Abstract
Analyses of space-time statistical features of a flavescence dorée (FD) epidemic in Vitis vinifera plants are presented. FD spread was surveyed from 2011 to 2015 in a vineyard of 17,500 m2 surface area in the Piemonte region, Italy; count and position of symptomatic plants were used to test the hypothesis of epidemic Complete Spatial Randomness and isotropicity in the space-time static (year-by-year) point pattern measure. Space-time dynamic (year-to-year) point pattern analyses were applied to newly infected and recovered plants to highlight statistics of FD progression and regression over time. Results highlighted point patterns ranging from disperse (at small scales) to aggregated (at large scales) over the years, suggesting that the FD epidemic is characterized by multiscale properties that may depend on infection incidence, vector population, and flight behavior. Dynamic analyses showed moderate preferential progression and regression along rows. Nearly uniform distributions of direction and negative exponential distributions of distance of newly symptomatic and recovered plants relative to existing symptomatic plants highlighted features of vector mobility similar to Brownian motion. These evidences indicate that space-time epidemics modeling should include environmental setting (e.g., vineyard geometry and topography) to capture anisotropicity as well as statistical features of vector flight behavior, plant recovery and susceptibility, and plant mortality.
Highlights
Flavescence dorée (FD) is one of the most important and damaging phytoplasma diseases of grapevine (Vitis vinifera L.) causing yellowing or reddening and downward rolling of leaves, drying of inflorescences and bunches, and lack of cane lignification (Caudwell, 1990)
Analyses presented here for the characterization of FD epidemic point patterns over a time frame of 5 years using area- and distance-based methods, as well as correlation functions, highlighted that epidemic patterns over the field scale did not generally satisfy the Complete Randomness Hypothesis (CRS) but rather showed either sparse or aggregated infection points, the former being typical of the initial epidemic stages and the latter being typical of the epidemic exponential growth phase
A moderate spatial preferential patter caused by row effect was found from multidirectional and bidirectional twodimensional autocorrelation and cross-correlation functions
Summary
Flavescence dorée (FD) is one of the most important and damaging phytoplasma diseases of grapevine (Vitis vinifera L.) causing yellowing or reddening and downward rolling of leaves, drying of inflorescences and bunches, and lack of cane lignification (Caudwell, 1990). While FD was initially detected in France in the 1950s (Caudwell, 1957), it is present in several grapevine growing areas of Europe (EFSA-PLH Panel on Plant Health, 2014), including the Piemonte region in northwestern Italy (Gotta and Morone, 2001; Marzachì et al, 2001). In this region, disease incidence is locally very high, possibly due to an increasing S. titanus population on wild grapes as noticed by the authors and in Pavan et al (2012). In spite of the remarkable resources devoted to control S. titanus presence with mandatory insecticide applications and compensate yield losses (more than €34 million in the European Union), the vector is expanding its presence in western, eastern, and southern Europe (Chuche and Thiery, 2014; Digiaro et al, 2014)
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