Susceptible Olive Cultivars Research Articles

Colletotrichum infections during flower development and fruit ripening in four olive cultivars

Olive anthracnose, caused by Colletotrichum, is an important disease in olive-growing regions, with the most destructive symptoms being fruit rot and blossom blight. Susceptibility of fruit to Colletotrichum increases with maturity, but differences between cultivars and Colletotrichum species have been reported, still information on flower susceptibility during development is scarce. The susceptibility of the olive cultivars Arbequina, Coratina, Frantoio and Picual was evaluated during flower development and fruit maturity to Colletotrichum acutatum s.s., C. nymphaeae, C. fioriniae, C. theobromicola and C. alienum. Susceptibility to anthracnose begins in early stages during flower development and increases during blossoming. Flowers of Arbequina, Coratina and Picual were susceptible, whereas those of Frantoio were moderately susceptible. Green fruit developed less anthracnose than mature fruit. At the green fruit stage, Arbequina and Frantoio were the most susceptible, Coratina was intermediate, and Picual was moderately susceptible, while no differences were found among the cultivars at mature fruit stages. No mayor differences were found among the Colletotrichum species with exception of C. theobromicola, which caused greatest severity at the green fruit stage. Future research should focus on developing anthracnose management strategies to minimize the disease progress from early stages of flower development and fruit ripening, especially in the most susceptible olive cultivars.

Optimisation of sampling and testing for asymptomatic olive trees infected by Xylella fastidiosa in Apulia region, Italy

Early detection of Xylella fastidiosa outbreaks in Apulian olive groves is crucial, especially in buffer zones and pathogen-free areas where olive trees are asymptomatic. Three studies were conducted. Two were on the spatial and temporal progression of X. fastidiosa infections in tree canopies of asymptomatic or mildly symptomatic olive trees of tolerant (‘Leccino’) and susceptible (‘Cellina di Nardò’ and ‘Ogliarola salentina’) cultivars. Despite different trends in pathogen infection rates and concentrations between ‘Leccino’ and susceptible olive cultivars over the study period, results showed that sampling was most effective in the mid-upper part of tree canopies throughout the year, excluding the warmest and coldest periods. Stem xylem tissues were the most appropriate for detecting the pathogen compared to lower parts of mature leaves with petioles, using serological and molecular assays. Based on these results, a third study was conducted to compare molecular and serological tests (qPCR, real-time LAMP, DAS-ELISA, DTBIA) for detection of X. fastidiosa in the mid-upper part of asymptomatic branches of infected ‘Leccino’ trees that were sampled in an appropriate collection time, using stem xylem tissue as the most appropriate matrix for testing. The molecular methods showed the greatest sensitivity, with no undetermined results, while among the serological assays, DTBIA was more sensitive than DAS-ELISA. An improved protocol for monitoring asymptomatic olive trees is recommended.

Xylella fastidiosa subsp. paucaST53 exploits pit membranes of susceptible olive cultivars to spread systemically in the xylem

AbstractXylella fastidiosa subspecies pauca strain De Donno (XfDD) ST53 is the causal agent of olive quick decline syndrome, a severe disease first described in Apulia, Italy. Although the two local cultivars Cellina di Nardò and Ogliarola Salentina showed high susceptibility, traits of resistance to the bacterium were found in the cultivar Leccino. Previous studies in field‐grown olives suggested that vascular occlusions and anatomophysiological properties of the different cultivars played a role in the olive response to XfDD. The present investigation reports observations at the early stage of the infection on artificially inoculated olives. Electron microscope studies showed that XfDD exploits the pit membranes (PMs) of the susceptible cultivar Cellina di Nardò to spread systemically. In this cultivar, PMs were degraded upon XfDD infection, suggesting activity of bacterial cell wall‐degrading enzymes. Moreover, occluded vessels contained an amorphous electrondense matrix resembling gum. Conversely, in Leccino, occluded vessels were mainly filled by callose‐like granules that tightly entrapped XfDD cells. In addition, PMs from Leccino had a compact undegraded structure that was not permeable to XfDD. Our study suggests that exploitation of PMs is a key event in the infection process of X. fastidiosa subsp. pauca ST53 in susceptible olive cultivars.

Susceptible and resistant olive cultivars show differential physiological response to Xylella fastidiosa infections.

Olive quick decline syndrome (OQDS) is a severe disease, first described in Italy in late 2013, caused by strains of Xylella fastidiosa subsp. pauca (Xfp) in susceptible olive cultivars. Conversely, resistant olive cultivars do not develop OQDS but present scattered branch dieback, which generally does not evolve to severe canopy decline. In the present study, we assessed the physiological responses of Xfp-infected olive trees of susceptible and resistant cultivars. Periodic measurements of stomatal conductance (gs) and stem water potential (Ψstem) were performed using a set of healthy and Xfp-infected plants of the susceptible “Cellina di Nardò” and resistant “Leccino” and “FS17” cultivars. Strong differences in Δgs and ΔΨstem among Xfp-infected trees of these cultivars were found, with higher values in Cellina di Nardò than in Leccino and FS17, while no differences were found among healthy plants of the different cultivars. Both resistant olive cultivars showed lower water stress upon Xfp infections, compared to the susceptible one, suggesting that measurements of gs and Ψstem may represent discriminating parameters to be exploited in screening programs of olive genotypes for resistance to X. fastidiosa.

Verticillium wilt resistant and susceptible olive cultivars express a very different basal set of genes in roots

BackgroundOlive orchards are threatened by a wide range of pathogens. Of these, Verticillium dahliae has been in the spotlight for its high incidence, the difficulty to control it and the few cultivars that has increased tolerance to the pathogen. Disease resistance not only depends on detection of pathogen invasion and induction of responses by the plant, but also on barriers to avoid the invasion and active resistance mechanisms constitutively expressed in the absence of the pathogen. In a previous work we found that two healthy non-infected plants from cultivars that differ in V. dahliae resistance such as ‘Frantoio’ (resistant) and ‘Picual’ (susceptible) had a different root morphology and gene expression pattern. In this work, we have addressed the issue of basal differences in the roots between Resistant and Susceptible cultivars.ResultsThe gene expression pattern of roots from 29 olive cultivars with different degree of resistance/susceptibility to V. dahliae was analyzed by RNA-Seq. However, only the Highly Resistant and Extremely Susceptible cultivars showed significant differences in gene expression among various groups of cultivars. A set of 421 genes showing an inverse differential expression level between the Highly Resistant to Extremely Susceptible cultivars was found and analyzed. The main differences involved higher expression of a series of transcription factors and genes involved in processes of molecules importation to nucleus, plant defense genes and lower expression of root growth and development genes in Highly Resistant cultivars, while a reverse pattern in Moderately Susceptible and more pronounced in Extremely Susceptible cultivars were observed.ConclusionAccording to the different gene expression patterns, it seems that the roots of the Extremely Susceptible cultivars focus more on growth and development, while some other functions, such as defense against pathogens, have a higher expression level in roots of Highly Resistant cultivars. Therefore, it seems that there are constitutive differences in the roots between Resistant and Susceptible cultivars, and that susceptible roots seem to provide a more suitable environment for the pathogen than the resistant ones.

Highly infested soils undermine the use of resistant olive rootstocks as a control method of verticillium wilt

AbstractVerticillium wilt of olive (VWO) is probably the most devastating fungal disease for olive trees worldwide, and currently the main cultivars are susceptible or moderately susceptible to this disease. The evaluation of resistant cultivars as rootstocks to control the disease has scarcely been explored, and mainly in short‐term studies under controlled conditions, which usually do not correspond with field evaluations. The main objective of this study was to assess the responses to VWO of different scion × rootstock combinations of the olive cultivars Picual, Arbequina, Changlot Real, and Frantoio in a long‐term field experiment with a soil highly infested with the defoliating pathotype of Verticillium dahliae. The results showed that grafting the susceptible cultivar Picual onto resistant rootstocks delayed the onset of the disease symptoms; however, after 4 years, it was observed that all combinations that contain Picual (a) were extensively colonized by V. dahliae; (b) developed severe symptoms of the disease; and (c) had plant mortality similar to Picual growing on its own roots. This result highlights the importance of long‐term field experiments to evaluate VWO and shows that grafting susceptible olive cultivars onto resistant ones does not provide a durable control of VWO under high inoculum potential, as V. dahliae is able to progress through the resistant rootstock and then extensively colonize and kill the susceptible scion. However, the high inoculum potential observed in this study does not allow us to consider the evaluated resistant cultivars as completely ineffective under lower inoculum densities.

Complete Genome Sequence of the Olive-Infecting Strain Xylella fastidiosa subsp. pauca De Donno.

ABSTRACTWe report here the complete and annotated genome sequence of the plant-pathogenic bacterium Xylella fastidiosa subsp. pauca strain De Donno. This strain was recovered from an olive tree severely affected by olive quick decline syndrome (OQDS), a devastating olive disease associated with X. fastidiosa infections in susceptible olive cultivars.

Pre-breeding for resistance to Verticillium wilt in olive: Fishing in the wild relative gene pool

This study aimed to identify new sources of resistance to Verticillium wilt in olive. We evaluated various types of genotypes: wild olive trees (Olea europaea subsp. europaea var. sylvestris), genotypes belonging to related subspecie (Olea europaea subsp. guanchica) and genotypes coming from crosses between Picual cultivar and wild olive trees. Fifty-six genotypes were inoculated by dipping roots and then screened under controlled conditions to test their resistance to a highly virulent Defoliating isolate of Verticillium dahliae. Picual (susceptible) and Frantoio (resistant) were control cultivars. Wide variability and significant differences were obtained in the evaluated disease parameters. The Relative Susceptibility Index (RSI), summarizing the disease parameters, was used for final classification of genotypes. Thirteen genotypes were categorized as resistant: eight wild olives from different locations, two genotypes belonging to subsp. guanchica populations from Canary Islands and three genotypes obtained from one of the crosses Picual x wild. The identification of high levels of resistance to V. dahliae among wild olive genotypes may be helpful for the management of this disease. The resistant genotypes could be used as rootstocks for susceptible olive cultivars or parents in future breeding programs.

Resistance of olive cultivars to Botryosphaeria dothidea, causal agent of olive fruit rot in Montenegro

Fruit rot has caused severe damage in olive orchards in Montenegro in recent years. Symptoms of the disease in commercial orchards include necrotic, depressed and clearly delimited spots on fruit. Identity of fungal isolates from infected fruit was confirmed by morphological and molecular evidence as Botryosphaeria dothidea. Pathogenicity was confirmed by fulfillment of Koch's postulates. Seventeen olive cultivars were assessed for susceptibility to the causal agent. Two native cultivars, Crnjaka and Gloginja, were highly resistant, whereas the most widely grown domestic variety, Žutica, was susceptible. Introduced cultivars Pendolino and Cassanesse were highly resistant and Giarraffa and Ascolana tenera were the most susceptible olive cultivars. This report is the first comparison of cultivar resistance to olive fruit rot in the Balkan region, and the first assessment of resistance in olive cultivars indigenous to this region.

Control of Verticillium wilt of olive by resistant rootstocks

The potential of grafting susceptible cultivars onto resistant rootstocks was evaluated for the control of Verticillium wilt in olive. Clones of the cultivars ‘Frantoio’ (highly resistant), ‘Coratina’ (susceptible) and ‘Leccino’ (highly susceptible) were used as scions and rootstocks in all combinations, inoculated by root-dipping with a defoliating (D) pathotype of Verticillium dahliae, and incubated in the greenhouse for 3 months. Ninety days after inoculation, non-grafted plants of ‘Frantoio’ and all cultivars grafted onto ‘Frantoio’ were devoid of wilt symptoms, while non-grafted plants of ‘Coratina’ (AUDPC = 203.3) and ‘Leccino’ (AUDPC = 167.3) were characterized as susceptible. In contrast, when ‘Frantoio’ was grafted onto susceptible rootstocks, plants became susceptible (AUDPC of 139.8 and 108.9 on ‘Frantoio’ grafted onto ‘Coratina’ and Leccino’, respectively). No significant difference occurred for vascular browning severity and frequency of V. dahliae reisolation from trunk tissues, though the pathogen was less abundant in the xylem of ‘Frantoio’. Microscopic observations on plugging of xylem vessels are presented. Grafting of susceptible olive cultivars ‘Leccino’ and ‘Coratina’ onto resistant rootstock ‘Frantoio’ provided an excellent control of Verticillium wilt. It is postulated that the resistance of ‘Frantoio’ occurs mainly in the roots and involves biochemical mechanisms rather than plant structural characteristics.

Phenolic Responses of Resistant and Susceptible Olive Cultivars Induced by Defoliating and Nondefoliating Verticillium dahliae Pathotypes.

Verticillium wilt is the most serious olive disease worldwide. The olive-infecting Verticillium dahliae pathotypes have been classified as defoliating (D) and nondefoliating (ND), and the disease is mainly controlled in olive orchards by using resistant or tolerant cultivars. Limited information is available about the nature of resistance in most of the olive cultivars. In the present study, the phenolic responses of the susceptible to V. dahliae olive cv. Amfissis and the resistant cv. Koroneiki upon D and ND V. dahliae infection were monitored in relation to the fungal DNA levels in the vascular tissues with the purpose to explore the defense mechanisms of olive trees against V. dahliae. Quantitative polymerase chain reaction revealed that the decrease in symptom severity shown in Koroneiki trees was associated with significant reduction in the growth of both V. dahliae pathotypes in the vascular tissues compared with Amfissis. In Koroneiki trees, the levels of o-diphenols and verbascoside were positively associated with the DNA levels of the D and ND pathotypes. In addition, a positive association was observed between the levels of verbascoside and the fungal DNA level in Amfissis trees, whereas a negative association was revealed between the fungal DNA level and the total phenols and oleuropein content in both cultivars. The levels of verbascoside were clearly higher in Koroneiki trees compared with Amfissis trees, indicating for the first time in the literature the involvement of verbascoside in the defense mechanism of olive trees against V. dahliae.

Relationship Between the Inoculum Density of Verticillium dahliae and the Progress of Verticillium Wilt of Olive.

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.

Olive scab: a review1

Olive scab or leaf spot, caused by the fungus Spilocaea oleagina, is widespread in the Mediterranean region. Losses arise mostly from defoliation of severely infected trees, with consequently reduced yield. Symptoms are mainly confined to leaves and appear as dark brown, circular, zonate spots surrounded by yellow haloes (‘peacocks eye’). S. oleagina shows a typical subcuticular growth, forming flat colonies within the cutinized layer of the thick epidermal cell wall. This habit has been associated with a defence reaction of the host involving mobilization and breakdown of the phenolic glucoside oleuropin and inhibition of pectolytic enzymes produced by the pathogen. The disease is particularly severe in densely planted groves of susceptible olive cultivars and in nurseries. Infections may occur throughout the year, except during hot and dry summers, when favourable temperatures (opt. 16–21°C) and rains occur. Conidia, formed at the apex of short ampulliform conidiophores, are usually carried by rain droplets, but recent data show that humid air currents and insects also contribute to limited aerial dissemination. Usually, the incubation period is about 2 weeks; however, if infection is followed by a hot season, it may last several weeks. Spots already formed in spring may stop growing in summer and resume their growth and sporulation in autumn. Chemical control schedules include fungicide (especially copper) treatments during the main infection seasons (spring and autumn).