Spatiotemporal Analysis of Spread of Infections by Verticillium dahliae Pathotypes Within a High Tree Density Olive Orchard in Southern Spain

Abstract

The development of Verticillium wilt epidemics in olive cv. Arbequina was studied from November 1999 to May 2003 in a drip-irrigated, nontillage orchard established in a soil without a history of the disease at Córdoba, southern Spain. Disease incidence measured at 1-month-intervals increased from 0.2 to 7.8% during this period. Verticillium dahliae infecting the trees was characterized as defoliating (D) or nondefoliating (ND) pathotypes by a specific, multiplex-polymerase chain reaction (PCR) assay. Of the symptomatic trees, 87.2 and 12.8% were infected by the D or ND pathotypes, respectively. Dynamics of disease incidence were described by a generalized logistic model with a multiple sigmoid pattern. In the fitted model, the infection rate was highest in the winter to spring period and decreased to minimum values in the summer to fall period. Binary data of disease incidence was analyzed for point pattern and spatial correlation, either directly or after parsing them in contiguous quadrats. Overall, ordinary runs analysis indicated a departure from randomness of disease within rows. The binomial index of dispersion, interclass correlation, and Taylor's power law for various quadrat sizes suggested aggregation of diseased trees within the quadrat sizes tested. Spatial analysis by distance indices showed a nonrandom arrangement of quadrats containing infected trees. Spatial pattern was characterized by the occurrence of several clusters of infected trees. Increasing clustering over time was generally suggested by stronger values of clustering index over time and by the increase in the size of patch clusters. Significant spatial association was found in the clustering of diseased trees over time across cropping seasons; however, clustering was significant only for infections by D V. dahliae, indicating that infections by the D pathotype were aggregated around initial infections. The number and size of clusters of D V. dahliae-infected trees increased over time. Microsatellite-primed PCR assays of a representative number of V. dahliae isolates from diseased trees indicated that the majority of infecting D isolates shared the fingerprinting profile with D V. dahliae isolated from soil of a naturally infested cotton field in close proximity to the orchard, suggesting that short distance dispersal of the pathogen from this soil to the olive orchard may have occurred.

Verticillium wilt of olive: a case study to implement an integrated strategy to control a soil-borne pathogen

“…At late stages of the disease MS are formed in dead or dying plant tissues and pathogen biomass could be formed outside the fading vascular tissue. The incorporation and subsequent decomposition of plant debris, particularly of infected leaves (Wilhelm and Taylor 1965;Tjamos and Botseas 1987;Tjamos and Tsougriani 1990), facilitates MS release into soil, closing the parasitic phase of V. dahliae and contributing to the increase of the number of infective propagules within (Navas-Cortés et al 2008). Dormant MS will remain in soil until suitable conditions favor germination and and the formation of new infections and start a new parasitic phase (Pegg and Brady 2002).…”

“…Indeed, MS present in infested soil can be dispersed within and among olive orchards by wind (Easton et al 1969), transport of plant residues from affected crops to pathogen-free areas (Schnathorst and Sibbett 1971a, b), or use of cultivating machinery (Al-Ahmad and Mosli 1993; Serrhini and Zeroual 1995), which, in addition, can cause root damage and thus facilitate infection by the pathogen (Tjamos 1993). Similarly, the spread of contaminated plant tissues debris, particularly leaves (Wilhelm and Taylor 1965;Tjamos and Botseas 1987;Tjamos and Tsougriani 1990;Navas-Cortés et al 2008), or by pruning operations (Fig. 6b) (López-Escudero et al 2008) has been documented.…”

“…Indeed, temporal and spatial analyses of the disease in an olive orchard (about 400 trees/ha, approximately 4 ha, susceptible cv. Arbequina, drip-irrigated, flat plot, nontilling) was carried out during 5 years at Southern Spain (Navas-Cortés et al 2008). Assessment of V. dahliae isolates infecting the trees was performed by PCRbased and DNA fingerprinting tools as well, to track inoculum sources and spread of pathogen genotypes (D and ND pathotypes) throughout the orchard.…”

“…Main conclusions of this study were: 1) VWO epidemics in a newly-established, high tree-density, irrigated olive orchard could occur within a relatively short period of time; 2) V. dahliae D and ND pathotypes infecting olive trees have different disease spatial patterns; 3) Infections by the D isolates are more concerning since they can spread rapidly by windblown infected leaves that fall early and numerously from diseased trees; 4) Actions should be taken to avoid sources of primary pathogen inoculum (i.e. propagules in infected planting material and/or infested soil from neighboring fields); 5) Dispersal of inoculum (infested soil and/or infected fallen leaves) by human activities was considered highly relevant; and 6) The dispersal of V. dahliae propagules by surface water can be considered of minor importance, and just restricted to surface rain, under a situation of drip irrigation in a flat terrain (Navas-Cortés et al 2008). It is tempting to speculate whether scenarios like this one, where diverse triggering and overlapping factors are interacting, could serve to describe VWO epidemics in other types of olive orchards, or even in larger areas (i.e., at a regional scale).…”

Scientific opinion on the pest categorisation of Verticillium dahliae Kleb

“…the transport of infested soil by water or wind (Easton et al, 1969;Thanassoulopoulos et al, 1980;Soesanto, 2000); and the dissemination of infected host plant tissues, particularly cotton and olive leaves and olive inflorescences, by wind (Wilhelm and Taylor 1965;Schnathorst and Sibbett 1971;Tjamos and Botseas 1987;Tjamos and Tsougriani 1990;Navas-Cortés et al, 2008;Trapero et al, 2011).…”