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.
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.