|Keith, Ronald - UH MANOA - CTAHR|
|Nishijima, Wayne - UH MANOA - CTAHR|
Submitted to: Hawaii Papaya Industry Association
Publication Type: Other
Publication Acceptance Date: September 21, 2007
Publication Date: N/A
Technical Abstract: Internal yellowing (IY) is a bacterial disease of ripening papaya fruit that is caused by the enteric bacterium, Enterobacter cloacae. The disease is characterized by yellow discoloration of flesh, tissue softening and a foul or rotten odor that reduces the quality of fresh fruit and value-added products such as fresh or frozen papaya cubes. E. cloacae-infected fruit can also affect the coliform bacterial counts of papaya cube products that must not exceed the product-specific food safety guideline limit of 100 colony forming units per gram (cfu/g). Resistant papaya cultivars are needed to reduce or minimize the risk of E. cloacae-infected fruit. Fruit of four papaya cultivars collected from packinghouses or fields on the island of Hawaii were examined to determine the population of E. cloacae in naturally-infected fruit. Also, fruit inoculation studies were conducted to determine how varying populations of E. cloacae affect IY response in resistant (‘R’) and susceptible (‘S’) cultivars. One-third to ½ ripe fruit of six cultivars were inoculated with two E. cloacae strains (101 to 1012 cfu/ml concentrations) or water to establish IY incidence and responses (IY1, IY2, or IY3 for slight, moderate, or severe symptoms, respectively). IY was observed in 6% of 237 ‘Kapoho Solo’ fruit and E. cloacae population ranged from 38 cfu/g (fruit with slight symptoms) to 4.17 x 105 cfu/g (fruit with severe symptoms). ‘Rainbow’ fruit had 3% IY, while IY was not observed in red-flesh ‘Sunrise’ or ‘SunUp’. E. cloacae was not isolated from these 3 cultivars. Inoculations in the 109 to 1010 cfu/ml dose range produced the most consistent IY incidence results that clearly separated ‘R’ and ‘S’ cultivars. ‘Sunrise’ was the most ‘R’ (3% IY) followed by ‘Rainbow F6’ and ‘SunUp’ (< 5% IY). ‘Kapoho’ (90% IY) and ‘Laie Gold’ and ‘Kamiya’ (50 to 57% IY) were ‘S’. For IY severity responses, only ‘Kapoho Solo’ among the 3 ‘S’ cultivars produced the IY3 category (severe) symptoms at all dose treatments. ‘SunUp’ was the only ‘R’ cultivar that developed the most severe symptoms, which occurred only at 1011 cfu/ml concentration of the bacterium. The hierarchy of resistance to E. cloacae of the six evaluated cultivars was ‘Sunrise’, followed by ‘Rainbow F6’ and ‘SunUp’. ‘Kamiya’, ‘Laie Gold’ and ‘Kapoho Solo’ were less resistant, in that order. We conclude that high E. cloacae populations (exceeding 105 cfu/g) that can occur in ‘Kapoho Solo’ fruit pose a potential risk for future or developing fresh-cut fruit industry. ‘Rainbow F6’, ‘Sunrise’ and ‘SunUp’ produced low incidences of IY when challenged with E. cloacae and are potential candidates for use in processed papaya products such as fresh or frozen papaya cubes.