Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: August 4, 2003
Publication Date: N/A
Interpretive Summary: Citrus Huanglongbing (HLB), previous called greening disease, is a bacterial disease of citrus that is devastating to commercial and residential citrus throughout Asia. The disease is vectored by an insect that was introduced into South America in the 1930s and into the Florida about 2000. With the vector now present in the US, the disease is a considerable threat to commercial citrus in North America if introduced from Asia. To gain more information about the disease it was studied in mainland China, the Philippines, and Taiwan. Both the rate of disease increase and the patterns of spread were studied. The understanding of the dynamics of this disease helps to predict outcomes if the disease is introduced into the US and also will help regulatory agencies to develop eradication/containment strategies.
Technical Abstract: The epidemiology of Huanglongbing (HLB) has been observed, but only relatively few quantitative epidemiological studies have been conducted due to the perennial nature of the disease requiring a dedication to data collection over multiple years combined with the inability until recently of detection by ELISA or PCR. Studies were conducted to establish preliminary rates of disease increase of HLB under endemic conditions in the presence of vector populations, and develop preliminary temporal models to estimate the expected longevity of infected citrus groves. Both the logistic models adequately described disease progress in these plots over time. The relative increase of the HLB disease is slow and multi years in duration, even when vector populations are high and inoculum sources are prevalent. One interpretation is that vector transmission in the field must therefore be inefficient. Spatial pattern analysis was undertaken to better understand the relationships among infected trees near and at distance to each other and thereby gain some understanding of vector spread of HLB. The combined analyses indicate two 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, i.e., encompassing as many as 1672 trees. Over longer distances, secondary foci develop. These 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.