ARS | Publication request: Demonstrating Pathogenicity of Enterobacter cloacae on Macadamia and Identifying Associated Volatiles of Gray Kernel of Macadamia in Hawaii

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 22, 2007
Publication Date: October 1, 2007
Citation: Nishijima, K.A., Wall, M.M., Siderhurst, M.S. 2007. Demonstrating pathogenicity of Enterobacter cloacae on macadamia and identifying associated volatiles of gray kernel of macadamia in Hawaii. Plant Dis. 91:1221-1228.

Interpretive Summary: Gray kernel is an important disease of macadamia that affects the quality of kernels with gray discoloration and a permeating, foul odor that can render entire batches of nuts unmarketable. We report on the successful production of gray kernel in raw macadamia kernels artificially inoculated with strains of the bacterium Enterobacter cloacae. Our studies show that E. cloacae can be a major cause of this disease. Some of the conditions that are conducive to the production of gray kernel in artificially inoculated kernels were identified as: incubation period of 2 months at 30ºC, anaerobic atmosphere, presence of free water, and 1 to 2 days of air exposure at 30ºC after kernels are removed from shells. In addition, headspace analysis was conducted and major volatiles were identified from control nuts (ethanol and acetic acid) and infected, gray nuts [ethanol, acetic acid, and additional compounds: 3-hydroxy-2-butanone (acetoin), 2, 3-butanediol, phenol, and 2-methoxyphenol (guaiacol)]. The information on the volatiles involved in gray kernel is a first-time report. The demonstration that gray kernel likely is a bacterial disease that can be caused by E. cloacae, acknowledges that it is a food quality as well as a food safety concern for the macadamia industry.

Technical Abstract: Gray kernel is an important disease of macadamia 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 non-gray kernel samples also were analyzed. Ethanol and acetic acid were present in non-gray 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). It is believed to be the first report of the identification of volatile compounds associated with gray kernel.