Suppression of the Symptoms of American Leaf Spot of Coffee with Calcium Hydroxide. 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. Plant Dis. 72:688-690. Accepted for publication 18 March 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0688.

Detached and undetached leaves of coffee were scratched and sprayed at 276 k Pa with suspensions of calcium hydroxide containing 2 ml L^-1 of a spreader-sticker. The leaves received 0.04, 0.08, or 0.1 mg of calcium hydroxide cm^-2. The scratched points were inoculated with the gemmae of Mycena citricolor and the leaves were scored for lesion development 1 and 2 wk after inoculation. Calcium hydroxide spray significantly decreased the number and diameter of lesions and delayed their development, with lesion development being completely inhibited by a spray of 0.1 mg cm^-2. Scanning electron microscopy revealed numerous calcium oxalate crystals on lesions that developed on calcium hydroxide-sprayed and inoculated leaves as well as in calcium hydroxide-supplemented distilled water cultures of M. citricolor. It is suggested that the mode of action of calcium hydroxide in disease control is by way of neutralization of oxalic acid secreted by the pathogen. The results suggest that calcium hydroxide could control the disease on a field scale.