Monitoring and spatiotemporal analysis of ppv-m spread in two apricot orchards in southern greece
Two apricot orchards established in Southern Greece with healthy material in 1992 and 1993 have been monitored since 1996 for Plum pox virus (PPV) spread. Assessments on the trees for PPV identification were made each year; trees with symptoms were tested by using monoclonal antibodies, whereas symptomless trees were tested by using polyclonal antibodies. The two orchards were established in different areas: orchard 1 in an area with high inoculum pressure and orchard 2 in a naturally isolated site surrounded by hills. In addition, orchard 1 was comprised of two plots: plot 1a was in direct proximity to an old infected apricot orchard; plot 1b was established a year later and was separated from plot 1a by a hedge. In all cases, only the PPV strain M was identified. Differences in the rate of PPV spread were observed among plots and/or orchards. Within seven years, PPV incidence in plot 1a reached 85.4%, in plot 1b 31.8% and in orchard 2 26.2%. The difference between plot 1a and 1b was attributed to the hedge effect that slowed down aphid flights and virus spread. In the case of orchard 2, the slow PPV spread, due to natural isolation of the orchard, was expected. To further analyse these data spatiotemporaly, the coordinates of the infected trees were determined based on a defined coordinate system and a distance-based Monte Carlo test was performed on the data of each sampling year to find whether infected trees were scattered randomly or not. The statistic used for the Monte Carlo test was the mean of the distance between infected trees and their nearest infected neighbours. The statistical analysis revealed that in plot 1a the pattern of PPV spread was a cluster in 1996 (P<0.01%) and that uniformity was established after 1997. In plot 1b a complete spatial random pattern (P=0.014) was obtained in 1996, but clusters appeared over the next years (P=0.003 for 1997, P=0.002 for 1998, P<0.01% for 1999), whereas in orchard 2 a complete spatial randomness (P<0.01) was obtained throughout the years.