The Effect of Short Irrigation Frequencies on the Development of Verticillium Wilt in the Susceptible Olive Cultivar 'Picual' under Field Conditions.

The effect of irrigation frequency (daily [T1], biweekly [T2], and dryland [T3]) on Verticillium wilt of olive was studied in two fields that were naturally infested with Verticillium dahliae in southern Spain and planted to 'Picual' olive. Disease onset (average 61 weeks after planting) and disease incidence (average 75.6%) did not differ among irrigation treatments in both fields. Irrigation consistently increased disease development regarding dryland treatment, but this effect varied over time. In experiment I, T1, the relative area under the disease progress curve was greater on all recording dates (ranging from 15.8 to 33.7) in comparison with T3 (average 6.6). Data for experiment II were similar to this on the most favorable dates for disease (March to April). The T2 treatment value varied over time depending on the season and experimental field, being difficult to differentiate from the values of T1 and T3. Significant correlation between disease incidence and severity increments during spring and fall with the soil water content of the same or previous favorable seasons was observed. Through these correlations, we detected soil water contents of 24.3% (experiment I) and 23.6% (experiment II), where the increments of disease parameters remained at zero. Therefore, scheduling irrigation treatments based on rainfall may be a feasible method for maintaining the soil moisture below levels that favor for disease development.

The Influence of Irrigation Frequency on the Onset and Development of Verticillium Wilt of Olive.

The influence of irrigation frequency on the onset and development of Verticillium wilt of olive (VWO) was studied. A split-plot design in microplots with naturally infested soil was established for studying four irrigation frequencies for three olive cultivars with differing levels of disease resistance. Final disease incidence (DI) and mortality in 'Picual' plants subjected to daily irrigation treatment (T1) reached values of 100 and 63%, respectively. For Picual-T1 samples, the area under the disease progress curve values were significantly different between 15 December 2012 and 15 July 2013 (14.8 to 42.8%) compared with the average results of the other treatments, which were weekly (T2), biweekly (T3), and deficit (T4) (0.4 to 11.5%). No significant differences between the irrigation treatments were observed in 'Arbequina', although the DI progressed consistently (60% in all treatments). In 'Frantoio', little disease developed. We conclude that a daily irrigation treatment encourages VWO development in susceptible Picual. Therefore, in susceptible cultivars growing in infested soils under daily irrigation regimes, the extension of irrigation frequency may reduce disease incidence. In the context of an integrated control, the use of resistant cultivars seems to be more relevant than the detrimental effects observed in frequent irrigation schedules.

Field Resistance to Verticillium Wilt in Selected Olive Cultivars Grown in Two Naturally Infested Soils.

The resistance of 11 olive cultivars to Verticillium dahliae was assessed in two experimental field trials. One-year-old rooted olive cuttings from the World Olive Germplasm Bank (IFAPA research center, Córdoba, Spain) were planted in a heavily infested field in Utrera (Sevilla province) and in a moderately infested field in Andújar (Jaén province) of southern Spain. Plants were assessed for Verticillium wilt resistance during 22 months based on disease severity and tree growth. Severe disease symptoms were observed 6 months after planting in both trials. Twenty months after planting in the heavily infested soil, V. dahliae had killed nearly all of the trees of 'Bodoquera', 'Cornicabra', 'Manzanilla de Sevilla', and 'Picual', demonstrating the elevated risk of planting susceptible cultivars in a soil heavily infested with V. dahliae. 'Arbequina', 'Koroneiki', 'Sevillenca', and especially 'Frantoio', 'Empeltre', and 'Changlot Real' showed a high level of disease resistance. However, all of them were affected by the disease. Although the field results confirmed the level of resistance previously obtained for these olive genotypes under controlled conditions, there were some discrepancies. This information will be useful in managing the disease and also in selecting new cultivars for the breeding of Verticillium wilt resistance.

First Report of Verticillium Wilt of Pistachio Caused by Verticillium dahliae in Spain.

Pistachio (Pistacia vera L.) trees in the Castilla La Mancha and Andalusia regions of central and southern Spain are grown close to olive orchards, which are often severely affected by Verticillium dahliae. During the last decade, wilt and death of one or several branches have been observed on pistachio (cv. Kerman) scions grafted on rootstock (P. terebinthus). Discoloration of vascular tissue was occasionally observed. In five surveyed orchards, incidence of affected trees was less than 2%. Wood chips with the bark removed from symptomatic trees were washed in running tap water, surface disinfested in 0.5% sodium hypochlorite for 1 min, and placed onto potato dextrose agar plates incubated at 25°C in the dark. Isolates were identified as V. dahliae on the basis of the characteristics of microsclerotia, conidiophores, and conidia. V. dahliae isolate V117 from olive was used as reference (1). The fungus was also isolated from soil in pistachio orchards using wet sieving and a modified sodium polypectate agar medium (1). Inoculum density varied from 0 to 4.73 microsclerotia per gram of soil. P. terebinthus seedlings were inoculated with two isolates of V. dahliae from pistachio trees by injecting the stems with 50 µl of a conidial suspension (107 conidia per ml) (2). Wilt symptoms of varying severity developed in 12 and 15 seedlings of the 20 pistachio seedlings inoculated with each of two isolates. No symptoms developed on the control seedlings. The pathogen was recovered from stem tissues of inoculated plants. To our knowledge, this is the first report of Verticillium wilt of pistachio in Spain. This study demonstrates the susceptibility of certain rootstocks to V. dahliae and the importance of using resistant rootstocks, such as UCBI (2), in pistachio plantations established on soils infested by V. dahliae. References: (1) F. J. López-Escudero and M. A. Blanco-López. Plant Dis. 91:1372, 2007. (2) D. P. Morgan et al. Plant Dis. 76:310, 1992.

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.

Effect of a Single or Double Soil Solarization to Control Verticillium Wilt in Established Olive Orchards in Spain.

Four soil solarization experiments were completed in three commercial olive orchards infested with Verticillium dahliae in southern Spain. Three of the experiments used lines of trees and one used individual plants. Plantations had different initial inoculum densities of the pathogen. Initial studies indicated that highly virulent (cotton-defoliating) isolates of the pathogen were present in Marinaleda (experiment I), which represents the first record of such isolates affecting olive trees in Europe. Solarization treatments were applied to lines of trees for either one (single) or two consecutive (double) years. Solarization significantly reduced pathogen populations in the top 20 cm of soil for at least 3 years in relation to control plots. Pathogen reduction after the single solarization obscured effects of the second solarization treatment. Decrease of inoculum density in soil by solarization did not correspond to a similar reduction in disease severity. Disease severity was reduced only in orchards with medium or high initial inoculum densities. A second soil solarization treatment did not improve the effect of single solarization on Verticillium wilt control. In orchards with low inoculum densities, soil solarization did not result in significant differences in disease incidence and severity, but improved recovery of trees from the disease. Soil-solarized plots remained free of weeds, but tress in solarized plots did not show significant growth increase measured by trunk perimeter.

First Report of Transmission of Verticillium dahliae by Infested Manure in Olive Orchards in Andalucia (Southern Spain).

Olive (Olea europaea L.) is a traditional tree crop in Andalucia, southern Spain. Over the past several years, new plantations have been established in areas where other crops susceptible to Verticillium dahliae, such as cotton, sunflower, and vegetables, usually have been grown. Surveys performed from 1989 to 1996 showed that Verticillium wilt, caused by V. dahliae, has become the most important soilborne fungal disease affecting young olive trees in this area (3). Manure is sometime used as a soil amendment before and after tree planting to increase organic matter content of soil. In 1996, manure was used to establish an olive plantation in Sevilla Province. The dung came from sheep fed on harvested fields of cotton, sunflower, and sugar beet. The remaining manure was left in a pile in the field. In 1998, 2 years after planting, many olive plants in the field showed Verticillium wilt symptoms. Samples were collected from the soil and the surfaces and core of the manure pile for isolation and quantitative assessment of the pathogen. Wet-sieving and sodium pectate agar selective medium were used (1). V. dahliae was isolated from both the manure and soil. Mean inoculum density in the samples taken from the pile and soil were 8.4 and 7.2 microsclerotia per gram, respectively. Although it has been demonstrated that V. albo-atrum survives the digestive process in animals (2), this is the first report on dispersion of V. dahliae by manure from sheep fed on infested stubble. The use of manure infested with V. dahliae may contribute to pathogen dispersion and to the increase in Verticillium wilt incidence in olive orchards in Andalucia. Because animal manure is a potential means for V. dahliae transmission, it is recommended that before manure is applied tests be performed, when feasible, to make sure it is pathogen-free. References: (1) E. J. Butterfield and J. E DeVay Phytopathology 67:1073, 1977. (2) H. C. Huang et al. Plant Dis. 70:218, 1986. (3) M. E. Sanchez Hernandez et al. Eur. J. Plant Pathol. 104: 347, 1998.