|Palle, S. R.|
|Da Graca, J. V.|
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: Gottwald, T.R., Palle, S., Miao, H., Seyran, M., Skaria, M., Da Graca, J. 2005. Assessment of the possibility of natural spread of citrus psorosis disease. International Organization of Citrus Virologists Proceedings. Interpretive Summary: Citrus psorosis is a virus disease of citrus that is not completely understood. The disease causes lesions on the trunk and stems of the tree and a general decline. Although a virus has been detected, the means of the disease to move within and between orchards has never been completely worked out. It is known that it can be moved by grafting and moving infected plant material, but there have been some indications that the disease also may have a vector, that is, an insect or some other means of moving the virus. This study was undertaken to see if any indications of the existence of a vector could be determined by doing several complex epidemiological analyses that might indicate the affect 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: Observations in Argentina, Texas and California several years ago on the increase in incidence of citrus psorosis symptoms in the field suggested a possible natural spread of the disease. Specifically, past and more recent observations on the incidence of psorosis in Texas in nucellar Redblush grapefruit trees and in originally virus-free Rio Red grapefruit trees also support the theory that natural transmission occurs. The incidence of psorosis symptoms in four orchards of grapefruit trees was recorded annually over a 3-year period, rating trees on a scale of 0 (no symptoms), 1 (some bark peeling and/or gumming), 2 (psorosis-like scaling) and 3 (classic psorosis bark-scaling). To assess the possibility of biotic cause of psorosis spread, the spatial arrangement of infected plants was examined at different spatial scales (single tree, groups of plants, and whole plantings) and stochastic simulation models were used to predict the likelihood and contribution of potential exogenous versus endogenous sources of inoculum. Aggregation was observed both at the single tree, and groups scale that decreased through time. Some evidence of relationships among groups of diseased trees was also observed and stochastic models indicated the likelihood of background (exogenous) sources of inoculum. These spatial and spatio-temporal findings are consistent with expected disease spread via vectors although two different possible cases of spread could be identified among the four plots. Citrus psorosis virus is the type member of the Ophiovirus genus, and there is evidence that other members are transmitted through the soil by Olpidium brassicae. An Olpidium-like fungus was found on the roots of psorosis-infected trees, and the virus was detected in zoospores by PCR.