Spatial and Temporal Analyses of Citrus Sudden Death as a Tool to Generate Hypotheses Concerning Its Etiology

Abstract

Citrus sudden death (CSD), a new disease of unknown etiology that affects sweet orange grafted on Rangpur lime, was visually monitored for 14 months in 41 groves in Brazil. Ordinary runs analysis of CSD-symptomatic trees indicated a departure from randomness of symptomatic trees status among immediately adjacent trees mainly within rows. The binomial index of dispersion (D) and the intraclass correlation (k) for various quadrat sizes suggested aggregation of CSD-symptomatic trees for almost all plots within the quadrat sizes tested. Estimated parameters of the binary form of Taylor's power law provided an overall measure of aggregation of CSD-symptomatic trees for all quadrat sizes tested. Aggregation in each plot was dependent on disease incidence. Spatial autocorrelation analysis of proximity patterns suggested that aggregation often existed among quadrats of various sizes up to three lag distances; however, significant lag positions discontinuous from main proximity patterns were rare, indicating a lack of spatial association among discrete foci. Some asymmetry was also detected for some spatial autocorrelation proximity patterns, indicating that within-row versus across-row distributions are not necessarily equivalent. These results were interpreted to mean that the cause of the disease was most likely biotic and its dissemination was common within a local area of influence that extended to approximately six trees in all directions, including adjacent trees. Where asymmetry was indicated, this area of influence was somewhat elliptical. Longer-distance patterns were not detected within the confines of the plot sizes tested. Annual rates of CSD progress based on the Gompertz model ranged from 0.37 to 2.02. Numerous similarities were found between the spatial patterns of CSD and Citrus tristeza virus (CTV) described in the literature, both in the presence of the aphid vector, Toxoptera citricida. CSD differs from CTV in that symptoms occur in sweet orange grafted on Rangpur lime. Based on the symptoms of CSD and on its spatial and temporal patterns, our hypothesis is that CSD may be caused by a similar but undescribed pathogen such as a virus and probably vectored by insects such as aphids by similar spatial processes to those affecting CTV.

Spatiotemporal Analysis of Spread of Infections by Verticillium dahliae Pathotypes Within a High Tree Density Olive Orchard in Southern Spain

“…Each of those spatial analyses provided discrete, but complementary, information about the spatial pattern of distribution of Verticillium wilt-diseased olive trees in the orchard, and consequently provided a global view of the disease expansion process. This approach, first proposed by Madden and Hughes (31), proved efficient for describing spatial dynamics in several pathosystems characterized by systemic or local infections (1,9,16,38,44).…”

“…Analyses of spatial pattern were performed at three levels of spatial scale as described elsewhere (1,9,37,44).…”

Comparative epidemiology of citrus tristeza in Cuba and citrus sudden death in Brazil

“…Aggregation of infected plants was found in the second and third level of spatial hierarchy (Batista et al, 2008). For CSD, a preliminary epidemiological study was carried out and the results were compared to previously described tristeza epidemics in the Caribbean Basin (Bassanezi et al, 2003). Symptomatic plants were aggregated in most spatial scales.…”

“…CSD is graft-transmissible (Yamamoto et al, 2004) but the search for graft-transmitted agents such as endogenous bacteria, viroids and viruses has produced negative results (Bassanezi et al, 2003). However, CTV and a new marafivirus (Tymoviridae) provisionally called Citrus sudden death associated virus (CSDaV) have been found in diseased trees.…”

“…Symptomatic plants were aggregated in most spatial scales. Based on symptoms and the similarities found between the temporal and spatial patterns of CSD in Brazil and the CTV/ T. citricida pathosystem in Costa Rica and the Dominican Republic, it was proposed that CSD may be caused by a similar pathogen (likely a virus) and is probably vectored by aphids (or an insect with similar behavior) (Bassanezi et al, 2003). More detailed analysis of CSD epidemiology led to similar conclusions (Jesus Junior & Bassanezi, 2004;Bassanezi et al, 2005a).…”