First report of association of Mucor circinelloides on noni (Morinda citrifolia) in Hawaii

Authors: Nishijima, Kate; Wall, Marisa; CHANG, LENG CHEE - University Of Hawaii; WEI, YANZHANG - Clemson University; WONG, DESMOND - University Of Hawaii

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2010
Publication Date: 3/1/2011
Citation: Nishijima, K.A., Wall, M.M., Chang, L., Wei, Y., Wong, D.K. 2011. First report of association of Mucor circinelloides on noni (Morinda citrifolia) in Hawaii. Plant Disease. 360. DOI:10.1094/PDIS-11-10-0815

Interpretive Summary: Noni (Morinda citrifolia) is a popular medicinal plant found in tropical or subtropical regions of the world. The fruit and juice extracts have properties reportedly therapeutic for various health-related conditions including diabetes, high blood pressure and certain types of cancer. In our studies on noni juice produced from fruit collected from the island of Hawaii, Mucor circinelloides f. circinelloides (MCC) was isolated consistently from extracts of fermented fruit. Fungus strains were identified by molecular methods as Mucor circinelloides f. circinelloides. Inoculation studies were conducted on wounded and non-wounded fruits with MCC strains collected in 2006, 2008 and 2009. Wounded fruit inoculated with any of the three MCC strains resulted in 85% to 100% infection of moderate severity, while non-wounded fruit had little or no infections. There were no infections in control treatments of non-wounded or wounded fruit. This is the first report of M. circinelloides as a wound pathogen of noni fruit.

Technical Abstract: Noni (Morinda citrifolia) is a popular medicinal plant found in tropical or subtropical regions of the world. The fruit and juice extracts have properties that are reportedly therapeutic for diabetes, high blood pressure, and certain types of cancer (1,4). In our studies on noni juice produced from fruit collected from the Kohala and Puna districts of the island of Hawaii from 2008 to 2010, Mucor circinelloides f. sp. circinelloides was isolated from 85% of 157 juice samples and observed with up to 75% incidence on fruit surfaces during fermentation processing in glass jars. Fungal growth, appearing 14 to 21 days in storage at 22°C, was pale yellow to tan brown and was associated with wounded surfaces. Single-spore strains, KN 06-2 (2006; ripe fruit puree) and KN 08-08 (2008; fermented juice; CBS 124110), identified by Centraalbureau voor Schimmelcultures by molecular methods were 97.3% similar in internal transcribed spacer sequence to the type strain (CBS 195.68). M. circinelloides f. sp. circinelloides strains (KN 08-08, KN 09-06, or KN 10-02) (2008 to 2010; fermented juice) were inoculated by pipetting an aliquot of 100 µl of fungus strain spore suspension (1 × 105 to 1.33 × 106 spores/ml) onto firm, yellow maturity noni fruit that were washed, surface disinfected, and either wounded (surface cuts) or nonwounded. Controls consisted of no inoculation and sterile distilled water (SDW) inoculation treatments. Ten to twenty each of wounded and nonwounded fruit comprised each inoculation treatment. Fruit were incubated in acrylic bins with a layer of distilled water at the bottom, and sealed with snap-on lids. The bins were incubated on a lab bench at 22 to 23°C under fluorescent lights. Fruits were evaluated for presence of fungal growth and severity of symptoms. To determine viability of spores on inoculated fruit without symptoms, surfaces were swabbed with sterile cotton swabs dipped in SDW, streaked on potato dextrose agar (PDA) plates, and incubated at 22°C under fluorescent lights. The inoculation experiment was conducted twice. Nonwounded fruit inoculated with M. circinelloides f. sp. circinelloides strains did not result in infections (KN 09-06 and KN 10-02) or produced slight mycelial growth (0 to 20%; KN 08-08). Wounded fruit inoculated with any of the three strains resulted in 85 to 100% infection of moderate severity. There were no infections in noninoculated or SDW treatments of nonwounded or wounded fruit. Koch's postulates were fulfilled with the reisolation of M. circinelloides f. sp. circinelloides from selected fruit exhibiting soft tissue, discoloration, and sporulating yellowish green mycelial growth. Swab washes from asymptomatic surfaces of inoculated nonwounded fruit resulted in the growth of M. circinelloides f. sp. circinelloides on PDA, proving viability of the spores and confirmed that the fungus is primarily pathogenic only on wounded fruit surfaces. To our knowledge, this is the first report of M. circinelloides as a wound pathogen of noni fruit. The quality of fermented noni juice may be affected by the presence of M. circinelloides f. sp. circinelloides but can be remedied by pasteurization that does not affect antitumor properties (unpublished data). This fungus is also a reported pathogen of mango (2) and peach (3).