SPATIAL PATTERN ANALYSIS OF CITRUS CANKER INFECTED PLANTINGS IN SAO PAULO BRAZIL AND IMPLICATION OF THE ASIAN LEAFMINER ON POTENTIAL DISPERSAL PROCESSES

Authors: Gottwald, Timothy; BASSANEZI, R. B. - FUNDECITRUS, ARARAQUARA; AMORIM, L. - UNIV. OF SAO PAULO, BR; BERGAMIN-FILHO, A. - UNIV. OF SAO PAULO, BR

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: 9/30/2005
Publication Date: 11/7/2005
Citation: Gottwald, T.R., Bassanezi, R., Amorim, L., Bergamin-Filho, A. 2005. Spatial pattern analysis of citrus canker infected plantings in sao paulo brazil and implication of the asian leafminer on potential dispersal processes. International Citrus Canker and Huanglongbing Workshop, Orlando, FL, Nov 7-11, 2005. C7, p.17.

Interpretive Summary:

Technical Abstract: Eradication of Asiatic Citrus Canker (ACC) has become increasingly difficult over the last decade following the introduction of the Asian leafminer into Brazil and Florida. This prompted epidemiological studies in both countries that resulted in changes in the eradication protocols. The objective of this study was to characterize the spatial patterns of ACC in commercial citrus plantings in Brazil, to improve understanding of disease dynamics post introduction of the leafminer. The spatial patterns of ACC were mapped in 326 commercial citrus plantings, and statistically assessed at various spatial dimensions. The presence of ‘within-group’ aggregation in each plot was examined via beta-binomial analysis for groups of trees parsed into 3X3 quadrats. The relative intensity of aggregation was expressed as a Beta-binomial index of dispersion (I') and heterogeneity among plots expressed as the intracluster correlation coefficient, '. The population of data sets was found to fall in to three I' categories, I' < 1.3, 1.3 ' I' ' 3.5, and I' > 3.5. These categories were related to other spatial characteristics. The binary form of Taylor’s power law was used to assess the overdispersion of disease across plots and was highly significant. When the overall population of plots was parsed into I' categories, the Taylor’s R2 improved in all cases. Although these methods assess aggregation well, they do not give information on the number of foci or aggregations within each plot. Therefore the number of foci/1000 trees was quantified and found to relate directly to the I' categories. The I' < 1.3 category could be explained by a linear relationship of the number of foci to disease incidence, whereas the 1.3 ' I' ' 3.5, and I' > 3.5 categories were most easily explained by a generalized beta function. Spatial autocorrelation was used to examine the spatial relationships ‘among groups’ composed of 3X3 quadrats and determine common distances between these groups of ACC-infected trees. Aggregation was found in >84% of cases at this spatial level and there was a direct relationship between increasing I' category and increasing core cluster size and aggregation at the among-group spatial hierarchy was generally stronger for the within-row than for the across-row orientation. Clusters of disease were estimated to average between 18 and 33 tree spaces apart, and the presence of multiple foci of infection was common. The effectiveness of the eradication protocol for removing all ‘exposed’ trees within 30 m surrounding each ‘ACC-infected tree’ was examined, and the distance of subsequent infected trees beyond this 30-m zone from the original focal infected tree was measured for each plot. A frequency distribution was compiled over all plots to describe the distance needed to circumscribe all of these outliers as a theoretical alternative to the 30-m eradication protocol. The frequency distribution was described by a monomolecular model (r2 = 0.98) and used to determine that 90, 95, and 99% of all new infected trees occurred within 296, 396, and 623 m of prior infected trees in commercial citrus plantings. These distances are very similar to those previously reported for ACC exposure in residential settings in Florida.