Effects of cultivar mixtures on scab control in apple orchards

The effects of two mixtures of resistant and susceptible apple cultivars on the development of scab caused by Venturia inequalis were observed in an experimental orchard over four years, initially for two years without fungicides against scab, and subsequently for two years with a moderate fungicide schedule. The row-by-row and within-row mixtures included a susceptible cultivar and a resistant cultivar in equal proportions. Without fungicides, the results showed a significant reduction of disease incidence over both years (7·3 to 21·3%), and severity in the second year (35·4%) in the within-row mixtures, compared to the monoculture of the susceptible cultivar. The best results were obtained when the within-row mixture was associated with moderate fungicide treatments; in this case the reduction in disease incidence reached 75·1% on leaves and 69·7% on fruits during the growth phase. The characteristics of the Venturia inaequalis / Malus  ×  domestica pathosystem and the results obtained in this experiment suggest a moderate but not negligible ability of cultivar mixtures for reducing epidemics of the disease.     

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.     

Population variation of apple scab (Venturia inaequalis) within mixed orchards in the UK

Apple scab, caused by Venturia inaequalis, is one of the most important apple diseases worldwide. To investigate between- and within-orchard fungal variability, 212 isolates were sampled from two mixed orchards, one of 10?years of age and the other of 45?years of age, in the UK and genotyped with AFLP and SSR markers. Populations of isolates from the two orchards did not differ significantly in terms of allele frequencies at the screened AFLP and SSR loci. However, groups of isolates from individual cultivars differed significantly within each orchard and there were also significant differences between groups of isolates from individual trees of the same cultivar in the same orchard. These differences were less pronounced in the younger mixed orchard than in the older one. The existence of tree-to-tree fungal variability indicates a possible role for conidia as a source of primary inoculum. Non-random mating may be one of the factors causing the significant differences among fungal populations from different cultivars. These results suggest that apparently ??susceptible?? cultivars have different background genetic resistance factors, which can be exploited for disease management in mixtures.     

Temporal and spatial propagule deposition patterns of the emerging fungal pathogen of chestnut Gnomoniopsis castaneae in orchards of north-western Italy

Two chestnut (Castanea sativa) orchards of north-western Italy were sampled with passive spore traps 35 times over 24 months. Samples were analysed through a newly developed quantitative PCR assay to quantify propagule loads of the emerging fungal pathogen Gnomoniopsis castaneae. Average propagule deposition patterns were assessed along with temporal and climatic variables, including sampling month and season, temperatures, relative humidity, precipitations, and wind. Machine learning algorithms combining information theory, fractal analysis, unbiased recursive partitioning, ordinary least squares and logistic regressions, were used to model propagule deposition patterns. The trained models were validated on independent data gathered from 24 samplings conducted in a third chestnut orchard during the same timeframe. Results showed that propagule deposition rate (DR) was variable within and among sites, with a site average ranging from 173 to 765 spores ⋅m⁻² ⋅h⁻¹. Propagule deposition was observed across all seasons, although the DR dropped substantially during wintertime (p < 0.05). Mean, maximum, and minimum temperatures, the growing degree days at 0 and 5℃ thresholds, and wind gust were all positively correlated (p < 0.05) with DR of G. castaneae. The trained models were all significant (p < 0.05), as well as their validation (p < 0.05). Fluctuations of propagule deposition throughout the year were consistent among sites and proved to be driven by temperatures. Wind gust was associated with the overall amount of propagules deposited at site level. In future, the increase in temperatures and strong winds as a result of climate change may boost the spread of G. castaneae.     

Virulence characteristics of apple scab (Venturia inaequalis) isolates from monoculture and mixed orchards

Isolates of Venturia inaequalis were sampled from monoculture and mixed orchards of three apple cultivars: Bramley, Cox and Worcester. In addition, single-ascospore progeny isolates were obtained from three crosses between pairs of isolates originating from the three cultivars in monoculture orchards. These field isolates and single-ascospore progenies were inoculated onto each of the three cultivars in a glasshouse. The patterns of infection showed that all three cultivars, commonly regarded as susceptible to scab, contained some specific resistance factors and that scab isolates from both mixed and monoculture orchards appeared to have co-evolved with cultivars. A much higher proportion of isolates from cv. Worcester in the mixed orchard were unable to infect any of the three cultivars than isolates from any other combination of cultivar and orchard type, but there was no other difference between isolates from mixed vs. monoculture orchards. Many isolates could infect both Bramley and Cox, or Cox and Worcester; but only a single isolate could infect both Bramley and Worcester. Results from the testing of 61 single-ascospore progeny isolates suggested that virulence towards Bramley, Cox and Worcester was controlled by at least one, two and three factors, respectively. Moreover, the exact nature of the interactions between these factors in determining virulence depended on the particular pair of isolates concerned.     

Dispersal gradient of M. fructicola conidia from peach orchard to an open field

European Journal of Plant Pathology - Dispersal gradient and the maximum distance that spores of M. fructicola can spread allow the quantification of the relative risk of disease spread from spores...     

Development and validation of a standard area diagram set to aid assessment of severity of loquat scab on fruit

A standard area diagram set (SAD) to aid visual assessment of loquat scab (caused by Fusicladium eriobotryae) severity on fruit was developed and evaluated for improving accuracy, precision and reliability of visual estimates. The SAD set contains eight black and white diagrams of diseased fruit with severity values from 2 % to 98 %. To evaluate the SADs, a group of 20 raters (comprising 10 ‘experienced’ and 10 ‘inexperienced’ raters) assessed the same set of 50 images three times, the first without SADs and the second and third using the SADs as an aid. Only for the group of inexperienced raters did SADs significantly improve accuracy (bias correction factor, C _b?=?0.93 without SADs and 0.98 with SADs), precision (correlation coefficient, r?=?0.88 without SADs and r?=?0.96 with SADs) and overall agreement (Lin’s concordance correlation coefficient, ρ _c?=?0.82 without SADs and ρc with SADs = 0.95) of the estimates. Accuracy and precision of the estimates by inexperienced raters were significantly higher than those obtained by the experienced raters, especially for the second assessment with SADs. Inter-rater reliability was improved when SADs were used by inexperienced raters, whereas a high degree of intra-rater reliability was obtained by both experienced and inexperienced raters when using SADs. The SADs developed in this study were useful for obtaining more accurate, precise and reliable assessments of loquat scab for inexperienced raters, and should be used as an aid for assessing scab in epidemiological studies or monitoring for decision-making purposes.     

Characterization of Botryosphaeriaceae species associated with diseased loquat (Eriobotrya japonica) in Spain

Loquat (Eriobotrya japonica) is an important subtropical fruit crop in Spain and other Mediterranean countries. In recent years, characteristic symptoms of branch canker and dieback have been observed in the main cultivated areas of loquat in Spain. The goal of this study was to identify and characterize the species of Botryosphaeriaceae associated with these symptoms. For this, 36 affected orchards were surveyed between 2010 and 2011 in six provinces of southeastern Spain. Eighty‐four isolates belonging to the family Botryosphaeriaceae were recovered from samples with symptoms. These isolates were characterized by means of phenotypical studies, DNA sequence analyses of the internal transcribed spacer (ITS) and part of the translation elongation factor 1‐α regions, and pathogenicity tests. Ten fungal species were identified: Diplodia malorum, Diplodia olivarum, Diplodia seriata, Diplodia pseudoseriata/Diplodia alatafructa, Diplodia sp., Dothiorella sarmentorum, Neofusicoccum mediterraneum, N. parvum, Spencermartinsia plurivora and S. viticola. In addition, Diplodia eriobotryicola and Dothiorella eriobotryae are newly described. The most frequent species isolated from cankers was D. seriata and, as far as is known, this is the first report of D. malorum, and species belonging to the complex D. pseudoseriata/D. alatafructa, in Spain. All species were pathogenic to 1‐year‐old loquat plants cv. Algerie, with Diplodia sp. and S. viticola as the most virulent.     

Survey of herbicide residues in soil and wells in three citrus orchards in Valencia, Spain

Soil samples taken in summer 1989 and winter 1990 at different depths from three citrus orchards of the Valencia region (Spain) with a long history of residual herbicide treatments were analysed with bioassays and chromatographic procedures. The herbicides involved were atrazine, bromacil, diuron, simazine, terbuthylazine, terbumetone, terbutryn and trifluralin. Water samples from wells in the orchards were also analysed. The concentrations of the herbicides were very low, often below the limit of determination, and were always highest in the upper layers of soil. After a very unusual period of heavy rain, small quantities of some chemicals were found in the well water. The conclusion was that in these orchards the herbicides applied at currently used rates are unlikely to accumulate in any layers of the soil.     

Dispersal of potato cyst nematodes measured using historical and spatial statistical analyses

Rates and modes of dispersal of potato cyst nematodes (PCNs) were investigated. Analysis of records from eight countries suggested that PCNs spread a mean distance of 5.3 km/year radially from the site of first detection, and spread 212 km over ≈40 years before detection. Data from four countries with more detailed histories of invasion were analyzed further, using distance from first detection, distance from previous detection, distance from nearest detection, straight line distance, and road distance. Linear distance from first detection was significantly related to the time since the first detection. Estimated rate of spread was 5.7 km/year, and did not differ statistically between countries. Time between the first detection and estimated introduction date varied between 0 and 20 years, and differed among countries. Road distances from nearest and first detection were statistically significantly related to time, and gave slightly higher estimates for rate of spread of 6.0 and 7.9 km/year, respectively. These results indicate that the original site of introduction of PCNs may act as a source for subsequent spread and that this may occur at a relatively constant rate over time regardless of whether this distance is measured by road or by a straight line. The implications of this constant radial rate of dispersal for biosecurity and pest management are discussed, along with the effects of control strategies.     

Dispersal of conidia of Ascochyta fabae f. sp. lentis from infected lentil plants by simulated wind and rain

Splash was shown to be an effective mechanism for short-range dispersal of conidia of Ascochyta fabae f. sp. lentis , the cause of ascochyta blight of lentil. The dispersal gradients were well described by the power law model in its linear form, In y = In a-b In x. In still air the slope of the linearized dispersal gradient, b , ranged from 2.83 to 4.07 and was steeper for 4.9 mm than for 3.9 mm incident drops. Nevertheless, for all drop sizes tested, fewer than 50% of the conidia were splashed more than 15 cm from the source. The pattern of conidium dispersal was similar for both drop sizes when horizontal windspeeds were 2.5 or 5 m/s. Wind significantly decreased the value of b (range 2.35-2.43 at 2.5 m/s, 1.71–1.91 at 5 m/s) and increased by about 2 m the maximum distance that conidia in ballistic droplets were deposited. In addition, the experiments suggested two other potentially important mechanisms for dispersal of the pathogen over longer distances, namely conidia in small air-borne droplets and windblown leaflets.     

Comparison of conventional and molecular methods for the detection of Rosellinia necatrix in avocado orchards in southern Spain

White root rot, caused by Rosellinia necatrix , is one of the most important diseases in avocado orchards and is particularly widespread on the Mediterranean seaboard of southern Spain. In this study, the presence of the pathogen in soil samples collected from the base of 47 plants showing different symptoms of canopy decline was assessed with a molecular detection method based on real-time Scorpion PCR. Results were compared with symptoms in the canopy and with the traditional method of isolation of R. necatrix from roots and/or bark. The fungus was isolated from 24 samples by the traditional method and from 37 soil samples by the molecular method (cycle threshold values 25·8 to 47·1), demonstrating the higher sensitivity and reliability of the molecular method. A single real-time PCR amplification was sufficient to detect R. necatrix in naturally infested soils. The avoidance of nested PCR has important practical implications because of the reduced costs and risk of cross contamination. Also, it enables faster sample analysis and is more appropriate for quantitative detection. A modified molecular method was also developed to detect R. necatrix in roots and in soils with very low populations of the pathogen.     

Effect of six sanitation treatments on leaf litter density, ascospore production of Venturia inaequalis and scab incidence in integrated and organic apple orchards

A two-year study was conducted to determine the effect of six sanitation treatments on leaf litter density (LLD), relative ascospore production of Venturia inaequalis and scab incidence on spur-leaf clusters, leaves and harvested fruits, on two cultivars with low and high scab susceptibilities, in Hungarian integrated and organic apple orchards. The following sanitation treatments were used: sprays of lime sulphur in autumn, collecting fallen leaves in autumn, straw mulch cover in late winter, sprays of lime sulphur followed by mulch cover, collecting fallen leaves followed by mulch cover, collecting fallen leaves followed by covering the orchard floor with plastic foil, and non-sanitized control. LLD decreased continuously in all treatment plots by 0–23% by mid-May in both orchards and years; however, LLD reduction was 1.4–4.2 times higher in the organic orchard compared to the integrated one. All treatments, except for the lime sulphur treatment, resulted in significant (P < 0.05) reduction of LLD and ascospore production in both the integrated and organic apple orchards compared to non-sanitized plots. The most efficient treatment was leaf collection combined with plastic foil cover, followed by leaf collection combined with mulch cover, leaf collection alone, mulch cover alone, and lime sulphur spray combined with mulch cover, with a reduction in the ascospore production of >95, 72–92, 56–79, 24–38, and 27–46%, respectively, in the mean of both orchards and years. However, only treatments of leaf collection applied alone, or in combination with mulch or with plastic foil cover reduced significantly (P < 0.05) leaf and/or fruit scab incidence by 18–57% compared to non-sanitized plots. These three leaf collection treatments are recommended in both integrated and organic orchards and the possibilities of successfully incorporating these methods into orchard management practices are interpreted.     

Occurrence of Monilinia laxa and M. fructigena after introduction of M. fructicola in peach orchards in Spain

Seventeen field surveys were done in four commercial orchards during six consecutive fruit-growing seasons from 2006 until 2011 in order to determine the current frequencies of occurrence of M. laxa, M. fructigena, and M. fructicola and their relative contributions to postharvest brown rot in peaches and nectarines in the Ebro Valley. The relative frequencies of occurrence of Monilinia spp. were determined on three sources of primary inoculum and on three sources of secondary inoculum. The major relative frequencies of Monilinia spp. were significantly recorded (P?=?0.05) from mummified fruit on the trees (approx. 42 %) and 7-day-old harvested fruit with brown rot (32 %), followed by that recovered from mummified fruit on the orchard bed (14 %), pruned branches on the orchard bed (8 %) and latent infections of immature fruit (3 %). We found that: (a) the relative frequency of M. fructicola has increased over the years to coexist on the same level as at the time M. laxa, (b) M. fructigena is no longer a cause of brown rot in harvested peaches, (c), a progressive reduction in the time of the first appearance of Monilinia airborne conidia (r?=??0.30, P?=?0.003), and the time of the first latent infection (r?=??0.44, P?=?0.0001) was detected along years after correlation analysis, and (d) these displacements are not associated with an increased incidence of brown rot disease. The M. fructicola increase was due to its significantly increased presence in 7-day-old harvested fruit with brown rot (r?=?0.73, P?=?0.0009), in latent infections of immature fruit (r?=?0.68, P?=?0.002), on pruned branches on the orchard bed (r?=?0.56, P?=?0.018), and on mummified fruit sampled on the trees (r?=?0.53, P?=?0.03). This progressive increase was accompanied by a progressive reduction in the relative frequency of occurrence of M. laxa in 7-day-old harvested fruit with brown rot (r?=??0.55, P?=?0.021) and M. fructigena on mummified fruit sampled on the trees (r?=??0.51, P?=?0.03).     

不同类型果园苹果绵蚜及其天敌种群动态和空间分布

苹果绵蚜是一种重要的外来入侵性害虫,近年来危害逐年加重并不断扩展蔓延。本文调查研究了粗放园与化防园苹果绵蚜及其天敌的种群动态规律,分析了苹果树上苹果绵蚜及其天敌空间分布差异。结果表明,粗放园中苹果绵蚜及天敌数量均高于化防园。2007年粗放园与化防园中平均虫落数量为40.9个/株和4.9个/株,2008年为19.6个/株和2.1个/株。粗放园中苹果绵蚜趋向于整个树冠分布,主枝、树干相对较少;在化防园中苹果绵蚜向树的下部转移,主枝上分布较多,树冠分布较少。化防园中天敌数量低,对苹果绵蚜控制作用减弱。     

Effects of temperature, relative humidity and duration of wetness period on germination and infection by conidia of the pear scab pathogen (Venturia nashicola)

Experiments were conducted to determine the effects of temperature, relative humidity (RH) and duration of wetness period on in vitro germination of conidia and infection of detached pear leaves by Venturia nashicola , the causal agent of pear scab. Conidia germinated only in near-saturation humidity (RH > 97%). The final percentage germination (24 h after inoculation) at 100% RH without free water was less than half that in free water. Conidia germinated over the range of temperatures tested (5–30°C); the optimum temperature for germination was ≈21°C. Changes in percentage germination of conidia over time were fitted by logistic models at each individual temperature. Polynomial models satisfactorily described the relationships between two (rate and time to 50% of maximum germination) of the three logistic model parameters and temperature. The minimum length of the wetness period for successful infection of detached pear leaves by conidia was observed at several temperatures. The shortest length of wetness period required for infection was 7 h at 22°C. Two polynomial models fitted well the relationship between the minimum wetness duration required for infection, and temperature.     

Temporal patterns of airborne conidia of Podosphaera leucotricha, causal agent of apple powdery mildew

In each of the five years 1969 and 1971-1974 inclusive a volumetric spore trap was used in an apple orchard to monitor changes in the number of airborne conidia of Podosphaera leucotricha , the causal agent of apple powdery mildew. The number of trapped conidia varied greatly between years. Time-series analyses, using autoregressive integrated moving average (ARIMA) models, revealed that the temporal pattern of the number of airborne conidia was similar in all years, generally following a diurnal pattern with an afternoon peak. A strong correlation between consecutive hourly counts indicated that the number of trapped conidia depended on the strength of sporulating sources. Using the time-series transfer function (TF) method, it was shown that in each year the most important weather variables influencing the number of airborne conidia were vapour pressure deficit (VPD) and rainfall. Variation between years in the dynamic effects of these variables on conidium numbers was detected, and may reflect weather differences between years. Stepwise regression analysis was applied to the combined daily data for 1973 and 1974 using a subset of weather variables as independent variables, chosen on the basis of TF analysis. A resulting regression model accurately predicted the temporal pattern of conidium numbers (expressed as a percentage of the maximum daily number trapped in the same year) in both years. When this model was applied to the other three years there was good agreement between predicted and observed temporal patterns. Application of this regression model for practical disease control is discussed.     

Role of Cylindrosporium-type conidia of Mycosphaerella pomi (Pass.) Lindau: cause of Brooks fruit spot of apple, as an infection source in apple orchards

Ascospores of Mycosphaerella pomi, the pathogen of Brooks fruit spot of apple, were produced in pseudothecia on previously infected and overwintered apple leaves from late April through early August in Aomori Prefecture, Japan. In June 2003, the ascospores were germinating and producing Cylindrosporium-type conidia on apple fruit and leaf surfaces in an orchard. After ascospores were sprayed on apple leaves, Cylindrosporium-type conidia developed on the leaf surfaces. Such Cylindrosporium-type conidia caused typical symptoms of Brooks fruit spot on apple trees after inoculations. These results suggested that the Cylindrosporium-type conidia also serve as an infection source, in addition to the ascospores, for Brooks fruit spot in apple orchards.