Effect of Foliar and Soil Magnesium Application on Bacterial Leaf Spot of Peppers. J. B. Jones, Assistant Plant Pathologist, Institute of Food and Agricultural Sciences, University of Florida, Agricultural Research and Education Center, Bradenton 33508-9324. S. S. Woltz, Professor of Plant Physiology, and J. P. Jones, Professor of Plant Pathology, Institute of Food and Agricultural Sciences, University of Florida, Agricultural Research and Education Center, Bradenton 33508-9324. Plant Dis. 67:623-624. Accepted for publication 12 November 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-623.

Early Calwonder pepper plants were grown in Myakka fine sand amended with calcium carbonate or dolomite. Plants were sprayed weekly with magnesium chloride (2.4 g/L) or were left unsprayed. Inoculation consisted of infiltration of Xanthomonas campestris pv. vesicatoria at 8 × 10³ colony-forming units per milliliter in leaves (one per plant) in each treatment in early October; later in November, the same plants were spray-inoculated with X. c. pv. vesicatoria at 10⁸ colony-forming units per milliliter. Disease development was most severe on plants grown in the dolomite-amended soil that were sprayed with magnesium. Unsprayed plants grown in dolomite-amended soil or magnesium-sprayed plants grown in calcium carbonate-amended soil were intermediate in disease, whereas unsprayed plants grown in soil amended with calcium carbonate had the lowest disease severity. Tissue analysis revealed that magnesium levels were positively correlated with disease development. Because disease development followed similar trends when both inoculation procedures were used with the four treatments, it appears that the mechanism of reduced susceptibility in plants with low magnesium levels was, to a large extent, internal rather than completely external (the physical and chemical nature of the leaf surface).