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Epidemiology of Botrytis Blight of Macadamia Racemes. J. E. Hunter, Associate Plant Pathologist, University of Hawaii, Beaumont Agricultural Research Center, Hilo 96720; K. G. Rohrbach(2), and R. K. Kunimoto(3). (2)Plant Pathologist, Pineapple Research Institute, Wahiawa, 96786; and (3)Assistant in Plant Pathology, University of Hawaii, Beaumont Agricultural Research Center, Hilo, 96720. Phytopathology 62:316-319. Accepted for publication 7 October 1971. DOI: 10.1094/Phyto-62-316.

Incidence of Botrytis blight of macadamia varied greatly depending upon meteorological conditions and abundance of inoculum and susceptible flowers. Botrytis cinerea developed only on senescent flower parts, with no evidence of infection of immature buds. This was attributed to lack of nutrient exudates, which are required for spore germination, on the surface of immature racemes. Exudates from mature and senescent racemes stimulated spore germination. Botrytis cinerea incidence was correlated with the number of hours per week of temperatures between 18 and 22 C, relative humidity of 95-100%, and the presence of water as detected with a leaf-wetness recorder. Temperatures above 22 C had a significant negative correlation with the incidence of Botrytis blight. Absence of Botrytis blight during some periods of cool, wet weather was attributed to the heavy tropical rains stripping the senescent flower parts from the racemes. Normally, these are the main source of nutrients for the fungus. Heavy rains, which may exceed 1 inch in 1-2 hr, also suppress blight development by removing spores from racemes and the atmosphere. Detection of inoculum increase in the orchard was best determined by direct observation of senescent racemes, rather than by examination of Hirst spore trap slides for spores.