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ARS Home » Southeast Area » Fort Pierce, Florida » U.S. Horticultural Research Laboratory » Citrus and Other Subtropical Products Research » Research » Publications at this Location » Publication #310378
Title: High incidence of preharvest colonization of huanglongbing-symptomatic Citrus sinensis fruit by Lasiodiplodia theobromae (Diplodia natalensis) and exacerbation of postharvest fruit decay by that fungus

Author
item Zhao, Wei
item Bai, Jinhe
item McCollum, Thomas
item Baldwin, Elizabeth - Liz

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2014
Publication Date: 1/1/2015
Citation: Zhao, W., Bai, J., Mccollum, T.G., Baldwin, E.A. 2015. High incidence of preharvest colonization of huanglongbing-symptomatic Citrus sinensis fruit by Lasiodiplodia theobromae (Diplodia natalensis) and exacerbation of postharvest fruit decay by that fungus. Applied and Environmental Microbiology. 81(1):364-372. https://doi.org/10.1128/AEM.02972-14.
DOI: https://doi.org/10.1128/AEM.02972-14

Interpretive Summary: Huanglongbing (HLB), presumably caused by bacterium Candidatus Liberibacter asiaticus (CLas), is one of the most devastating citrus diseases associated with excessive pre-harvest fruit drop. This is the first report on the correlation of HLB disease and Diplodia infection. The results showed high incidence of pre-harvest colonization of Diplodia in HLB-affected sweet orange fruit tissues, and the Diplodia infection exacerbated postharvest fruit decay and may contribute to HLB-related pre-harvest fruit drop. Our finding implied that fungicide application to control Diplodia may alleviate the HLB-associated pre-harvest fruit drop problem.

Technical Abstract: Huanglongbing (HLB), presumably caused by bacterium Candidatus Liberibacter asiaticus (CLas), is a devastating citrus disease associated with excessive pre-harvest fruit drop. Lasiodiplodia theobromae (Diplodia) is the causal organism of citrus stem end rot (SER). The pathogen infects citrus fruit under the calyx abscission zone (AZ-C), and is associated with cell wall hydrolytic enzymes similar to plant enzymes involved in abscission. By means of DNA sequencing, Diplodia was found in CLas positive juice from HLB-symptomatic fruit (S), but not in CLas negative juice. Therefore, the incidence of Diplodia in fruit tissues, impact on HLB-related postharvest decay and implications for HLB-related pre-harvest fruit drop were investigated in Hamlin and Valencia orange. qPCR results revealed significantly (P<0.001) greater incidence of Diplodia in AZ-C of the HLB-symptomatic (S, CLas Ct value <30) than in asymptomatic (AS, CLas Ct value >30) fruit. In agreement with the qPCR results, two weeks following exposure to ethylene, the incidence of SER in S fruit was 66.7% (Hamlin) and 58.7% (Valencia); while for AS fruit, the decay rates were 6.7% (Hamlin) and 5.3% (Valencia). Colonization of Diplodia in S fruit AZ-C was observed by scanning electron microscopy and confirmed by PCR test and morphology of conidia in isolates from the AZ-C after surface sterilization. Diplodia Ct values were negatively correlated with ethylene production (R=-0.838 for Hamlin; R=-0.858 for Valencia) in S fruit, and positively correlated with fruit detachment force (R=0.855 for Hamlin; R=0.850 for Valencia), suggesting that Diplodia colonization in AZ-C may exacerbate HLB-associated pre-harvest fruit drop.