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

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.

Quality changes in macadamia kernel between harvest and farm-gate

“…Conditions also favoured bacterial infection, which may also have increased unsound kernel. Grey kernel disease of macadamia in Hawaii has been shown to be associated with a bacterium, Enterobacter cloacae 21. This is an important macadamia disease in Hawaii that causes grey discolouration and a foul odour of kernels.…”

“…This is an important macadamia disease in Hawaii that causes grey discolouration and a foul odour of kernels. The foul odour can contaminate batches of kernels 21 . Enterobacter cloacae , ubiquitous in nature, is an opportunistic pathogen in humans and is commonly found on or in plants in high‐moisture environments 21.…”

“…The foul odour can contaminate batches of kernels 21 . Enterobacter cloacae , ubiquitous in nature, is an opportunistic pathogen in humans and is commonly found on or in plants in high‐moisture environments 21. Although this pathogen has not been identified in Australian macadamias, its ubiquitous nature suggests the risk of it becoming a problem.…”

Enterobacter spp.: A new evidence causing bacterial wilt on mulberry

“…In the past 10 years people started to pay more and more attention on Enterobacter species which are thought to be important new opportunistic human pathogens and potential dangerous food pathogens [4,5]. However, fewer studies have been done on plant pathogens of Enterobacter, among which five species were reported to cause plant diseases [6,7]. No record of Enterobacter spp.…”

Evaluating Hawaii-grown Papaya for Resistance to Internal Yellowing Disease Caused by Enterobacter cloacae

“…cloacae is widely distributed in the environment (Richard, 1984;Sanders and Sanders, 1997), occurring on or in water, soil, plants, humans, and animals (Richard, 1984). It is also a cross-domain pathogen that causes infections in humans (Sanders and Sanders, 1997) as well as various plant hosts such as elm (Carter, 1945;Murdoch and Campana, 1983), mulberry (Wang et al, 2008), orchid (Takahashi et al, 1997), coconut (George et al, 1976), corn (Rosen, 1922), bulb onion (Bishop and Davis, 1990;Cother and Dowling, 1986), macadamia (Nishijima et al, 2007a), papaya (Nishijima et al, 1987), and mung bean sprouts (Wick et al, 1987). The ability of E. cloacae to cause infections in humans and the occurrence of this bacterium in food crops could pose a food safety risk if contaminated products were ingested in high concentrations or by immune-suppressed individuals.…”