Submitted to: International Organization of Citrus Virologists Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/18/2005
Publication Date: 3/18/2005
Citation: Castle, W.S., Gottwald, T.R. 2005. Spatio-temporal analysis of tree decline losses among navel orange trees on swingle citrumelo rootstock in two central florida citrus groves. International Organization of Citrus Virologists Proceedings. Interpretive Summary: Citrus blight is a serious disease of citrus in many tropical citrus growing countries. The disease has been investigated for over 100 years yet its cause remains unknown. The disease causes a general decline due to a plugging of the vascular system and eventually death of the affected tree. Some researchers believe it is caused by a contagion but other researchers believe it is abiotic, that is, perhaps nutritional or a genetic defect in citrus trees themselves. It is thought that it can move by root grafting between trees but no other means of movement is known. There has been some speculation and indications that the disease also may have a vector, that is, an insect or some other means of moving the contagion if it exists. This study was undertaken to see if any indications of the existence of a vector could be determined by several complex epidemiological analyses that might indicate the effect of a vector and if one is indicated, help to identify what it might be. The study was partially successful in that the interaction of some kind of vector was indicated in several cases. However, the type of vector was not entirely clear although it was not inconsistent with movement of other virus diseases whose vector is an aphid.
Technical Abstract: The incidence of decline in two blocks of navel orange [Citrus sinensis (L.) Osbeck] trees on Swingle citrumelo [C. paradisi Macf. x Poncirus trifoliata (L.) Raf.] rootstock planted in 1977 and 1991, respectively, was mapped annually. Trees were growing in typical sand soils of Central Florida. Declining trees began to appear in the mid and late 1990s in the older and younger groves, respectively. Selected trees in each grove were assayed each year for trunk water uptake, blight P-12 protein, and other possible causes of decline. Most trees tested were positive for blight especially in the younger grove, and displayed typical canopy symptoms for blight. Some trees in the older grove had abnormal bud unions suggesting a different cause for decline related to a possible mechanical pinching effect from a severe overgrowth of the scion by the rootstock. Those symptoms did not appear until the trees were ca. 15 years old. Spatial analyses were conducted at the individual tree, local area (small groups of trees) and orchard levels. Blight incidence was inconsistently aggregated at the individual tree level and more consistently at the local level for 2X2, 3X3, and 4X4 tree group sizes for all plot/year combinations tested. SADIE analysis, used to test for aggregation at the plot level, also demonstrated little evidence for aggregation for all plot/year. Stochastic spatio-temporal modeling was used to examine the likelihood of blight movement from local versus background interactions and to help interpret the biotic versus abiotic cause of blight. The likelihood maps of the spatio-temporal model indicated spread of blight occurred within a local vicinity of a few trees although not necessarily to nearest neighboring trees and that there was some tendency for background longer range transmission. Therefore, although the causality of blight remains unclear, and there is no conclusive evidence for vector transmission, there is some indication that the dynamics of the spread of blight are similar in many regards to the CTV/Toxoptera citricida pathosystem. If a vector is involved in blight transmission, there is no evidence whatsoever that it is an aphid, however, the likelihood is that it acts with spatial dynamics similar to Toxoptera citricida.