll. In the 2 to 4 youngest inoculated leaves, resulting necrotic lesions were usually less than 1 mm in diameter, but in older leaves, lesions enlarged to average about 8 mm in diameter by 14 days after inoculation. Expansion of heavily inoculated leaves that had reached less than 75% of their potential size on the day of inoculation was severely inhibited following inoculation. A cicatrice of densely packed, angular cells developed around infections on these leaves, stopping further advancement of the fungus. A similar response occurred around mechanical wounds on young leaves. Both cell division and inhibition of expansion apparently had roles in cicatrization. No walling-off response occurred around lesions incited by A. alternata or mechanical wounding in older tobacco leaves. Walls of cicatrice cells had greater staining affinity than walls of other host cells. Chloroplasts were absent from cells in the walling-off layers and cells in chlorotic halos around lesions in older leaves.

Additional keywords: Nicotiana tabacum, leaf growth, brown spot.