|Da Graca, John|
Submitted to: HortScience
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/8/2013
Publication Date: 1/2/2014
Publication URL: http://handle.nal.usda.gov/10113/58802
Citation: Kunta, M., Da Graca, J.V., Malik, N.S., Louzad, E.S., Setamou, M. 2014. Quantitative distribution of Candidatus Liberibacter asiaticus in the aerial parts of the field grown HLB-infected citrus trees in Texas. HortScience. 49:65-68. Interpretive Summary: The multi-billion dollar US citrus industry is being destroyed by citrus greening disease named Huanglongbing (HLB). In fact, HLB is considered as the most destructive disease worldwide that has caused devasting economic damage to citrus industries in several countries. Unfortunately, there is no known cure for this disease at this time. One reason for the lack of remedy for the disease is that so far the disease causing bacteria (Candidatus Liberibacter spp) have never been isolated and cultured; and hence, the traditional culture based methods of counting CFU (colony forming units) grown on a plate cannot be employed for bacterial quantifications. It has ben suggested that knowledge of pathogen distribution inside the tree is essential to choose an appropriate sample for diagnostic tests, understand the virulence mechanism, and to develop disease management strategies. Therefore, molecular techniques were used to assess the quantities of disease causing bacteria in various tissues of the citrus cultivars. Lowest bacterial quantities were consistently recorded in young shoots and leaf blade, especially leaf margins. These finding provide important information to develop effective and efficient methods of managing HLB.
Technical Abstract: The Asian citrus psyllid, Diaphorina citri Kuwayanna, one of the vectors for citrus greening disease or Huanglongbing (HLB) has been present in Texas for over a decade, but the detection of the disease is recent. HLB has been confirmed in only two adjacent commercial citrus groves of grapefruit and sweet orange. A study was conducted to compare the population of Candidatus Liberibacter asiaticus (CLas) cells in different plant parts including peduncle, columella, leaves, seeds, young shoots, flower buds, flowers, and bark of six year-old known infected grapefruit and sweet orange trees. The bacterial population was derived using a previously described grand universal regression equation Y=13.82-0.2866X, where Y is the log of the target copy number and X is the Ct (threshold cycle) of the assay. Overall, significantly higher CLas cells were recorded in grapefruit than sweet orange. The bacterial concentration also varied with the plant part such as the peduncle and columella midrib that had a significantly higher titer of CLas compared to other plant parts. The results obtained are in agreement with previous studies conducted on Florida samples, but a new finding was that the lowest bacterial titer was consistently recorded in young shoots and leaf blade, especially leaf margins.