Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2003
Publication Date: 6/1/2003
Citation: Dallot, Sylvie, Gottwald, Tim R., Labonne, Gerard, Quiot, Jean-Bernard. 2003. Spatial Analysis of the Sharka Disease (PPV-M) Within Peach Orchards of Southern France Investigated at Various Hierarchical Scales. Phytopathology. Interpretive Summary: Plum Pox virus causes one of the most serious diseases of Prunus fruit trees worldwide and has caused major losses across Europe and is now affecting the Americas resulting in quarantines and eradication programs. New outbreaks have been detected in Pennsylvania, Canada and was previously detected in Chile. The virus is transmitted by aphids and moved over long distances by movement of plant material by man. There are several strains of the virus but the most common are the D and M strains, the M strain is severe and fast moving whereas the D strain in not very severe and is slower moving.. In North America only the D strain has been found to date and the D strain is the primary strain that has been previously investigated relative to spread. This paper represents the first analysis of the spread and movement of the M strain and was conducted in fruit orchards in the south of France. We applied a number of sophisticated epidemiological statistical procedures to the data collected and were able to describe and model the disease. By analyzing the spatial patterns of the disease we have a more complete understanding of how the disease is dispersed, can develop more accurate sampling plans, and more effective strategies for removal of diseased trees to augment eradication efforts.
Technical Abstract: The spatial pattern of Sharka disease, caused by the Plum pox virus strain M, was investigated in 18 peach plots located in two distinct areas of Southern France. PPV infections were monitored visually for each individual tree during one to three consecutive years. Point pattern and correlation type approaches were undertaken using the binary data directly or after parsing them in contiguous quadrats of 4, 9 and 16 trees. Ordinary runs generally evidenced a low but variable proportion of rows with adjacent symptomatic trees. The beta-binomial distribution and related indices indicated aggregation of infected peach trees within the quadrats for 15 of the 18 plots tested for at least one assessment date of each. When aggregation was detected, it was indicated at all quadrat sizes and tended to be a function of disease incidence, as shown by the binary form of Taylor's power law. Spatial relationships among quadrats of 2x2 trees of similar status were evidenced in a majority of plots by spatial autocorrelation. The size and shape of the main proximity patterns indicated that the local area of influence of PPV spread was often ellipsoidal and extended from 2 to 12 trees in the row direction and 4 to 6 trees in the across and diagonal directions. Relationships at longer distances, discontinuous to the main proximity pattern, were often suggested by the detection of reflected clusters, located 60 to 90 meters away from the origin. A good agreement was generally observed between the results of SADIE analysis and spatial autocorrelation. Aggregation and clustering indices provided complementary information to further investigate the spatial patterns of infected trees at the plot scale. The combination of the different approaches undertaken revealed a wide range of spatial patterns of PPV-M infected trees among the various plots tested. The mechanism of primary virus introduction, the age and structure of the orchards when infected and the diversity of vector species had a strong influence on the secondary spread and the resulting spatial patterns of PPV-M that developed in peach orchards.