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Demonstrating Pathogenicity of Enterobacter cloacae on Macadamia and Identifying Associated Volatiles of Gray Kernel of Macadamia in Hawaii

October 2007 , Volume 91 , Number  10
Pages  1,221 - 1,228

K. A. Nishijima, M. M. Wall, and M. S. Siderhurst, Pacific Basin Agricultural Research Center (PBARC), USDA-ARS, P.O. Box 4459, Hilo, HI 96720



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Accepted for publication 22 May 2007.
ABSTRACT

Gray kernel is an important disease of macadamia (Macadamia integrifolia) that affects the quality of kernels, causing gray discoloration and a permeating, foul odor. Gray kernel symptoms were produced in raw, in-shell kernels of three cultivars of macadamia that were inoculated with strains of Enterobacter cloacae. Koch's postulates were fulfilled for three strains, demonstrating that E. cloacae is a causal agent of gray kernel. An inoculation protocol was developed to consistently reproduce gray kernel symptoms. Among the E. cloacae strains studied, macadamia strain LK 0802-3 and ginger strain B193-3 produced the highest incidences of disease (65 and 40%, respectively). The other macadamia strain, KN 04-2, produced gray kernel in 21.7% of inoculated nuts. Control treatments had 1.7% gray kernel symptoms. Some abiotic and biotic factors that affected incidence of gray kernel in inoculated kernels were identified. Volatiles of gray and nongray kernel samples also were analyzed. Ethanol and acetic acid were present in nongray and gray kernel samples, whereas volatiles from gray kernel samples included the additional compounds, 3-hydroxy-2-butanone (acetoin), 2,3-butanediol, phenol, and 2-methoxyphenol (guaiacol). This is believed to be the first report of the identification of volatile compounds associated with gray kernel.


Additional keywords:anaerobic storage, bacterial fermentation, Enterobacteriaceae, food safety, spoilage, water activity

The American Phytopathological Society, 2007