ARS | Publication request: Huanglongbing Epidemiology: An international perspective

Submitted to: Meeting Abstract
Publication Type: Proceedings/Symposium
Publication Acceptance Date: July 14, 2006
Publication Date: July 22, 2006
Citation: Gottwald, T.R., Aubert, B., Zhao, X., Bergamin-Filho, A., Bassanezi, R., Amorim, L. 2006. Huanglongbing Epidemiology: An international perspective. Proceedings of Huanglongbing-greening International Workshop, July 14-21, 2006, Ribeiro Preto, Brazil. P. 35-36.

Technical Abstract: Prior to the discovery of Huanglongbing (HLB) in Brazil and Florida in 2004 and 2005 respectively, very few quantitative epidemiological studies had been conducted, and thus the increase and spread of the disease remains incompletely characterized. The main issue is the perennial nature of the disease which necessitates dedication to data collection over multiple years. This is often problematic, because of the concern of continued spread resulting in obligatory or preferred diseased mitigation strategies such as tree removal to attempt to control or contain the disease, which then makes tracking disease increase and spread very difficult or impossible. This limitation has been coupled with the inability, until recently, of detection by PCR, requiring a reliance on visual assessment for disease monitoring. Monitoring the occurrence of HLB by visual symptoms alone is problematic. The lag in time between transmission by psyllid vectors or propagation and the onset of symptoms can be quite variable and quantifying the severity of disease symptoms in individual trees may not be a true indication of pathogen concentration. Additionally, due to the temporal variation in symptom expression, trees infected at the same time may express symptoms over one or more years. This variable lag period compromises the accuracy of spatial and temporal studies. Symptoms observed are the expression of infections that have occurred over one or more years in the past. Thus we are visually assessing a `fuzzy history¿ of infection that has occurred over a prior time period equivalent to the lag period. Nevertheless, epidemiological information can be used to predict the economic and physical life of a given planting and provide a means to investigate the efficacy of potential control interventions. Considering the above caveats, epidemiological data has been collected from a few locations in Reunion Island (RI), southern China (LCF, LARI), Taiwan, the Philippines and more recently in São Paulo, Brazil and southern Florida, USA. However, to date an assessment of disease progression has only been accomplished in China and Reunion Island and resulted in an estimated reduced longevity of HLB-infected sweet orange and mandarin groves. HLB epidemics are multiyear in duration, but rarely progress to an asymptote before removal of the planting occurs. Therefore, both the exponential and the logistic models adequately described disease progress over time. In the LCF plot, an asymptote was reached after 13 yrs. However, for groves that became unproductive, disease incidence never reached asymptotic levels prior to removal, but logistic model predictions calculated that HLB would reach asymptotic disease after 7 and 13 years. Spatial analysis of HLB data has been less problematic and was undertaken to better understand the relationships among infected trees near to and at distance from each other and thereby gain some understanding of vector spread of HLB. Combined interpretations of spatial analyses indicate two spatial mechanisms of vector spread of HLB, within local areas and over longer distances. Within local areas, aggregations of infected trees occur that at times can be quite large (up to 1672 trees). In this case vectors apparently spread the disease to either adjacent or nearby trees. Spatial autocorrelation also identifies a prevalence of reflected clusters or areas of aggregation that are discontinuous with the main cluster. These are interpreted as indicative of the presence of secondary foci that are quite prevalent and are at a distance of about 25-50 m from the main cluster of disease and each other. Such a pattern of widely spaced foci perhaps indicates a spatial mechanism associated with longer distance vector movement. Longer regional scale vector transmission has not been investigated.