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Background and aims: The olive root endophyte Pseudomonas fluorescens PICF7 is an effective biocontrol agent of Verticillium wilt of olive (VWO) . Colonization of olive roots either by strain PICF7 or by Verticillium dahliae triggers differential systemic transcriptomic responses, many of them related with defense-related genes. The aims were to develop an olive split-root system for assessing VWO development and biocontrol effectiveness of strain PICF7 in plants with a divided root architecture, and for evaluating systemic defense responses during this tripartite interaction when strain PICF7 and V. dahliae are spatially separated. Methods: An olive split-root system was generated and disease development, biocontrol effectiveness and systemic genetic responses in these plants upon strain PICF7 and V. dahliae colonization were compared to those reported and observed in olive plants grown under standard conditions (single pots). Specific defense-related genes, previously identified during PICF7- and/or V. dahliae-olive root interactions were selected and their expression patterns assessed in above-ground tissues by real-time qPCR analyses. Results: Symptoms of VWO developed similarly both in split-root and single-root plants. However, even though PICF7 triggered systemic defense responses in aerial tissues prior to the infection by V. dahliae, effective biocontrol was not observed under these experimental conditions. While most of studied genes showed similar expression patterns along time in both systems (i.e. split root and single pot), some of them (e.g. the caffeoyl-O-methyltransferase coding gene) varied depending on whether strain PICF7 and V. dahliae were spatially separated or shared the same compartment. Conclusions: A successful split-root system was generated to investigate genetic events taking place during the tripartite interaction olive- V. dahliae-P. fluorescens PICF7. VWO biocontrol by strain PICF7 must rely on mechanisms other than induction of systemic resistance responses. The expression pattern of specific defense-related olive genes depended on whether or not the biocontrol agent and the pathogen share the same root/soil region. [ABSTRACT FROM AUTHOR]

Developing verticillium wilt resistant genotypes is currently a major objective in olive breeding. In this study, 6017 genotypes derived from 48 crosses obtained by open pollination and crosses between olive cultivars, wild olive genotypes and other Olea species and Olea europaea subspecies were individually evaluated for verticillium wilt resistance. More than 800 genotypes were identified as resistant to the disease based on the absence of symptoms. High genetic variability and wide segregation in resistance were observed. The inheritance of resistance was studied, and the best parents and crosses to breed resistant genotypes were identified. According to the results, verticillium wilt resistance in olive appears to be a quantitative trait. The results obtained by comparing the level of resistance between different crosses as well as by estimating heritability suggest that it is possible to breed for verticillium wilt resistance in olive. [ABSTRACT FROM AUTHOR]

The resistance of 28 Spanish olive cultivars to Verticillium dahliae was evaluated in an experiment conducted under greenhouse conditions, by impregnating plant roots with a semisolid fluid mass of a mixture of culture medium and the conidia and mycelium of the fungus. Five-month-old olive plants were inoculated with a cotton defoliating isolate of V. dahliae. 'Frantoio' and 'Picual' were used as resistant and susceptible reference cultivars, respectively. Cultivars were assessed on the basis of final values of the area under the disease progress curve, mean severity of symptoms, and mortality at 26 weeks following inoculation. Verticillium wilt disease developed more slowly and reached lower values of these parameters than those normally recorded in previous studies conducted in growth chambers, using root-dip inoculation in a conidial suspension of the pathogen. However, most of the evaluated cultivars exhibited susceptible or moderately susceptible reactions to the infections caused by V. dahliae. In particular, a group of eight cultivars, from the same group as 'Picual', such as 'Manzanilla de Abla', 'Manzanilla del Centro' and 'Negrillo de Iznalloz', were significantly more susceptible than 'Frantoio'. Conversely, 'Escarabajillo', 'Menya' and 'Sevillana de Abla' exhibited a high level of resistance to the disease, no dead plants, and vegetative recovery. Field experiments are currently being carried out to confirm the level of resistance assigned to these last genotypes. If confirmed, these genotypes will act as potential resistant genitors for inclusion in current olive breeding programs or for use as resistant rootstocks. [ABSTRACT FROM AUTHOR]

An experiment was conducted in microplots which were artificially infested with a defoliating isolate of Verticillium dahliae using seven different treatments of inoculum densities ranging from 0 to 10 microsclerotia per gram of soil (ppg). The experiment was conducted in Andalucía (southern Spain), and the susceptible Spanish olive cv. Picual was used to determine the relationship between pathogen inoculum density and the progress of Verticillium wilt of olive (VWO). The inoculum, produced on a sodium pectate cellophane medium, was found to efficiently infect olive trees. Symptoms first appeared 30 weeks after the trees were transplanted into infested soil. Periods of increasing disease incidence in the following seasons and years were mainly during spring and autumn, particularly in the second year after planting. Olive trees exhibited a high susceptibility to the defoliating pathotype of the pathogen, even at very low inoculum levels; in fact, diseased plants were encountered throughout the experiment regardless of the inoculum density treatment. Inoculum densities greater than 3 ppg in the soil resulted in final disease incidence greater than 50% for the trees after 2.5 years. Therefore, these inoculum densities must be considered very high for olive trees. There were no differences in final disease incidence, mean symptom severity, or area under the disease progress curve between plots infested with 10 or 3.33 ppg, whereas other treatments exhibited lower values for each of these disease parameters. The temporal variations of disease incidence and severity were highly correlated for the higher inoculum density treatments, with r² values ranging from 0.92 to 0.84 for disease incidence and from 0.93 to 0.88 for severity. However, r² was slightly lower for the treatments involving lower inoculum densities of the pathogen in microplots. The slopes of the linear regression curves were statistically different for nearly all the inoculum density treatments. Positive correlation was found between the initial inoculum density and final disease incidence values after the study period that was accurately explained by mathematical models. The results suggest that susceptible olive cultivars should not be planted in soils infested with virulent defoliating pathotypes of V dahliae. Results also clarify that inoculum density levels obtained from field soil analyses can be used for establishing a risk prediction system with a view to controlling VWO in olive tree plantations. [ABSTRACT FROM AUTHOR]

Verticillium wilt of olive (Olea europaeaL.), caused byVerticillium dahliae, is nowadays the most serious olive disease in Spain. The disease increments are being observed particularly in young olive plantations, favoured by several factors including inadequate cultural practices and crop production intensification, such as irrigation. Thus, three olive orchards affected by Verticillium wilt, with disease incidence ranging 30–50%, were selected to determine if the drip irrigation could favour the increase of pathogen in soil. Pathogen in soil was quantified in wet zones around the drippers and in dry zones out of them. Inoculum density in all experiments was higher in wet than in dry areas. After 4 months of watering, soil pathogen population increased considerably in wet and dry areas but inoculum density remained higher in the wet soil. [ABSTRACT FROM AUTHOR]