Preliminary spatial pattern analysis of Huanglongbing in São Paulo, Brazil

Abstract

3 USDA, ARS, USHRL, 2001 S. Rock Road, Fort Pierce, FL, 34945 USA ABSTRACT. Huanglongbing (HLB) was reported in 2004 in Sao Paulo, Brazil. Assessments of diseased trees by visual symptoms were made in 36 plots from 8 farms in the central citrus region of Sao Paulo State. A total of 155 HLB spatial maps (varying from 0.14 to 25.99% disease inci- dence) were analyzed, considering quadrat sizes of 2 × 2, 4 × 4, 6 × 6 and 8 × 8 trees, by ordinary runs analysis, binomial index of dispersion and binary form of Taylor's power law. Aggregation among HLB-symptomatic trees was detected by ordinary runs analysis, with clustering in both within- and across-rows directions. However the percentage of aggregation within- and across- rows was low. The binomial index of dispersion for various quadrat sizes suggested aggregation of HLB-symptomatic trees for about 40% of the plots. The relationship between log (observed vari- ance) and log (binomial variance) was highly significant for all four quadrat sizes. Estimated parameters of the binary form of Taylor's power law provided an overall measure of aggregation of HLB-symptomatic trees for all quadrat sizes tested. All power law estimates of b and A, were sta- tistically different from 1, which indicated a general and significant pattern of aggregation of symptomatic plants for all quadrat sizes tested. The degree of aggregation was also positively related to disease incidence. Data from 20 plots ranging in disease incidence were also analyzed by spatial autocorrelation to examine the association among groups of infected trees using the 2 × 2 quadrat size. In 14 of 20 cases, clusters of HLB-infected trees were found to be associated with secondary clusters whose centers were at distances ranging from 4.2 to 22.1 tree spaces distant, indicating psyllid vector movement resulting in transmission to nearby trees causing clusters and to trees at considerable distance initiating new foci of infection. Index words . Binomial analysis, disease incidence, infection foci, Taylor's power law.