Submitted to: Society of Citrus Nurserymen International Congress
Publication Type: Proceedings
Publication Acceptance Date: 3/24/1997
Publication Date: N/A
Interpretive Summary: Citrus tristeza virus (CTV) is one of the most destructive diseases of citrus worldwide. The virus is transmitted by aphid. Until recently in the U.S., the main vector has been the melon aphid. The most efficient vector of CTV is the brown citrus aphid, which was recently introduced into Florida in November 1995. This study contains data from over 160 research plots located in Florida, California, Spain, Costa Rica, the Dominican Republic, and Trinidad to study the different effects that these two aphids have on CTV increase and spread. This information will be used in the U.S. to help regulatory agencies and growers to better understand and prepare for increased economic losses due to CTV. It was found that CTV increased 3 to 4 times faster in the presence of the brown citrus aphid. It was also found that CTV-infected trees tend to be in aggregates or groups in a grove when the brown citrus aphid is present; whereas, CTV-infected trees were much more widespread when only the melon aphid was present. Such basic information on the study of aphid dynamics and their effect on CTV increase and spread will be used in future studies to help develop control strategies to lessen the effect of the brown citrus aphid on CTV-associated losses.
Technical Abstract: Data from research plots in commercial citrus plantations in Florida, Spain, Costa Rica, the Dominican Republic, and California have been used to develop models for the rate and the patterns of spread of citrus tristeza virus (CTV). In the presence of the melon aphid, and no brown citrus aphid, CTV progressed from low (5%) to high (95%) incidence levels in 8 to 15 years. In contrast, when the brown citrus aphid was predominant, this same increase often occurs in only 2 to 4 years. Spread of CTV was examined by analyzing spatial patterns of CTV to test for spread within rows versus spread across rows, for aggregation among adjacent trees, and for associations over longer distances. Where the melon aphid was the predominant vector, new infections were diffuse and not closely associated with existing infected trees. Where the brown citrus aphid was the dominant species, virus spread occurred to trees adjacent to existing infections, resulting in loose clusters of CTV infected trees. Moderate distance spread also occurred when both aphid species were present. We suspect that clustered spread was primarily caused by the brown citrus aphid, whereas, longer distance spread was probably due to the combined influence of both aphid species. The behavior of each aphid species and the species composition of the overall vector population greatly influenced the rate and patterns of CTV epidemics.