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Physiology and Biochemistry

Production of Oxalic Acid by Mycena citricolor, Causal Agent of the American Leaf Spot of Coffee. D. V. Rao, Postdoctoral fellow, Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5; J. P. Tewari, Associate professor, Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5. Phytopathology 77:780-785. Accepted for publication 9 October 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-780.

Light and scanning electron microscopy revealed numerous prismatic crystals in agar and broth cultures of Mycena citricolor. Based on their shape, solubility, energy-dispersive X-ray emission spectra, and ¹³C nuclear magnetic resonance studies, they were shown to be composed of calcium oxalate. Histochemical study of infected coffee leaf tissue revealed calcium oxalate crystals at the site of infection and in the necrotic lesion tissue. The latter contained more than 2 times the number of calcium oxalate deposits and more than 3’ times the amounts of oxalic acid relative to healthy controls. In addition, necrotic lesions similar to those incited by M. citricolor developed when drops of oxalic acid solutions were placed on coffee leaves. In broth culture, increases in oxalic acid levels followed the growth curve of the fungus. The results suggest a key role for oxalic acid in the pathogenesis of M. citricolor, and the demonstration of calcium oxalate formation in this study provides concrete evidence for the host-calcium sequestration hypothesis. It is postulated that oxalic acid, by lowering the pH, stimulates indoleacetic acid-oxidase and macerating enzyme systems resulting in tissue disintegration and premature leaf drop.