Novel methodologies in screening and selecting olive varieties and root-stocks for resistance to <Emphasis Type="Italic">Verticillium dahliae</Emphasis>

An innovative inoculation process, involving the drilling of a trunk hole in 3 year-old olive trees and injecting a dense conidial suspension of Verticillium dahliae, was developed to study differentiation in foliar symptom expression between olive cultivars tolerant or susceptible to the pathogen. It was demonstrated that V. dahliae conidia could be translocated and colonize the xylem at the same distance above and below the point of trunk injection in both cultivars. However, the pathogen could be subsequently isolated at statistically significant percentages in susceptible cv. Amphissis compared to the tolerant cv. Kalamon, indicating operation of resistance mechanisms in the vascular phase of the disease. Consequently symptom development in the susceptible cultivar was at least sixfold more intensive compared to the tolerant cultivar, 6–11 months after trunk inoculation. Perennial olive orchard experiments, aimed at selecting Verticillium-resistant root-stocks, were conducted by applying the novel method in 2–3 year-old root-stock suckers of Amphissis olive trees and in the tolerant cvs Lianolia of Corfu and Koroneiki. It was indicated that potentially resistant root-stocks could be obtained following the trunk drilling technique. Resistance differentiation between cvs Amphissis and Kalamon was further verified through root inoculation by various V. dahliae microsclerotial concentrations and demonstrated that the trunk drilling inoculation procedure is equally efficient in resistance evaluation of olives to Verticillium wilt. The trunk inoculation procedure could be useful in selecting and screening root-stocks for resistance to V. dahliae and other vascular pathogens and could elucidate resistance mechanisms in woody plants against vascular wilt diseases.     

Evaluation of Olive Cultivars for Resistance to Verticillium dahliae

Resistance of 23 important olive cultivars to Verticillium dahliae has been evaluated in four experiments under controlled conditions. Nine-month-old nursery olive plants were inoculated with a cotton non-defoliating (ND) (V4) or a cotton defoliating (D) (V117) isolate of V. dahliae. Resistance was evaluated by assessing symptom severity using a 0–4 rating scale and estimating the area under disease progress curves. The percentage of plants killed and of those which recovered from the disease were used as additional parameters for including a particular cultivar into a defined category. Most of the evaluated cultivars were susceptible, although at different levels, to both isolates of V. dahliae. All cultivars were more susceptible to the D pathotype than to the ND one. A group of 11 cultivars, including several important Spanish cultivars, were susceptible or extremely susceptible to both pathotypes of V. dahliae. A second group showed differences of resistance depending on the pathotype used. They were susceptible or extremely susceptible to the D pathotype but resistant or moderately susceptible to the ND one. Finally, 'Frantoio', 'Oblonga' and 'Empeltre' were moderately susceptible to the D isolate of V. dahliae and resistant to the ND one. The resistance of 'Empeltre' was evident by the plant ability to recover from infection with either isolates. 'Empeltre' is considered to be a valuable cultivar for inclusion in breeding programmes for resistance to Verticillium wilt.     

Recovery of Young Olive Trees from <Emphasis Type="Italic">Verticillium dahliae</Emphasis>

Natural recovery from wilt disease symptoms was evaluated in young olive trees root dip inoculated with Verticillium dahliae in a growth chamber over a 12 week period and, later on, when the trees were transplanted in a V. dahliae-free soil in a lathhouse during a period of 127 weeks. Recovery in an individual tree was considered when a plant showed symptom remission after having reached a maximum value of symptom severity. Recovery accounted for 53% of 464 trees that showed wilt symptoms during observations in the two environments. The remaining trees died. Recurrent wilt symptoms were not observed in recovered trees, and recovery was usually accompanied by the production of new green tissues. Recovery was clearly higher in trees inoculated with a non-defoliating (ND) isolate (86.4%) of the pathogen than in those inoculated with a defoliating (D) isolate (23.9%). The percentage of recovery and the level of resistance were significantly correlated. Recovery accounted for 92.1% of the cases in resistant and moderately susceptible cultivars, reaching 100% in plants inoculated with the ND isolate (Table 2); meanwhile it was three times lower (30.1% of the plants) in susceptible and extremely susceptible diseased trees. In the lathhouse, periodical tissue isolations for monitoring the progress of infections over a period of 127 weeks in recovered trees, showed that the pathogen could only be isolated from trees 19 weeks after inoculation. Pathogen isolation was significantly higher from susceptible and extremely susceptible cultivars (84.6%) than from resistant and moderately susceptible ones (33.3%). Results showed that if a tree overcomes infection by pathogen from a single inoculation, and it is able to begin a recovery process, it will not express wilt symptoms again in a pathogen-free environment. The pathogen remained inactive or dead over time in recovered trees. Thus, new infections from rootlets would be necessary for new symptom expression. Recovery from Verticillium wilt is an important natural mechanism that occurs in a high percentage of infected olive trees, and can complement the resistance of the cultivar, particularly in conditions of low inoculum densities of low virulence isolates of the pathogen in the soil.     

Assessment of real-time PCR as a method for determining the presence of <Emphasis Type="Italic">Verticillium dahliae</Emphasis> in different Solanaceae cultivars

Real-time PCR was used to detect and quantify Verticillium dahliae and to assess the susceptibility of four Capsicum annuum cultivars (Luesia, Padrón, SCM331 and PI201234) and the Capsicum chinense cv. C118 to this pathogen. The symptoms which developed after infection included stunting and yellowing, and were more acute in the cv. SCM331, which also suffered defoliation in later stages of the disease and in C118, which suffered severe stunting. Quantification of the pathogen DNA in roots 23 and 34 days post-inoculation (dpi) revealed that there were significantly higher amounts of Verticillium dahliae DNA in C118 than in the other cultivars, followed by SCM331, Padrón and PI201234. The lowest amounts of fungal DNA in roots were found in Luesia. In hypocotyls, the highest amounts of fungal DNA were found in SCM331, while Luesia, Padrón and PI201234 had much lower amounts, and C118 had intermediate levels. When a compatible versus an incompatible system was studied, using the near-isogenic tomato lines LA3030 (susceptible) and LA3038 (resistant to V. dahliae), we were able to detect fungal DNA in both lines. As expected, the fungus/plant DNA ratio was lower in LA3038 than in LA3030 and it decreased with time in LA3038. The amount of Verticillium dahliae DNA in the roots of LA3030 remained constant between days 23 and 34 post-inoculation, but increased 10-fold in collars. Finally, when real-time PCR was applied as a diagnostic method to samples from pepper plants, soil and water collected from farms in northwest Spain, we were able to detect V. dahliae DNA in these samples even when symptoms of the disease were not evident.     

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.     

Present status of verticillium wilt of olive in Andalucía (southern Spain)1

Verticillium wilt caused by Verticillium dahliae is a disease highly prevalent in newly established olive orchards in Andalucía, southern Spain. Two syndromes of the disease occur in Andalucia, namely apoplexy and slow decline. Apoplexy is characterized by quick dieback of twigs and branches while slow decline consists of rapid drying out of inflorescences together with leaf chlorosis and necrosis. Systematic disease observations carried out in two experimental orchards planted with susceptible cv. Picual indicated that natural recovery of diseased trees occurred over time. Infection and vascular colonization of olive plants by V. dahliae were studied in susceptible (Picual) and resistant (Oblonga) cultivars inoculated with a mildly virulent or a highly virulent cotton-defoliating isolate of V. dahliae. Disease symptoms developed 24–32 days after inoculation in cv. Picual, but at that time plants of cv. Oblonga remained free from symptoms. However anatomical observations and isolations indicated that systemic infections by the two isolates had occurred to a large extent in both cultivars.     

Detection of the Defoliating Pathotype of Verticillium dahliae in Infected Olive Plants by Nested PCR

Spread of Verticillium wilt into newly established olive orchards in Andalucía, southern Spain, has caused concern in the olive industry in the region. This spread may result from use of Verticillium dahliae-infected planting material, which can extend distribution of the highly virulent, defoliating (D) pathotype of V. dahliae to new areas. In this study, a molecular diagnostic method for the early in planta detection of D V. dahliae was developed, aimed especially at nursery-produced olive plants. For this purpose, new primers for nested PCR were designed by sequencing a 992-bp RAPD marker of the D pathotype. The use of the specific primers and different nested-PCR protocols allowed the detection of V. dahliae pathotype D DNA in infected root and stem tissues of young olive plants. Detection of the pathogen was effective from the very earliest moments following inoculation of olive plants with a V. dahliae pathotype D conidia suspension as well as in inoculated, though symptomless, plants.     

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

Verticillium 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.     

Sanitation of olive plants infected by Verticillium dahliae using heat treatments

Verticillium wilt, caused by the fungus Verticillium dahliae, is currently the most important disease affecting olive in the Mediterranean basin. There are no effective treatments for controlling this disease. The use of infected nursery stocks has largely contributed to the spread of the pathogen, and therefore the development of treatments to preventively sanitize the propagation stock is critical in the nursery industry. This study describes novel techniques to achieve this aim. The effects of several temperature–exposure time combinations were evaluated: (i) the survival of pathogen on culture medium (PDA); (ii) the pathogen viability on infected shoots and plants; (iii) the vegetative growth of plants of several cultivars; and (iv) the rooting ability of cuttings. The colonies of the pathogen growing in PDA were killed after 8 h and 60 min of exposure at 40 and 47°C, respectively. Temperatures ≥42°C for at least 2 h were lethal for the pathogen infecting the shoots. Likewise, moist hot air treatments at 42–44°C for 6–12 h eradicated the pathogen, without compromising the viability of the plants. Five olive cultivars were also evaluated and classified according to their thermotolerance as follows: sensitive (Chiquitita), moderately sensitive (Koroneiki, Frantoio and Picual) and heat tolerant (Arbequina). However, the optimized sanitation methods were applicable to all of the cultivars. Finally, heat treatments were applied to unrooted cuttings, which severely affected their rooting ability. Thus, this study developed a hot air treatment to produce V. dahliae‐free olive nursery plants.     

Epidemiology and effects of verticillium wilt on yield of olive trees (cvs. Barnea and Souri) irrigated with saline water in Israel

Epidemiology ofVerticillium dahliae on olive trees (cvs. ‘Barnea’ and ‘Souri’) irrigated with saline water was studied over a period of 3 years. Disease incidence in cv. Barnea increased from 18.9% to 40.5% in the first year, while disease severity decreased from 4.2 to 2.4 (on a scale of 2–10). In the first month after pruning of diseased branches (June 1999), disease incidence was 26.9% and it increased to 60.6% by May 2000. No changes in disease severity were observed from this time until the end of the study. In cv. Souri, disease incidence doubled during the 3 years of the study (from 22.1% to 44.8%), and disease severity increased from 2.4 to 2.9. The disease caused significant reduction of yield in cv. Barnea in 1999 and 2000 (48.3% and 12.0%, respectively). In cv. Souri yield was measured only in 2001, where a reduction of 66.8% was observed.V. dahliae was isolated from both diseased and symptomless trees. A significant decrease in the isolation rate ofV. dahliae from diseased cv. Barnea trees occurred between the first year (47.9%) and the third year (5.3%); positive isolations were obtained in all seasons, without differences between seasons. In cv. Souri, the fungus was isolated in significantly higher rates during the winter and spring (55% and 50%, respectively). Our findings strongly support the assumption that natural recovery occurs in cv. Barnea 4–5 years after planting, but not in cv. Souri. http://www.phytoparasitica.org posting July 10, 2003.     

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.     

Agricultural factors affecting Verticillium wilt in olive orchards in Spain

In recent years, the spread of Verticillium wilt in olive orchards, caused by the soil-borne pathogen Verticillium dahliae, is often related to intensive modern farming of highly productive cultivars, planted at high densities, usually irrigated, and under a mechanised system. The effects of agricultural factors associated with olive orchards were investigated in an important olive-growing area in southern Spain, as tools in predicting outbreaks of the disease. A stratified double-sampling technique was designed to determine the number of olive orchards needed to survey. A sampling survey was conducted from 2002 to 2005 in 873 olive orchards randomly selected, the owners were interviewed for details of agronomic factors, and orchards were inspected for the presence or absence of the disease. Polymerase chain reaction assays were carried out for identifying V. dahliae pathotypes. Pathogen prevalence showed a significant linear correlation with the mean plant density (r ² = 0.93), associated predominantly with a less virulent non-defoliating pathotype (r ² = 0.96). Overall, irrigation × high density caused disease incidence to peak in super-high-density olive-tree-planting systems. Olive orchards that had V. dahliae, however, did not differ in pathogen prevalence regardless of the olive cultivars. Young olive orchards were significantly more affected by V. dahliae than were old ones, particularly orchards with trees 8 to 12 years old. Irrigation increased pathogen prevalence and disease incidence in very young orchards (<7 years old). The prevalence of the non-defoliating pathotype was statistically high in young orchards whereas the prevalence of a highly virulent defoliating pathotype was high in old orchards.     

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.     

Distribution of Verticillium dahliae through watering systems in widely irrigated olive growing areas in Andalucia (southern Spain)

Verticillium dahliae Kleb. causes Verticillium wilts in many herbaceous and woody species. Many hosts of the pathogen are commonly cultivated in Andalucía (southern Spain), particularly major crops such as cotton, vegetables, almond, peach and, particularly, olive, in which the fungus causes Verticillium wilt of olive. Infective structures of the pathogen (microsclerotia), produced in the late phases of the infection cycle in senescent tissues of the infected plants, can be spread over short or long distances by a number of dispersal methods. Irrigation water is one of the factors implicated in this spread of V. dahliae. Indeed, increasing irrigation dosages in crops or an inadequate irrigation schedule have been identified as cultural practices favouring Verticillium wilt onset and severity in olive and other hosts. Most of the cultivated areas in the Guadalquivir Valley of Andalucía are irrigated by pumping stations using modern infrastructures that supply water to thousands of hectares of farm land, which are usually associated with irrigation communities. This study demonstrates that the pathogen survives in the sediment and particles suspended in water used for irrigation in different facilities of an irrigation community, that were involved in distributing water (main canal and reception tank of a investigated pumping station, irrigation pools and sand from filters). Thus microsclerotia moves from the pumping station to individual plots (olive and cotton cultivated farm) as viable microsclerotia, free or embedded in soil particles and plant debris, suspended in the irrigation water, or deposited in the sludge in piping systems or water storage ponds. We have detected amounts of inoculum in the solid pellet samples in these facilities that ranged from 2.7 to 6.7 microsclerotia per gram. Besides this, water from drippers in cultivated plots released into the soil a variable amount of infective propagules of the pathogen over time that accounted for 3.75 microsclerotia/m³ in some of the recording times. Therefore, irrigation water becomes an important source of inoculum that is very effectively involved in medium and long-distance spread of the pathogen.     

Plant Phenology Influences the Effect of Mycorrhizal Fungi on the Development of Verticillium-induced Wilt in Pepper

Verticillium dahliae alters water status and consequently, growth and production of pepper plants. On the other hand, arbuscular mycorrhizal fungi (AMF) can reduce damage caused by specific soil-borne plant pathogens and improve drought resistance of pepper. Therefore, one objective of this research was to assess if AMF can modify the development of Verticillium-wilt in pepper plants. A second objective was to study the influence of plant phenology at the moment when V. dahliae was inoculated on the possible biocontrol of the disease by AMF. Results suggested that AMF reduce the deleterious effect of V. dahliae on pepper growth and yield. However, bioprotection against Verticillium-wilt was conditioned by plant phenology at the moment of pathogen attack. The highest efficacy of AMF occurred when V. dahliae was inoculated during the vegetative stage of plants. AMF allowed leaf relative water content to be maintained for longer and delayed both the appearance of disease symptoms and the decrease of photosynthesis in Verticillium-inoculated plants. These benefits on plant physiology increased pepper yield.     

Effect of Soil Plowing and Fertilization on the Susceptibility of Four Olive Cultivars to the Insect<Emphasis Type="Italic">Bactrocera oleae</Emphasis> and the Fungi<Emphasis Type="Italic">Sphaeropsis dalmatica</Emphasis> and<Emphasis Type="Italic">Spilocaea oleagina</Emphasis>

Susceptibility to the insectBactrocera oleae and the fungiSpilocaea oleagina andSphaeropsis dalmatica was investigated in four olive cultivars, two for table fruit production (Kalamon and Chondrolia Chalkidikis) and two for oil production (Lianolia and Koroneiki). Cv. Chondrolia Chalkidikis was the most susceptible to all three pathogens, followed by cv. Kalamon. Soil plowing and the organic fertilizer Bio-Trust^® (10-3-6+8% MgCO₃+10% CaCO₈) increased the susceptibility of all four tested olive cultivars to the insect and the two fungi. Correlations were found between the attacks byB. oleae and infections byS. oleagina andS. dalmatica on the four olive cultivars.     

Evaluation of resistance of Spanish olive cultivars to Verticillium dahliae in inoculations conducted in greenhouse

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.     

Tolerance to a non-host isolate of Verticillium dahliae in tomato

Tolerance to Verticillium spp. is a condition in which a host plant develops few symptoms despite substantial colonization by the pathogen. In the present paper we have shown that Craigella tomatoes are tolerant to a non-host isolate of V. dahliae, Dvd E6. Symptom expression was used to quantify disease and quantitative PCR to assess the amount of fungus in the stems. The classical incompatible and compatible interactions between Craigella resistant or Craigella susceptible near isolines and V. dahliae, race 1 were used for comparative purposes. Additional experiments using cytological assessment and quantitative PCR showed that in the tolerant interactions one plant defence response, vascular coating, was deployed as effectively as in resistant plants, limiting pathogen distribution. However, a second defence response, which causes the cyclical elimination of fungus from the stem in the classical interactions either does not occur or is substantially delayed in tolerant plants. Thus, the Verticillium population remains stable and substantial throughout the studied time course.     

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.     

Incidence of Verticillium wilt of artichoke in eastern Spain and role of inoculum sources on crop infection

Surveys of 94 artichoke fields throughout the artichoke production areas of Comunidad Valenciana (eastern Spain) were conducted from 1999 to 2002 to determine the incidence and distribution of Verticillium wilt.Verticillium dahliae was isolated from 80.9% of the sampled fields, and detected in all artichoke-growing areas, with a mean disease incidence of 53.8% infected plants. The disease was found to cause severe damage to cv. ‘Blanca de Tudela’, which is the most important artichoke cultivar grown in Spain, and was also observed on the seed-propagated cv. ‘Imperial Star’. In field trials to study the role of infected planting material and soil inoculum on infection of artichoke plants during the cropping season,V. dahliae was transmitted from infected stumps to the plants, confirming that the use of infected stumps could have greatly contributed to the dissemination of the pathogen. Inoculum density ofV. dahliae in soil had an effect on crop infection, in that a higher number of microsclerotia per gram of soil resulted in a higher percentage of infected plants. In addition, yield of cv. Blanca de Tudela was significantly affected byV. dahliae infection, showing that a higher percentage of infection corresponded with lower yield. http://www.phytoparasitica.org posting July 21, 2005.