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United States Department of Agriculture

Agricultural Research Service

Title: Concepts in Huanglongbing Epidemiology

Authors
item Gottwald, Timothy
item Bergamin Filho, A. - UNIV. DE SAO PAULO
item Bassanezi, R. - FUNDECITRUS
item Irey, M. - U.S. SUGAR CORP.
item Zhao, X. - CHINESE ACADEMY OF AG SCI
item Aubert, B. - CIRAD-FLHOR

Submitted to: Symposium Proceedings
Publication Type: Proceedings
Publication Acceptance Date: December 10, 2006
Publication Date: February 21, 2007
Citation: Gottwald, T.R., Bergamin Filho, A., Bassanezi, R., Irey, M., Zhao, X., Aubert, B. 2007. Concepts in Huanglongbing Epidemiology. Symposium Proceedings.

Interpretive Summary: A new disease for Florida citrus called Huanglongbing (HLB) or citrus greening was discovered in Florida in 2005 and in Brazil in 2004 and has spread across the southern half of Florida and to over 100 municipalities in the state of Sao Paulo Brazil. The disease is moved by an insect vector call as psyllid and causes yield and quality losses eventually killing the tree. This paper documents previous work in China and Reunion Island describing the increase and spread of the disease. The work describes two general scenarios for the increase of the disease, on e slow and one fast. In the slow scenario, the disease increase in incidence over about a 12-13 year period until all trees are infected in a planting. In the fast scenario the disease increases much more rapidly and can go from 0 to 100% infection in only 2-4 years. The experience in South Florida to date and in Brazil suggest that we are experiencing the faster scenario at the moment, and thus considerable concern is placed by the commercial citrus industries on this disease. The psyllid vectors also seem to transfer the disease both to nearby trees forming groups or clumps of diseased trees and also over longer distances forming secondary groups of diseased trees. These groups tend to be about 20-50 meters apart, indicating the relatively short flight of the disease carrying vector insects.

Technical Abstract: Huanglongbing (HLB) was discovered in Brazil and Florida in 2004 and 2005 respectively. Previously, very few quantitative epidemiological studies had been conducted, and thus the increase and spread of the disease remains incompletely characterized. The perennial nature of the disease necessitates a dedication to data collection over multiple years, to obtain spatio-temporal data sufficient to understand increase and spread of the disease. In most areas, removing HLB-diseased trees in the attempt to control or contain the disease is one of the main disease mitigation strategies. This 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 is not a true indication of pathogen incidence. Additionally, due to the temporal variation in symptom expression, trees infected at the same time may express symptoms over one or more years. 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 the incidence of HLB visual symptoms 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. In most cases 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.

Last Modified: 10/30/2014
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