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Importance of Mesophyll in Mature-Leaf Resistance to Cancrosis of Citrus. R. E. Stall, Professor, Plant Pathology Department, University of Florida, Gainesville 32611; G. M. Marcó(2), and B. I. Canteros de Echenique(3). (2)(3)Plant pathologists, Instituto Nacional de Tecnología Agropecuaria (I.N.T.A.), 3432 Bella Vista, Corrientes, Argentina. Phytopathology 72:1097-1100. Accepted for publication 15 January 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1097.

Resistance in citrus leaves to cancrosis, caused by Xanthomonas campestris pv. citri, has been thought to be determined by stomatal structure. However, fewer lesions of cancrosis developed in mature citrus leaves than in immature leaves after inocula containing 103 – 104 cells per milliliter were infiltrated into the mesophyll. Mature-leaf resistance was not demonstrated at any inoculum level if an injury was used with inoculation. There was a positive correlation between inoculum dosage and numbers of lesions that developed in infiltrated leaves of grapefruit. The slope of the regression line decreased as age of leaves increased. Levels of resistance among some citrus species and cultivars in the field were distinguished after infiltration of 14- to 21-day-old leaves with inocula containing 5 × 103 cells per milliliter, but were not distinguishable by infiltrating leaves that were older. In all citrus types tested, resistance of the mesophyll to infection increased as leaves matured.

Additional keywords: horizontal resistance.