Large-scale epidemiological study and spatial patterns of Verticillium wilt in olive orchards in southern Spain

Verticillium wilt (VW) is caused by the soilborne pathogen, Verticillium dahliae. The fact that available control measures are not completely effective, together with an increasing spread of the highly virulent defoliating; pathotype (D) in the last two decades, makes VW one of the most threatening olive tree disease. For better management, epidemiology and spatial patterns of VW were investigated in an important olive-growing area in southern Spain. A sampling survey was conducted from 2002 to 2005 in 873 olive orchards, which were assessed for the presence of V. dahliae. PCR assays were used to identify the highly virulent defoliating (D) and nondefoliating (ND) pathotypes of V. dahliae. Ordinary kriging, a geostatistical tool that uses regression techniques for interpolation of spatial data, was applied to obtain predictive V dahliae incidence maps and to evaluate the correlation between distance from the river and VW. Prevalence in olive orchards was 14.1%, with a disease incidence of 1.9% and mean incidence per olive orchard of 9.5%. Prevalence of the ND pathotype was higher (10.7%) than for the D pathotype (3.4%). VW was three times more prevalent in irrigated than in non-irrigated orchards (relative risk = 3.01) and there was a close association between the ND pathotype and irrigated orchards (relative risk = 2.03). Spatial pattern analysis showed several factors involved in the spread of the pathogen. VW was more prevalent in the north-west, suggesting the spreading of the disease from other bordering areas known to be contaminated, in southwest Spain. The D pathotype had a very restricted and aggregated distribution, suggesting the introduction of this pathotype in new olive orchards areas. The incidence map generated by kriging displayed an increase in north, northeast and south areas with numerous subareas with probabilities equalling or exceeding 53%, showing the effect of intensive agriculture (high plant density and irrigation) on incidence of the disease. Assessment of rivers-pathogen relationship revealed an association of VW with the distance to the nearest river, indicating that soils through the rivers' axis and previously cropped with horticultural crops susceptible to V dahliae, have implications for the establishment of new olive orchards and the occurrence of the disease. (C) 2008 Elsevier Ltd. All rights reserved.