|Bassanezi, R. B.|
Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2007
Publication Date: 5/1/2007
Citation: Gottwald, T.R., Bassanezi, R., Amorim, L., Bergamin-Filho, A. 2007. Spatial pattern analysis of citrus canker infected plantings in São Paulo, Brazil and implication of the asian leafminer on the potential dispersal processes. Phytopathology. 97:674-683. Interpretive Summary: The distribution of citrus canker infected trees in over 100 plantings in Brazil was examined to characterize how the disease spreads in commercial plantings. In 1992, the Asian leafminer, a moth insect, was introduced into Brazil and since then has been interacting with citrus canker disease by providing wounds for the bacteria to infect. This study examined the interaction between citrus canker and this insect and found that the disease is exacerbated by this moth by increasing faster and spreading farther. Also the disease is not as clustered or grouped in the presence of the moth but is more diffuse over a greater area. The distribution of canker in the State of Sao Paulo Brazil was related to the success of the Brazilian eradication program for this disease and helps to explains why the Brazilian protocol of removing all trees within 98 feet (30 meters) is not adequately controlling the disease. This finding changed the eradication protocol in Brazil for citrus canker. In addition, the dynamics of disease spread in Brazil was compared to the dynamics of the disease in Florida and found to be remarkably similar.
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, which lead to changes in the eradication protocols. The present study, undertaken in Brazil, was aimed at characterizing the spatial patterns of ACC in commercial citrus plantings, thereby attempting to gain better understanding of the dynamics of the disease 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-tree quadrats. The relative intensity of aggregation was expressed as a binomial index of dispersion (D) and heterogeneity among plots expressed as the intracluster correlation coefficient, '. The population of data sets was found to fall into three D categories, D < 1.3, 1.3 ' D ' 3.5, and D > 3.5. These categories were then 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 D categories, the Taylor’s R2 improved in all cases. Although these methods assessed 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 D categories. The lowest D category could be explained by a linear relationship of number of foci versus disease incidence, whereas the higher two categories were most easily explained by a generalized beta function for the same relationship. Spatial autocorrelation was then used to examine the spatial relationships ‘among groups’ composed of 3X3-tree 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 D 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 commonplace. The effectiveness of the eradication protocol of removing all ‘exposed’ trees within 30m 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 that would have been needed to circumscribe all of these outliers as a theoretical alternative protocol to the 30-m eradication protocol. The frequency distribution was well 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, respectively. These distances are very similar to previously reported distances determined for ACC in residential settings in Florida.