Characterization of resistance against the olive‐defoliating Verticillium dahliae pathotype in selected clones of wild olive

Verticillium wilt of olive is best managed by resistant cultivars, but those currently available show incomplete resistance to the defoliating (D) Verticillium dahliae pathotype. Moreover, these cultivars do not satisfy consumers' demand for high yields and oil quality. Highly resistant rootstocks would be of paramount importance for production of agronomically adapted and commercially desirable olive cultivars in D V. dahliae‐infested soils. In this work, resistance to D V. dahliae in wild olive clones Ac‐13, Ac‐18, OutVert and StopVert was assessed by quantifying the fungal DNA along the stem using a highly sensitive real‐time quantitative polymerase chain reaction (qPCR) protocol and a stem colonization index (SCI) based on isolation of V. dahliae following artificial inoculations under conditions highly conducive for verticillium wilt. Ac‐13, Ac‐18, OutVert and StopVert showed a symptomless reaction to D V. dahliae. The mean amount of D V. dahliaeDNA quantified in stems of the four clones ranged from 3.64 to 28.89 pg/100 ng olive DNA, which was 249 to 1537 times lower than that in susceptible Picual olive. The reduction in the quantitative stem colonization of wild olive clones by D V. dahliae was also indicated by a sharp decrease in the SCI. Overall, there was a pattern of decreasing SCI in acropetal progression along the plant axis, as well as correlation between positive reisolation and quantification of pathogen DNA. The results of this research show that wild olive clones Ac‐13, Ac‐18, OutVert and StopVert have a valuable potential as rootstocks for the management of verticillium wilt in olive.     

Reliable detection of unevenly distributed Verticillium dahliae in diseased olive trees

Verticillium wilt caused by Verticillium dahliae is one of the most threatening diseases of olive worldwide. For pre‐planting and post‐planting control of verticillium wilt in olive trees, availability of a rapid, reliable and non‐destructive method for detection of V. dahliae is essential. For such a method, suitable and easily performed sampling and efficient processing of samples for extraction of DNA are necessary. In this study, the suitability of young twig and leaf samples of olive trees, which are easy to collect and extract DNA from, were assessed for the detection of V. dahliae in routine procedures. The lower (about 50 cm from the tip) and top parts (about 5 cm from the tip) of twigs, as well as leaves from infected olive trees were screened for V. dahliae infection and distribution using real‐time PCR. The biomass of V. dahliae detected in individual twigs was highly variable, but there was no significant difference between mean quantities of V. dahliae DNA detected in top and lower parts of twigs. Furthermore, it was demonstrated that analysis of combined samples containing DNA extracted from five twigs of an infected tree accurately detected the presence of the pathogen. Similarly, testing combined samples of 5–10 leaves enabled reliable detection of the pathogen in an infected tree. The development of this assay enables reliable detection of V. dahliae in infected olive trees that can aid in management decisions for the implementation of integrated disease management.     

Genetic structure of Verticillium dahliae isolates infecting olive trees in Tunisia using AFLP,pathogenicity and PCR markers

Since 2006, verticillium wilt of olive induced by Verticillium dahliae has caused considerable economic losses in olive orchards in Tunisia. The genetic structure of V. dahliae isolates collected from different olive growing regions was investigated using virulence tests, vegetative compatibility grouping (VCG) and amplified fragment length polymorphism (AFLP) analyses. In total, 42 isolates of V. dahliae from diseased olive trees were tested. Cluster analysis and principal coordinate analysis revealed that geographic origin was the main factor determining the genetic structure of V. dahliae populations and both methods indicated a genetic separation between the central and coastal isolates. Isolates were divided into two major groups: the AFLP‐I group included all isolates from Sidi Bouzid, Kairouan, Kasserine and Sfax (centre of the country) and the AFLP‐II group included isolates from Monastir, Zaghouane, Sousse, Mahdia (coastal region), and two isolates from Sfax. Analysis of the molecular variance (amova ) indicated a significant level of genetic differentiation among (76%) and within (23%) the two populations. Analyses of both the defoliating (D) and non‐defoliating (ND) pathotypes and VCG markers indicated that most of the isolates belong to VCG 2A and 4B/ND pathotype. The disease severity was highly variable among the isolates tested (P < 0·05) with no evidence of association between aggressiveness and geographical origin of the isolates. Overall, results of this study revealed a clear association between the genetic diversity of the isolates and their geographic origin, but not between genetic diversity and virulence patterns.     

Variation of pathotypes and races and their correlations with clonal lineages in Verticillium dahliae

Understanding pathogenic variation in plant pathogen populations is key for the development and use of host resistance for managing verticillium wilt diseases. A highly virulent defoliating (D) pathotype in Verticillium dahliae has previously been shown to occur only in one clonal lineage (lineage 1A). By contrast, no clear association has yet been shown for race 1 with clonal lineages. Race 1 carries the effector gene Ave1 and is avirulent on hosts that carry resistance gene Ve1 or its homologues. The hypothesis tested was that race 1 arose once in a single clonal lineage, which might be expected if V. dahliae acquired Ave1 by horizontal gene transfer from plants, as hypothesized previously. In a diverse sample of 195 V. dahliae isolates from nine clonal lineages, all race 1 isolates were present only in lineage 2A. Conversely, all lineage 2A isolates displayed the race 1 phenotype. Moreover, 900‐bp nucleotide sequences from Ave1 were identical among 27 lineage 2A isolates and identical to sequences from other V. dahliae race 1 isolates in GenBank. The finding of race 1 in a single clonal lineage, with identical Ave1 sequences, is consistent with the hypothesis that race 1 arose once in V. dahliae. Molecular markers and virulence assays also confirmed the well‐established finding that the D pathotype is found only in lineage 1A. Pathogenicity assays indicated that cotton and olive isolates of the D pathotype (lineage 1A) were highly virulent on cotton and olive, but had low virulence on tomato.     

Effect of soil solarization on the survival of fungal antagonists of Verticillium dahliae1

Talaromyces flavus, a fungal antagonist of Verticillium dahliae, naturally occurring in clay loam artichoke fields or sandy loam olive groves, is able to survive following application of soil solarization. Survival was almost always linked to an increase in T. flavus populations detected in the rhizosphere of artichoke plants or olive trees with a verticillium wilt history as compared with the untreated control soils. It was evident that soil solarization resulted in the control of the disease in artichoke fields and the recovery of olive trees from V. dahliae infection. It was furthermore proved that solarization had a beneficial long-term effect in controlling V. dahliae for a period of 2 or 3 consecutive years. This could at least partially be attributed to the activity of T. flavus in inhibiting the germination of microsclerotia or causing their death. Aspergillus terreus, another potential V. dahliae antagonist, was also found to survive and occasionally increase following the application of the technique.     

Biological control agents (BCAs) of verticillium wilt: influence of application rates and delivery method on plant protection,triggering of host defence mechanisms and rhizosphere populations of BCAs

Verticillium dahliae causes severe yield reductions in a variety of important annual crops worldwide. Control of verticillium wilt has relied on soil fumigation; however, the use of the main soil fumigant, methyl bromide, has been banned in the European Union since 2010, creating a demand for novel crop protectants. As such, the use of biocontrol agents (BCAs) is an appealing management strategy. Prerequisites for the development of a successful BCA are an understanding of the modes of action of the antagonist, its ecological fitness and an efficient and economically feasible delivery system. Therefore, two BCAs (Paenibacillus alvei K165 or the nonpathogenic Fusarium oxysporum F2) and two release strategies (seed coating or amendment of the transplant soil plug) were assessed against verticillium wilt of aubergine (eggplant). Mixing the transplant soil plug with K165 or F2, at a rate of 10 and 20% (v/v), respectively, reduced verticillium wilt symptom development. Furthermore, a positive correlation was revealed between the release strategy and the BCA rhizosphere population. Correlation analysis also showed that disease severity was negatively correlated to the rhizosphere size of the BCA population. In addition, qPCR analysis showed that both BCAs induced the expression of the pathogenesis‐related (PR) proteins PR1 and PR4 in the stem of aubergines before and after inoculation with V. dahliae in a manner that suggests a link with the rhizosphere size of the BCA population.     

Application to soil of disinfectants through irrigation reduces Verticillium dahliae in the soil and verticillium wilt of olive

The application of disinfectants through drip irrigation could be a feasible practice against verticillium wilt (Verticillium dahliae) of olive. OX-VIRIN (activated peroxide) and OX-AGUA AL25 (quaternary ammonium compounds) are two disinfectants that have shown efficacy against V. dahliae in irrigation water and potential for reducing the disease in young olive plants. In this work, various post-planting application strategies incorporating OX-VIRIN (once a month, or twice a month on alternate or successive weeks) or OX-AGUA AL25 (once a month, or twice a month on alternate weeks) were assessed for their effect on V. dahliae in soil, disease in olive trees, and olive yield, in a 2-year pot-experiment under natural environmental conditions. The disinfectants were injected via metering pumps into a drip irrigation system that irrigated olive trees planted in V. dahliae-inoculated soil. All the application strategies significantly reduced the total inoculum density in soil compared to controls with no disinfectants and noninoculated soil. The microsclerotia density was also significantly reduced in disinfested soils by 73.6–86.8%, depending on the strategy. The symptoms and infection incidence were always lower in treatments subjected to disinfestation. The treatment with OX-AGUA AL25 applied twice a month on alternate weeks most reduced the symptoms (by 53.0%) and colonization index (by 70.8%) with respect to untreated water control. This soil disinfestation also significantly strengthened the symptom remission. Tree growth and production were negatively affected by soil inoculation (reduced by 45.6% and 88.7%, respectively), but not so by disinfectants, which even relieved the reduction in inoculated soils, especially when OX-AGUA AL25 was applied.     

Interactions between Trichoderma harzianum and defoliating Verticillium dahliae in resistant and susceptible wild olive clones

Verticillium wilt (VW) in olive is best managed by an integrated disease management strategy, of which use of host resistance is a key element. The widespread occurrence of a highly virulent defoliating (D) Verticillium dahliae pathotype has jeopardized the use of commercial olive cultivars lacking sufficient resistance to this pathogen. However, the combined use of resistant wild olive rootstocks and Trichoderma spp. effective in the biocontrol of VW can improve the management of VW in olive. In vivo interactions between D V. dahliae and Trichoderma harzianum were studied in olive and wild olive plants displaying different degrees of resistance against this pathogen using confocal microscopy. This multitrophic system included wild olive clones Ac‐4 and Ac‐15, olive cv. Picual, and the fungal fluorescent transformants T. harzianum GFP22 and V. dahliae V138I‐YFP, the latter being obtained in this study. In planta observations indicated that V138I‐YFP colonizes the roots and stems of the olive and wild olive genotypes, and that GFP22 grows endophytically within the roots of them all. YFP fluorescence signal quantifications showed that: (i) the degree of root and stem colonization by the pathogen varied depending upon the susceptibility of the tested wild olive genotype, being higher in Ac‐15 than in Ac‐4 plants; and (ii) treatment with T. harzianum GFP22 reduced the extent of pathogen growth in both clones. Moreover, root colonization by strain GFP22 reduced the percentage of pathogen colonies recovered from stems of olive and wild olive plants.     

Prospects and strategies in controlling verticillium wilt of olive1

Control of verticillium wilt of olive currently depends on preventive measures. Since systemic fungicides are unable to prevent or control the disease, its control should primarily be based on cultural methods, including irrigation systems which restrict dissemination of Verticillium dahliae propagules by irrigation water and avoidance of intercropping with hosts susceptible to V. dahliae. Since leaves from affected olive trees contribute, through formation of microsclerotia, to the inoculum in the soil, pruning should be practised prior to branch defoliation. As for resistant olive rootstocks or cultivars, promising verticillium-wilt resistance has been found in two rootstocks selected in California (US). However, these have to be tested under local conditions before they can be released to Mediterranean growers, while further search for other resistant rootstocks is needed. Soil solarization applied to individual diseased trees in established olive groves could substantially contribute to recovery or long-lasting symptom remission in the treated trees. This effect is mainly attributed to the decrease or eradication of V. dahliae microsclerotia in the treated soil but also to heat-tolerant fungal antagonists of the pathogen. Using herbicides to control weeds, and limiting soil rotovation, can restrict symptom development. Biological control can also be considered as a promising trend in controlling the disease by searching, testing and exploiting potential fungal or bacterial antagonists.     

Disruption of Cerevisin via Agrobacterium tumefaciens‐mediated transformation affects microsclerotia formation and virulence of Verticillium dahliae

Agrobacterium tumefaciens‐mediated transformation (ATMT) was used to obtain a large number of Verticillium dahliae (Vd991) T‐DNA insertion mutants that were randomly integrated. Insertion mutants that produced significantly fewer microsclerotia were chosen for further analysis. Mutant T0065 was identified as having the Cerevisin gene interrupted by T‐DNA, and it was named cerevisin. The cerevisin protein showed a high amino acid sequence similarity with the vacuolar protease B. The mutant strain cerevisin displayed decreased production of microsclerotia and conidia, significantly reduced growth rate and reduction in virulence compared to the wild type. Moreover, the composition of secreted proteins differed between the cerevisin mutant and the wild type. Loss of function of Cerevisin decreased secretion of proteins of low molecular weight (14–25 kDa). Upon treatment with the secreted proteins of the mutant, the degree of leaf wilting decreased, indicating that Cerevisin is implicated in production of these proteins, which are putative pathogenicity factors of V. dahliae. The results suggest that Cerevisin is involved in controlling multiple processes of development and metabolism, plays an important role in vegetative growth and microsclerotia formation and affects virulence of V. dahliae.     

Role of co‐infection by Pectobacterium and Verticillium dahliae in the development of early dying and aerial stem rot of Russet Burbank potato

Potato early dying (PED) is a disease complex primarily caused by the fungus Verticillium dahliae. Pectolytic bacteria in the genus Pectobacterium can also cause PED symptoms as well as aerial stem rot (ASR) of potato. Both pathogens can be present in potato production settings, but it is not entirely clear if additive or synergistic interactions occur during co‐infection of potato. The objective of this study was to determine if co‐infection by V. dahliae and Pectobacterium results in greater PED or ASR severity using a greenhouse assay and quantitative real‐time PCR to quantify pathogen levels in planta. PED symptoms caused by Pectobacterium carotovorum subsp. carotovorum isolate Ec101 or V. dahliae isolate 653 alone included wilt, chlorosis and senescence and were nearly indistinguishable. Pectobacterium wasabiae isolate PwO405 caused ASR symptoms including water‐soaked lesions and necrosis. Greater Pectobacterium levels were detected in plants inoculated with PwO405 compared to Ec101, suggesting that ASR can result in high Pectobacterium populations in potato stems. Significant additive or synergistic effects were not observed following co‐inoculation with these strains of V. dahliae and Pectobacterium. However, infection coefficients of V. dahliae and Ec101 were higher and premature senescence was greater in plants co‐inoculated with both pathogens compared to either pathogen alone in both trials, and V. dahliae levels were greater in basal stems of plants co‐inoculated with either Pectobacterium isolate. Overall, these results indicate that although co‐infection by Pectobacterium and V. dahliae does not always result in significant additive or synergistic interactions in potato, co‐infection can increase PED severity.     

Variability and selection of verticillium wilt resistant genotypes in cultivated olive and in the Olea genus

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.     

Control of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> at a strawberry nursery by paddy-upland rotation

Effects of crop rotation between rice paddy fields and strawberry nurseries on the control of Verticillium wilt of strawberry were studied. For detecting Verticillium dahliae, the causal agent of Verticillium wilt, in soil, eggplant was used as an indicator plant. We were thus able to detect as low as 1 microsclerotium/g dry soil. In field surveys of Chiba and Hokkaido from 2000 to 2003, V. dahliae was detected in 9 of 10 upland fields but in none of 21 paddy-upland fields. In Hokkaido during 2000–2007, strawberry mother plants were planted, and plantlets were produced in upland and paddy-upland fields to assess V. dahliae infestation. Verticillium wilt of strawberry had never occurred in 72 tested paddy-upland fields, compared to 13.2–73.9% of plantlets infected with V. dahliae in upland fields. In a pot experiment in a greenhouse, two flooding treatments or two paddy rice cultivations suppressed Verticillium wilt symptoms on eggplant. In field experiments, one paddy rice cultivation in Chiba and two in Hokkaido prevented development of Verticillium wilt symptoms on eggplant. Verticillium wilt of strawberry was controlled completely with one paddy rice cultivation in infested fields in Chiba. In these field experiments, the number of microsclerotia of V. dahliae decreased under the flooding conditions for paddy rice cultivation. Based on the reduction in microsclerotia, a crop rotation system with paddy rice for 3 years (three times), green manure for 1 year, and strawberry nursery for 1 year was designed for Hokkaido.     

Assessment of the effect of surface drip irrigation on Verticillium dahliae propagules differing in persistence in soil and on verticillium wilt of olive

For efficient integrated management of verticillium wilt in olive (VWO), it is important to establish whether irrigation treatments (with Verticillium dahliae‐free water) that mitigate the disease in V. dahliae‐infested soil, also reduce the levels of more and less persistent propagules of the pathogen in the soil. Effects of irrigation on VWO and V. dahliae propagules were evaluated under natural environmental conditions. Potted plants were irrigated (pathogen‐free water) to two ranges of soil water content (RWC; high and low) at three surface drip‐irrigation frequencies (daily, weekly, and daily during some periods and otherwise weekly). VWO and total inoculum density (ID), density of less persistent micropropagules (MpD) and more persistent sclerotia in wet soil (S_wD), and sclerotia density for air‐dried soil (S_dD) were monitored. A logistic model (multiple sigmoid) of disease incidence revealed the lowest parameter values in treatments irrigated daily. Daily frequency of irrigation showed significantly lower disease incidence (39.2%) and disease intensity index (43.9%) and MpD (88.0%) values as areas compared with other frequencies, regardless of the RWC. High RWC significantly reduced (70.8–84.9%) ID, S_wD and S_dD as areas, but significantly increased (18.0%) the incidence of infected plants (IIP), regardless of the irrigation frequency. The disease incidence was not correlated with temperature. Daily irrigation to low RWC mitigated the VWO and the IIP, kept soil to the lowest MpD and resulted in the lowest S_dD level at the end of the trial. Results suggested that less persistent propagules could have played a part in the disease development.     

Effect of different organic amendments on lettuce fusarium wilt and on selected soilborne microorganisms

This study evaluated the effect of different organic amendments on lettuce fusarium wilt caused by Fusarium oxysporum f. sp. lactucae in pots under controlled conditions. Their effects on the density of the pathogen, on the total fungi and on fluorescent Pseudomonas spp. were also evaluated after two subsequent lettuce crops. A significant reduction in the severity of the symptoms of F. oxysporum f. sp. lactucae was found after the use of Brassica carinata pellets (52–79% reduction) and compost (49–67% reduction), while Brassica green manure and cattle and chicken manure only provided partial control of fusarium wilt. However, variations in effectiveness were observed for the same treatment in repeated trials. In general, an increase was observed in Pseudomonas and a decrease in fungal populations in the growing medium, which was obtained by mixing a blonde sphagnum peat and a sandy loam soil with B. carinata pellets and compost after two consecutive cropping cycles. Prolonging the Brassica and compost treatments from 30 to 60 days did not significantly affect disease severity, plant growth or the microbial population of the total fungi or Pseudomonas. The largest lettuce biomass was obtained in the non‐inoculated growing medium amended with brassica flour, chicken manure, B. carinata pellets and compost, as a consequence of fertilization. The treatment with B. juncea green manure, B. carinata (pellets and flour) and compost applied 30 days before planting led to promising results and merits further investigation for use under field conditions.     

Increased resistance to Verticillium dahliae in Arabidopsis plants defective in auxin signalling

Auxin signalling and transport participate in plant–microbe interactions as positive or negative regulators of disease resistance. The present study investigated the responses of Arabidopsis thaliana plants impaired in the auxin receptors TIR1, AFB1 and AFB3 and the auxin transporter AXR4, upon infection by the soilborne root pathogen Verticillium dahliae. Fewer symptoms were recorded in afb1, afb3 and axr4 plants compared to the wild type (wt). qPCR analysis revealed that the decrease in symptom severity in afb1, afb3 and axr4 was correlated with reduction in the growth of the pathogen in the plants. Therefore, afb1, afb3 and axr4 are partially resistant to V. dahliae. Upon V. dahliae infection, the expression of TIR1, AFB1, AFB3 and AXR4 was up‐regulated in roots, while indole‐3‐acetic acid levels were similar to mocks. The partial resistance of afb1, afb3 and axr4 against V. dahliae can be attributed in part to the up‐regulation of defence‐related genes, as it was observed that afb1 and axr4 had higher PR1 levels than wt, while afb3 had higher PDF1.2 levels than wt. The findings of the present study suggest that the auxin signalling defective mutants afb1, afb3 and axr4 show increased resistance against V. dahliae.     

Verticillium wilt in nursery trees: damage thresholds,spatial and temporal aspects

Verticillium wilt can cause high losses in tree nurseries. To be able to predict disease and unravel disease dynamics over time and space, the relationship between verticillium wilt and soil inoculum densities of Verticillium dahliae and the nematode Pratylenchus fallax was studied in two 4-year field experiments with Acer platanoides and Catalpa bignonioides in the Netherlands. Best-fit regression equations showed that pre-planting inoculum densities of V. dahliae can be used to predict verticillium wilt over a period of at least 4 years. Pratylenchus fallax contributed significantly to disease severity in A. platanoides in some years. Disease can already occur at the detection limit of the pathogens. The 5% infection thresholds for V. dahliae were at 1 (A. platanoides) vs. 3 (C. bignonioides) colony-forming units (CFU) g⁻¹ soil. Analysis of spatial relationships indicated that diseased plants had a higher influence on neighbouring plants at low V. dahliae inoculum densities (<5 CFU g⁻¹ soil) than at high densities (≥5 CFU g⁻¹ soil). Seventy-four percent of the diseased plants recovered during the following year. After that year, recovered plants had a significantly higher probability of becoming diseased again than plants that were healthy during the two previous years, at high inoculum densities of V. dahliae, indicating that inoculum density in the soil, rather than incomplete recovery, was the most important factor for disease development.     

Spread of verticillium wilt by nursery plants in olive groves in the Halkidiki area (Greece)1

Olive groves have been established during the last decade in the coastal area of Halkidiki and in the northern part of the Kassandra peninsula (GR). Visual symptoms of verticillium wilt were observed recently all over this area, particularly in young trees of cv. Hondrolia Halkidikis but not in the occasional old trees already in the orchards. The symptoms appeared regardless of the previous crop (cotton or not), so the nurseries supplying the farmers with young trees were assumed to be the primary source of infection. A survey of olive nurseries in the area was conducted during 1991. Eleven out of the total 18 nurseries in the area were located in the Verticillium dahliae-infested zone and 9 of these were sampled. Three-year-old olive seedlings were selected and isolations were made from the xylem onto PDA acidified with 0.25% lactic acid. V. dahliae-infected seedlings were detected in 4 out of the 9 nurseries examined. The presence of infected young trees in nurseries covering 15.6% of the total nursery area provided strong evidence that nurseries may be responsible for the recent expansion of verticillium wilt in the area.     

Symptom development, pathogen isolation and Real-Time QPCR quantification as factors for evaluating the resistance of olive cultivars to Verticillium pathotypes

Verticillium wilt is the most serious olive disease in the Mediterranean countries and worldwide. The most effective control strategy is the use of resistant cultivars. However, limited information is available about the level and source of resistance in most of the olive cultivars and there are no published data using microsclerotia, the resting structures of Verticillium dahliae, as the infective inoculum. In the present study, we correlated symptomatology and the presence of the fungus along with the DNA relative amount (molecules μl⁻¹) of a defoliating (D) and a non-defoliating (ND) V. dahliae strain in the susceptible cv. Amfissis and the tolerant cvs Kalamon and Koroneiki, as quantified by the Real-Time QPCR technology. The viability of the pathogen in the plant tissues was confirmed by isolating the fungus on PDA plates, while symptom assessment proved the correlation between the DNA relative amount of V. dahliae in plant tissues and cultivar susceptibility. It was further demonstrated that the D and ND strains were present at a significantly higher level in cv. Amfissis than in cvs Kalamon and Koroneiki. It was finally observed that the relative amount of the pathogen in roots was lower than in stems and shoots and declined in plant tissues over time. These data constitute a valuable contribution in evaluating resistance of olive cultivars or olive root-stocks to V. dahliae pathotypes.     

Morphophysiological response of young olive trees to verticillium wilt under different surface drip irrigation regimes

The effects of irrigation on verticillium wilt in olive, in terms of morphological, biomass and physiological parameters were evaluated on pot‐grown trees maintained in the field for 3 years. Plants inoculated and noninoculated with Verticillium dahliae were irrigated to high and low range of soil water content (HR and LR) at daily (DF; about 2 days/event), weekly (WF) and daily‐weekly (DWF) drip‐irrigation frequency. Morphological parameters, relative biomass and biomass water‐use efficiency were higher at LR than at HR (with few exceptions) and at DF than at other frequencies in noninoculated and inoculated plants, but the fungus reduced those parameters by 17.0–38.5%. Lower root weight ratio, relative biomass and shoot length as area originated at HR in noninoculated plants, could be favourable to the accumulation of root infections and the amount of fungus per tissue length in inoculated plants because higher infection was known at HR. Moreover, higher aerial biomass and length promoted by irrigation at DF could prevent the more severe expression of symptoms, which occurred at WF and DWF in the presence of Verticillium. Negative correlations were found between indicated parameters and disease. Lower water stress (S_Ψ), and higher stomatal conductance (g_s) and net photosynthesis at DF in noninoculated plants could limit the disease by improving water status, as S_Ψ was increased by the fungus only at WF and DWF, and g_s and disease were negatively correlated. LR‐DF treatment minimized the disease and kept the growth, water‐use efficiency and physiological parameters in inoculated plants to levels close to noninoculated plants.