Spread of Plum pox virus strain M in two apricot orchards in southern Greece

Spread of Plum pox virus (PPV) strain M has been monitored for five years in two apricot orchards, established in 1992 and 1993 in Southern Greece with PPV-free material. 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. In addition, orchard 1 was comprised of two plots: plot la was in direct proximity to an old heavily infected apricot orchard; plot 1b was established a year later and was separated from plot la by a hedge. In addition, aphids alighting on apricot trees were recorded in 1999 and 2000 by the sticky tree method. Aphis gossypii and Hyalopterus pruni were the most abundant virus vectors captured. In both orchards, only strain M was identified by using monoclonal antibodies. The rate of PPV spread in these orchards was followed from 1996 to 2000. Within eight years from the date of the initial establishment, PPV incidence in plot la reached 88.6%, in plot 1b 41% and in orchard 2 40.8%. The difference between plot la and 1b was attributed to the hedge effect that reduced the number of incoming viruliferus aphids. In orchard 2, slow virus spread, due to natural isolation of the orchard, was expected. To further analyse these data spatiotemporaly, a distance-based Monte Carlo test was performed on the data of each year to find out whether infected trees were scattered randomly or not. The statistical analysis revealed that initial infections followed a complete spatial random pattern, while loose clusters appeared over the next years, to finally reach a uniformity representing high infection levels.