Zhao Wei, Bai Jinhe, McCollum Greg, Baldwin Elizabeth
USDA, ARS Horticultural Research Laboratory, Ft. Pierce, Florida, USA.
USDA, ARS Horticultural Research Laboratory, Ft. Pierce, Florida, USA
Appl Environ Microbiol. 2015 Jan;81(1):364-72. doi: 10.1128/AEM.02972-14. Epub 2014 Oct 24.
Huanglongbing (HLB), presumably caused by the bacterium "Candidatus Liberibacter asiaticus," is a devastating citrus disease associated with excessive preharvest 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 "Ca. Liberibacter asiaticus"-positive juice from HLB-symptomatic fruit (S) but not in "Ca. Liberibacter asiaticus"-negative juice. Therefore, the incidence of diplodia in fruit tissues, the impact on HLB-related postharvest decay, and the implications for HLB-related preharvest fruit drop were investigated in Hamlin and Valencia oranges. Quantitative PCR results (qPCR) revealed a significantly (P < 0.001) greater incidence of diplodia in the AZ-C of HLB-symptomatic (S; "Ca. Liberibacter asiaticus" threshold cycle [CT] of <30) than in the AZ-C of in asymptomatic (AS; "Ca. Liberibacter asiaticus" CT of ≥30) fruit. In agreement with the qPCR results, 2 weeks after exposure to ethylene, the incidences of SER in S fruit were 66.7% (Hamlin) and 58.7% (Valencia), whereas for AS fruit the decay rates were 6.7% (Hamlin) and 5.3% (Valencia). Diplodia colonization of 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 preharvest fruit drop.
黄龙病（HLB）可能由“亚洲韧皮杆菌”引起，是一种毁灭性的柑橘病害，与采前果实大量脱落有关。可可毛色二孢（Lasiodiplodia theobromae）（毛色二孢）是柑橘蒂腐病（SER）的致病病原体。该病原菌在萼片离层区（AZ-C）侵染柑橘果实，且与类似于参与脱落过程的植物酶的细胞壁水解酶有关。通过DNA测序，在有HLB症状果实（S）的“亚洲韧皮杆菌”阳性果汁中发现了毛色二孢，但在“亚洲韧皮杆菌”阴性果汁中未发现。因此，对哈姆林和巴伦西亚橙子果实组织中毛色二孢的发生率、对与HLB相关的采后腐烂的影响以及对与HLB相关的采前落果的影响进行了研究。定量PCR结果（qPCR）显示，有HLB症状（S；“亚洲韧皮杆菌”阈值循环数[CT]<30）的果实AZ-C中毛色二孢的发生率显著（P<0.001）高于无症状（AS；“亚洲韧皮杆菌”CT≥30）果实的AZ-C。与qPCR结果一致，暴露于乙烯2周后，S果实中蒂腐病的发生率分别为66.7%（哈姆林）和58.7%（巴伦西亚），而AS果实的腐烂率分别为6.7%（哈姆林）和5.3%（巴伦西亚）。通过扫描电子显微镜观察到S果实AZ-C中有毛色二孢定殖，并通过PCR检测和表面灭菌后从AZ-C分离的分生孢子形态进行了确认。S果实中毛色二孢的CT值与乙烯产量呈负相关（哈姆林R=-0.838；巴伦西亚R=-0.858），与果实脱离力呈正相关（哈姆林R=0.855；巴伦西亚R=0.850），这表明AZ-C中的毛色二孢定殖可能会加剧与HLB相关的采前落果。
