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Physiology and Biochemistry

Occurrence of Magnesium Oxalate Crystals on Lesions Incited by Mycena citricolor on Coffee. D. V. Rao, Postdoctoral fellow, Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5; J. P. Tewari, Professor, Department of Plant Science, University of Alberta, Edmonton, Alberta, Canada T6G 2P5. Phytopathology 79:783-787. Accepted for publication 21 February 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-783.

Numerous rhombic crystals were observed on the lower surface of 3-wk-old lesions on coffee leaves incited by Mycena citricolor. Scanning electron microscopy showed that these crystals exist singly or as aggregates. The single crystals exhibited rhombic configuration, and crystal aggregates were organized as druses or crystal conglomerates. Energy-dispersive X-ray microanalysis and mass spectrometry provided evidence that the crystals were composed of magnesium oxalate. Similar rhombic crystals also were observed on the upper surface of 3-wk-old lesions, either present singly or coexisting with prismatic calcium oxalate crystals. It is postulated that magnesium oxalate crystals, which were absent in the control treatments, were formed in the infected tissue through sequestration of leaf magnesium by fungal oxalic acid. This is the first report on formation of magnesium oxalate subsequent to infection by a fungal pathogen.