Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/12/2010
Publication Date: 9/15/2010
Citation: Nishijima, K.A., Wall, M.M., Fitch, M.M., Sugiyama, L.S., Keith, R.C., Nishijima, W.T. 2010. Evaluating Hawaii-Grown Papaya for Resistance to Internal Yellowing Disease Caused by Enterobacter cloacae. HortScience. 45(9):1357-1364. . HortScience. Interpretive Summary: Internal yellowing (IY), a bacterial disease of papaya fruit caused by Enterobacter cloacae, is a limiting factor in value-added products such as fresh or frozen papaya cubes. Identifying or developing papaya germplasm that are resistant to this disease can help reduce the risk of bacterial contamination and ensure compliance with food safety regulations. We developed methods to differentiate resistant and susceptible papaya cultivars. Red-fleshed ‘Sunrise’ and ‘SunUp’ papaya were resistant while yellow-fleshed ‘Rainbow’ (an F1 hybrid between ‘SunUp’ and ‘Kapoho Solo’) and I-Rb F5/F6, an advanced inbred line derived from ‘Rainbow’, were moderately resistant. Yellow-fleshed ‘Laie Gold’ F1 and ‘Kapoho Solo’ were susceptible and highly susceptible, respectively, to IY disease. Our studies also determined that the bright yellow coloration of IY infected fruit can be measured with a colorimeter and infected tissue can be distinguished from healthy papaya tissue in red-or yellow-fleshed fruit using color analysis.
Technical Abstract: Papaya (Carica papaya L.) cultivars and breeding lines were evaluated for resistance to Enterobacter cloacae (Jordan) Hormaeche & Edwards, the bacterial causal agent of internal yellowing disease (IY), using a range of concentrations of the bacterium. Linear regression analysis was performed and IY incidence was positively correlated with increasing inoculum concentrations for susceptible cultivars Kapoho Solo and Laie Gold but not for resistant cultivars or lines. It was determined that the inoculum concentration of 9 to 10 Log10 colony-forming units per milliliter (cfu/mL) was able to reliably differentiate resistant and susceptible papaya germplasm. Red-fleshed cultivars SunUp and Sunrise were the most resistant papaya groups evaluated at this dose concentration. Yellow-fleshed cultivars, Kapoho Solo and Laie Gold, were susceptible to E. cloacae. ‘Rainbow’, an F1 hybrid between ‘SunUp’ and ‘Kapoho Solo’ that is yellow-fleshed, was moderately resistant to E. cloacae, exhibiting limited symptoms of the disease. Yellow-fleshed I-Rb F5/F6, an advanced inbred line derived from ‘Rainbow’, is resistant and offers the potential of improving resistance of yellow-fleshed commercial cultivars. A colorimeter was used to objectively measure internal flesh color and distinguish between infected and non-infected tissue in red- and yellow-fleshed papayas using L*C*H* color space analysis. Symptomatic tissue (72.4 and 79.0º) had higher hue angle means than non-symptomatic tissue (62.8 and 75.0º) for all cultivars or lines in red- and yellow-fleshed papayas, respectively. Yellow (“Y”) hue color also distinguished infected tissue from non-infected tissue. Symptomatic tissue that had Y hue color resulted in 79 to 81º hue angle means among red- or yellow-fleshed papayas. Our results demonstrated the usefulness of colorimetry to help detect infected papaya tissue. In surveys of naturally infected papaya, high populations (8.57 x 107 cfu/g) of E. cloacae were recovered in infected fruit of ‘Kapoho Solo’ and represent a food safety concern for fresh and processed papaya. In isolations from inoculated fruits, we observed decreases of approximately 1 to 2 Log10 cfu/g in final bacterial populations when high-dose range inoculum concentrations (9 to 12 Log10 cfu/mL) were used. This dose range may represent a saturation range for E. cloacae inoculation.