Scab Severity Research Articles

Establishing suppressive conditions against soilborne potato diseases with low rates of fish emulsion applied serially as a pre-plant soil amendment

Soil amendment with fish emulsion (FE) can suppress potato scab and verticillium wilt, but the effective broadcast rates (20 000 L ha−1) may not be feasible for commercial use. The aim of this 4-year study was to establish suppressive conditions against soilborne potato diseases and increase tuber yield with economically feasible rates of FE. Diluted FE (1000 and 2000 L ha−1 or 0.05 and 0.1%) was applied to field plots or micro-plots twice a year before planting and after harvesting potatoes starting in autumn of 2007 and ending in spring of 2010. In the micro-plots, FE (0.1%) added to an infested potato soil (site BL) consistently reduced scab severity by 44.9% (2008), 44.8% (2009) and 30.9% (2010) compared with the control. Scab severity on tubers harvested from these plots was also low in 2011 when no further FE was applied, but the effect was not significant. In the field, FE (2000 L ha−1) consistently reduced scab severity by 45% (2008), 53% (2009), 44% (2010) and 38% (2011); reduced the percentage of tubers with deep-pitted scab by 48% (2008), 51% (2009), 66% (2010) and 77% (2011); and increased the percentage of marketable tubers by 37% (2008), 83% (2009), 20% (2010) and 8% (2011). These field plots also showed a low percentage of Verticillium dahliae-infected potato plants during the first 3 years of the field trial. Fish emulsion treatments increased total tuber yield by 7–20% in the first 3 years, but there was no effect in the fourth year when no FE was applied. Marketable tubers were consistently higher in the plots amended with FE (2000 L ha−1). Fish emulsion soil treatments increased soil bacteria but the numbers dropped to the control level in the fourth year when no FE application was made. There was no change in total fungal numbers. The results suggest that economically feasible rates of FE can provide disease suppression and enhance tuber yield and the effect may last a year or more after the last FE application.

Impact of a cultivar mixture on scab, powdery mildew and rosy aphid in an organic apple orchard

The impact of a cultivar mixture within the rows including the cvs Melrouge (low susceptibility to scab) and Pitchounette (Rvi6 resistant) was assessed during 2008–2009 in an organic apple orchard located in Rhône Valley (Drôme, France), on three important pests and diseases: scab, powdery mildew and rosy apple aphid. In this area, the resistance of Rvi6 gene was effective since the orchard plantation in 2004. The impact of the cultivar mixture was assessed per se, but also associated with sanitation practices to decrease the primary inoculum of scab and mildew, and compared to disease development on pure stands of Melrouge. The cultivar mixture showed a good efficiency to decrease the scab incidence on leaves and fruits; a significant effect on scab severity was observed in 2009 for the cultivar mixture plus sanitation practices. Our data confirmed the different susceptibility levels of the two cultivars for powdery mildew. Against powdery mildew, a favourable effect of cultivar mixture, at the condition it is associated with sanitation, was observed in case of a moderate level of the disease (2008). For rosy apple aphid, a favourable effect of cultivar mixture was observed on Melrouge in 2008 when Pitchounette has a good resistance level. In the context of organic production, the impact of the cultivar mixture seemed favourable for the control of the three pests and diseases.

Characterizing a novel strain ofBacillus amyloliquefaciensBAC03 for potential biological control application

To identify and characterize a bacterial strain BAC03, evaluate its biological control activity against potato common scab (Streptomyces spp.) and characterize an antimicrobial substance produced by BAC03. Bacterial strain BAC03, isolated from potato common scab suppressive soil, was identified as Bacillus amyloliquefaciens by analysing sequences of fragments of the recA, recN, cheA and gyrA genes. BAC03 displayed an antagonistic activity against Streptomyces spp. on agar plates using a co-culture method. In glasshouse assays, BAC03 applied in potting mix significantly reduced common scab severity (P < 0·05) and potentially increased the growth of potato plants (P < 0·05). An antimicrobial substance extracted from BAC03 by ammonium sulfate precipitation was identified as an LCI protein using liquid chromatography-mass spectrometry. The antimicrobial activity of either a BAC03 liquid culture or the ammonium sulfate precipitate fraction was stable under a wide range of temperatures, and pH levels, as well as following incubation with several chemicals, but was reduced by all proteinases tested. Bacillus amyloliquefaciens strain BAC03 displayed a strong antimicrobial activity, that is, the suppression of potato common scab, and may potentially enhance the plant growth. LCI protein is associated with some of the antimicrobial activity. Bacterial strain BAC03 has the potential to be developed as a commercial biological control agent for potato common scab.

Effect of Manure Application Timing on Potato Yield, Quality, and Disease Incidence

This study was established to determine whether the timing of liquid dairy manure application prior to planting potatoes influenced potato crop performance and the occurrence and severity of common scab (Streptomyces spp.) and verticillium wilt (Verticillium dahliae). Four potato (Solanum tuberosum L.) varieties with varying levels of resistance to common scab and verticillium wilt were planted on a site treated with manure applied at 280,500 L ha−1 to separate plot areas 18, 6, or 1 month before potato planting in each of 2 years. Total tuber yield for 2001 showed a significant interaction between time of manure application and variety, with more scab-susceptible varieties (Russet Norkotah and W-1151R) showing lower yields compared to the fertilizer control when the manure application was made 1 month before planting. In 2002, the proportion of U.S. No. 1 tubers was significantly lower when manure was applied 1 month before planting, especially for W-1151R. Tuber dry matter was significantly reduced by manure application in both years, but was not consistently related to time of manure application. Total yield was significantly decreased as common scab levels increased, and this relationship was primarily driven by the low yield/high scab levels associated with W-1151R and Russet Norkotah planted 6 or 1 month after manure application in 2001. Late-season verticillium evaluations showed highest levels in 2001 when manure was applied 1 month before planting to susceptible varieties Superior and Russet Norkotah. This experiment showed that when manure was applied immediately ahead of potato planting yields, and quality of some varieties decreased in 1 of 2 years. Therefore, it may be safer to apply manure at least 18 months prior to planting potatoes.

Relationship between Spongospora subterranea f. sp. subterranea soil inoculum level, host resistance and powdery scab on potato tubers in the field

The relationship between initial soil inoculum level of Spongospora subterranea f. sp. subterranea (Sss) and the incidence and severity of powdery scab on potato tubers at harvest was investigated. In all experiments soil inoculum level of Sss (sporeballs/g soil) was measured using a quantitative real‐time PCR assay. Of 113 commercial potato fields across the UK, soil inoculum was detected in 75%, ranging from 0 to 148 Sss sporeballs/g soil. When arbitrary soil inoculum threshold values of 0, &lt;10 and &gt;10 sporeballs/g soil were set, it was observed that the number of progeny crops developing powdery scab increased with the level of inoculum quantified in the field soil preplanting. In four field trials carried out to investigate the link between the amount of inoculum added to the soil and disease development, disease incidence and severity on progeny tubers was found to be significantly (P &lt; 0·01) greater in plots with increasing levels of inoculum incorporated. There was a cultivar effect in all years, with disease incidence and severity scores being significantly greater in cvs Agria and Estima than in Nicola (P &lt; 0·01).

Development and validation of standard area diagrams to aid assessment of pecan scab symptoms on fruit

Pecan scab (Fusicladium effusum) causes losses of pecan nutmeat yield and quality in the southeastern United States. Disease assessment relies on visual rating, which can be inaccurate and imprecise, with poor inter‐rater reliability. A standard area diagram (SAD) set for pecan scab on fruit valves was developed. A set of 40 images of diseased fruit valves with known severity was assessed twice by 23 raters. The first assessment was conducted without SADs, and the second assessment was made using the SADs as an aid. SADs improved rater accuracy (correction factor, Cb = 0·86 and 0·97, without and with SADs, respectively) and agreement (Lin’s concordance correlation coefficient, ρc = 0·79 and 0·89 without and with SADs, respectively) with true values. SADs improved inter‐rater reliability (intra‐class correlation coefficient, ρ = 0·77 and 0·96 without and with SADs, respectively). The least accurate and precise raters without SADs improved more using SADs compared to the most accurate and precise raters. Experienced raters had significantly higher accuracy and precision compared to inexperienced raters, but only when unaided by the SAD set. There was no significant difference in time to assess images without SADs, but experienced raters using SADs were faster compared to inexperienced raters. There was a slight tendency for faster raters to assess more slowly, and slower raters to assess faster when using SADs. SADs improve rater estimates of pecan scab severity on fruit, and this SAD set should be useful for assessment where greater precision, accuracy and inter‐rater reliability are required.

Suppression of Pecan Scab by Nickel

The economic cost of pecan scab, caused by Fusicladium effusum G. Winter, can substantially limit profitability of pecan [Carya illinoinensis (Wangenh.) K. Koch] cultivation in humid environments. Laboratory, greenhouse, and field studies found nickel (Ni) to inhibit growth of F. effusum and reduce disease severity on fruit and foliage of orchard trees. Nickel was toxic to the fungus in vitro at concentrations applied to orchard trees, and Ni sprays reduced scab severity on foliage of pecan seedlings in greenhouse experiments. Host genotype appears to influence Ni efficacy with fruit tissue of cultivars of intermediate resistance (i.e., ‘Desirable’) being most responsive to treatment and those most susceptible to scab (i.e., ‘Wichita’ and ‘Apache’) being least responsive. Addition of Ni as a nutritional supplement applied in combination with fungicides applied as air-blast sprays to commercial orchards reduced severity of scab on both leaves and fruit depending on cultivar and date of disease assessment (e.g., scab severity on fruit was reduced by 6% to 52% on ‘Desirable’ in an orchard setting). Nickel-supplemented fungicide sprays to ‘Desirable’ trees in commercial orchards also increased fruit weight and kernel filling, apparently from improved disease control. Although the efficacy of Ni was typically much less than that of triphenyltin hydroxide (TPTH), a standard fungicide used in commercial orchards, Ni treatment of tree canopies for increasing tree Ni nutrition slightly lowered disease severity. These studies establish that foliar Ni use in orchards potentially reduces severity of scab on foliage and fruit in scab-prone environments. The inclusion of Ni with fungicides for management of pecan scab might reduce disease severity over that conferred by fungicide alone, especially if targeted cultivars possess at least a moderate degree of scab resistance. Similar benefit from Ni sprays might also occur in host–fungi interactions involving other crops.

Evaluation of a phosphite fungicide to control pecan scab in the southeastern USA

The efficacy of phosphite, a potential elicitor of systemically acquired resistance (SAR) was compared to the protectant fungicide triphenyltin hydroxide (TPTH) to control pecan scab caused by Fusicladium effusum. Efficacy was evaluated in four field experiments over a two-year period involving biweekly foliar applications of both fungicides to trees of five susceptible cultivars of pecan (Carya illinoinensis) and assessment of disease severity on foliage and fruit. Both phosphite and TPTH reduced scab severity on foliage equally well compared to the non-treated control, with the exception of one of the TPTH treatments in 2010. Both phosphite and TPTH provided equally good control of disease early in fruit development (Jul/Aug). However, by the final assessment (Sep/Oct), scab severity on phosphite-treated trees was most often greater than those receiving TPTH and in 2010, severity was equivalent to the non-treated control. Despite a suggested lack of late-season protection with phosphite, there was no difference in fruit volume between phosphite and TPTH-treated plots in 2009, and no difference in nut volume in 2010, although there were treatment differences in kernel weight and fruit weight in 2010. Phosphite-treated trees showed some symptoms of phytotoxicity. Regression analysis demonstrated the effect of scab on yield loss and confirmed the value of scab control on susceptible cultivars. In-vitro tests showed that phosphite is toxic to scab at rates applied in the field, thus implying direct fungitoxicity. Results indicate that phosphite provides useful control of pecan scab on both foliage and fruit early in the growing season, but might not provide prolonged late-season protection compared to an industry standard (i.e., TPTH).

Assessment of Application Timing of Bacillus spp. to Suppress Pre- and Postharvest Diseases of Apple.

Four isolates of Bacillus spp. were tested in a 2-year field study for biological control of pre- and postharvest diseases of apple. For the preharvest test, bacteria were applied to 'Golden Delicious' and 'Rome Beauty' trees in May or May and June. Foliar apple scab severity was assessed weekly. After harvest, fruit were wounded and then either left untreated or given a postharvest application of the bacteria. Wounded fruit were then inoculated with the bitter rot pathogen and lesion size was measured over 8 days. Bacillus megaterium isolate A3-6, B. mycoides isolate A1-1, and B. cereus isolate FLS-5 applied in May or May and June significantly reduced fruit and foliar apple scab severity in both years. A postharvest application of the bacteria (alone or in combination with a preharvest application) resulted in the greatest suppression of bitter rot on both cultivars (P < 0.04). The May + June + postharvest application of isolate A3-6 resulted in the greatest suppression of bitter rot, with an average of 45 and 95% reduction in lesion size compared with nontreated controls on 'Golden Delicious' and 'Rome Beauty' fruit, respectively. Results from this study indicate that preharvest applications of the bacteria were able to reduce foliar and fruit scab and an additional postharvest application was effective in reducing bitter rot severity.

Isolation of antagonistic Streptomyces sp. against a potato scab pathogen from a field cultivated with wild oat

Fungi, actinomycetes, and bacteria isolated from soil and plant samples from a potato field in which wild oat (Avena strigosa) had been pre-cultivated were screened for microorganisms that can be used as biocontrol agents for common scab of potato. Of 342 isolates assessed in initial pot trials for their suppressive effect on the severity of potato scab caused by Streptomyces turgidiscabies, 26 isolates were selected as antagonistic candidates based on their ability to reduce disease severity, then tested in a second pot trial. Of the 26, five actinomycetes, isolated from either the rhizosphere soil of wild oat or the soil adhering to potato stolons and tubers, were selected as antagonists. A comparison of partial sequences of 16S rRNA genes from the five isolates indicated that they belong to the genus Streptomyces. Of these five, WoRs-501 most strongly inhibited in vitro mycelial growth of S. turgidiscabies and was also the most effective in suppressing potato scab in a third field pot trial. In that pot trial, a 10% (v/v) mix of WoRs-501 (6.2 × 108 colony-forming units [CFU]/g dry mass) decreased the disease severity by 78–94% in comparison with the untreated control at 5 × 104 to 5 × 106 CFU S. turgidiscabies/g dry soil. WoRs-501 also grew well in vitro at a wide range of pH levels and temperatures. These results suggest that WoRs-501 is a promising candidate for biocontrol of potato scab.

Suppression of Common Scab of Potato Caused by Streptomyces turgidiscabies Using Lopsided Oat Green Manure.

Field experiments were conducted to determine the effect of green manure as fallow on common scab of potato caused by Streptomyces turgidiscabies. Significantly fewer diseased tubers were harvested from soil incorporated with lopsided oat or woolly pod vetch compared with those from oat and continuous potato cultivation in a planter experiment. Each field experiment consisted of lopsided oat cultivated during the spring and summer prior to the potato planting. Comparisons were also made with several other treatments, including cultivation of woolly pod vetch, oat, soybean, sugar beet, and potato ('Yukirasya', which is resistant to potato common potato scab) and soil application of Ferosand (Fe, mainly FeSO4, to decrease the soil pH). In field experiments conducted during 1999-2000, treatment with lopsided oat followed by lopsided oat or woolly pod vetch was significantly more effective at suppressing the disease severity than oat and continuous potato cultivation (P < 0.001). An increase in the marketable tuber ratio was also more significant than for oat and continuous potato cultivation (P < 0.001). In field experiments conducted during 2000-01, lopsided oat cultivation alone and with the application of Ferosand (1.8 t/ha) or resistant potato cultivar treatment were significantly more effective at suppressing the disease severity and incidence than sugar beet cultivation (P < 0.001), even under high disease intensity in the field. However, potato yield had a tendency to reduce after lopsided oat treatment with an application of Ferosand (1.8 t/ha) compared with lopsided oat alone or the application of Ferosand at 600 kg/ha, due to low pH conditions. In field experiments conducted during 2001-02, the lowest severity and incidence of common scab of potato were observed in soil treated with lopsided oat than with other treatments (P < 0.05 and P < 0.001, respectively). These findings suggest that lopsided oat used as fallow green manure can reduce the severity of common scab and increase potato yield.

Thermographic assessment of scab disease on apple leaves

Phytopathogenic fungi may affect both the cuticular and the stomatal conductance of plant tissue resulting in significant modifications of leaf temperature. Venturia inaequalis colonizes apple leaves below the cuticle (subcuticularly) causing scab disease. The suitability of digital infrared thermography for sensing and quantifying apple scab was assessed by investigating the effects of V. inaequalis on the water balance of apple leaves in relation to the disease stage and the severity of scab. Transpiration was measured by infrared thermo-imaging to evaluate spatial heterogeneity of the leaves in response to localized infections. Fungal development was assessed microscopically. Subcuticular growth of the pathogen caused localized decreases in leaf temperature before symptoms appeared that significantly increased the maximum temperature difference (MTD) of leaves. The MTD increased with scab development and was strongly correlated to the size of infection sites (r²linear = 0.85) and overall disease severity (% diseased leaf area, r²square = 0.71). In later stages of the disease, the MTD decreased because of leaf senescence. Thermographic measurements revealed differences in disease severity resulting from disease stage, resistance of host tissue and differences in the aggressiveness of V. inaequalis isolates. Subcuticular growth of the pathogen was beyond the area of conidia production, therefore, the area of leaf with increased transpiration was larger than the scab lesions; the proportion decreased from >70% in the early stages to <20% for mature lesions. Leaf transpiration was increased by all stages of scab development, therefore, MTD may be used not only for the differentiation between diseased and non-diseased leaves, but also for disease quantification, e.g. in screening systems and monitoring in precision agriculture.

Rotation and Cover Crop Effects on Soilborne Potato Diseases, Tuber Yield, and Soil Microbial Communities.

Seven different 2-year rotations, consisting of barley/clover, canola, green bean, millet/rapeseed, soybean, sweet corn, and potato, all followed by potato, were assessed over 10 years (1997-2006) in a long-term cropping system trial for their effects on the development of soilborne potato diseases, tuber yield, and soil microbial communities. These same rotations were also assessed with and without the addition of a fall cover crop of no-tilled winter rye (except for barley/clover, for which underseeded ryegrass was substituted for clover) over a 4-year period. Canola and rapeseed rotations consistently reduced the severity of Rhizoctonia canker, black scurf, and common scab (18 to 38% reduction), and canola rotations resulted in higher tuber yields than continuous potato or barley/clover (6.8 to 8.2% higher). Addition of the winter rye cover crop further reduced black scurf and common scab (average 12.5 and 7.2% reduction, respectively) across all rotations. The combined effect of a canola or rapeseed rotation and winter rye cover crop reduced disease severity by 35 to 41% for black scurf and 20 to 33% for common scab relative to continuous potato with no cover crop. Verticillium wilt became a prominent disease problem only after four full rotation cycles, with high disease levels in all plots; however, incidence was lowest in barley rotations. Barley/clover and rapeseed rotations resulted in the highest soil bacterial populations and microbial activity, and all rotations had distinct effects on soil microbial community characteristics. Addition of a cover crop also resulted in increases in bacterial populations and microbial activity and had significant effects on soil microbial characteristics, in addition to slightly improving tuber yield (4% increase). Thus, in addition to positive effects in reducing erosion and improving soil quality, effective crop rotations in conjunction with planting cover crops can provide improved control of soilborne diseases. However, this study also demonstrated limitations with 2-year rotations in general, because all rotations resulted in increasing levels of common scab and Verticillium wilt over time.

A LOW INPUT STRATEGY FOR SCAB CONTROL IN ORGANIC APPLE PRODUCTION

The effectiveness of compounds such as sulphur, lime sulphur, copper and potassium bicarbonate for controlling primary scab was investigated in field experiments over two growing seasons in Belgium on high and medium scab-susceptible cultivars, 'Pinova' and 'Pirouette', respectively, and two Vf scab-resistant cultivars, 'Initial' and 'Topaz'. With the aim of reducing the amount of fungicide to be applied, two strategies were evaluated: (i) timing fungicide applications so that spraying occurred during the infection process, as determined by the RIMpro software warning system and (ii) using a tunnel sprayer machine for treatment applications. In both years of the study, although there was high disease pressure, low rates of elemental sulphur (<40 kg ha -1 year -1 ), combined with low rates of copper (<3 kg has -1 year -1 ) provided the best scab control and reduced scab severity on the fruit of `Pinova' by ≥97% compared with a water control. Lime sulphur was more effective than wettable sulphur and appeared to be effective at temperatures below 10°C. Potassium bicarbonate significantly reduced apple scab and was, in some cases, as effective as wettable sulphur, using the same dosage. All treatments increased the yield of the scab-susceptible cultivars as well as that of the scab-resistant cultivars. Under these conditions, none of the treatments caused phytotoxicity, increased fruit russet, affected the summer density of the predatory mite, Typhlodromus pyri or resulted in undesirable residues on fruit at harvest.

Biological control of Fusarium dry rot and other potato tuber diseases using Pseudomonas fluorescens and Enterobacter cloacae

Dry rot caused by various species of Fusarium is a disease of significant importance in potatoes. Field trials were conducted in 2005 and 2006 in New Brunswick, Canada to assess the efficacy of Pseudomonas fluorescens and Enterobacter cloacae applied as a seed treatment in suppressing Fusarium dry rot of potato ( Solanum tuberosum L.) ‘Russet Burbank’ under field conditions. In 2005, the trial consisted of five treatments namely (1) non-treated, non-inoculated control; (2) non-treated control inoculated with Fusarium sambucinum; (3) seed inoculated with F. sambucinum and treated with P. fluorescens; (4) seed inoculated with F. sambucinum and treated with E. cloacae; and (5) seed inoculated with F. sambucinum and treated with the fungicide fludioxonil. In 2006, a mustard meal treatment was added. After harvest, tubers were assessed for disease severity of dry rot in addition to other tuber diseases, including silver scurf ( Helminthosporium solani), and common scab ( Streptomyces scabiei). In addition, tubers were graded and assessed for total yield, tuber size, tuber number, and tuber weight. Significant reduction in dry rot severity was obtained with all treatments compared to the non-treated control inoculated with Fusarium sambucinum. The highest dry rot reduction averaged over the two years of the study was for the fludioxonil treatment (55.7%) followed by the treatment with mustard meal (47.5%; 2006 only), Pseudomonas fluorescens (35%) and Enterobacter cloacae (26.5%). All treatments significantly reduced the severity of common scab and silver scurf compared to the non-treated, non-inoculated control. In both years, on an average, seed treated with P. fluorescens and E. cloacae produced higher total number of tubers. Both total and marketable tuber yields were significantly higher for the E. cloacae treatment compared to the non-treated, inoculated control. The results of this study suggest that P. fluorescens, E. cloacae and mustard meal are viable options for controlling potato tuber diseases along with fludioxonil. This is the first study to investigate the effect of these bacteria on potato diseases under field settings.

Effect of Systemic Inducing Resistance and Biostimulant Materials on Apple Scab Using a Detached Leaf Bioassay

A detached leaf bioassay was used to evaluate several systemic inducing resistance agents, a range of biostimulant products and a conventional triazole fungicide (myclobutanil) on apple scab (Venturia inaequalis) development under laboratory conditions. None of the biostimulant products (seaweed extract, betaine, molasses, humic acid, yucca extract, and plant hormone/vitamin complex) evaluated in this study inhibited germination of apple scab conidia, subsequent formation of appressoria or reduced leaf scab severity compared to water treated controls. All SIR agents used in this investigation (potassium phosphonate, potassium phosphite, harpin protein, salicylic acid, salicylic acid derivative) inhibited germination of apple scab conidia, subsequent formation of appressoria and reduced leaf scab severity. The synthetic fungicide myclobutanil resulted in the greatest levels of germination inhibition, reduced appressorium development and leaf scab severity. Results suggest application of an appropriate SIR product may provide a useful addition to existing methods of apple scab management; however, use of biostimulants as scab protectant compounds appears limited.

The Influence of Calcium Sprays to Reduce Fungicide Inputs Against Apple Scab [Venturia inaequalis (Cooke) G. Wint.

A goal of pathogen management in the arboriculture industry should be to eliminate or reduce the amount of active ingredients of synthetic fungicides used within a growing season. The aim of this research paper was to evaluate a nonfungicidal pathogen management system by investigating the hypothesis that replacing an apple scab fungicide spray program with calcium reduces fruit and leaf scab severity. In addition, a separate study investigated if a relationship existed between calcium concentrations within foliar tissue of resistant, intermediate, and sensitive apple species and varieties toward scab infection. A number of commercially available calcium products were applied to apple cv. Crown Gold trees at four distinct growth stages (bud break, 90% petal fall, early fruitlet, two weeks after early fruitlet). A comparative evaluation of the synthetic fungicide penconazole commercially used for scab control was also conducted. The experiment was performed in 2006 and repeated in 2007 at the University of Reading Experimental Field Site (UK). Application of calcium sprays significantly reduced the leaf and fruit scab severity of apple cv. Crown Gold however; in a separate study no relationship existed between foliar calcium content and susceptibility to apple scab attack between resistant, intermediate, and sensitive Malus species. Greatest protection in both field trials was provided by the synthetic fungicide penconazole. Within the calcium products evaluated, greatest protection in both field trials was provided by calcium chloride and calcium hydroxide. The integration of calcium foliar sprays into existing scab management practices offers a useful addition to reduce scab severity on ornamental apples that has applicability against other foliar diseases frequently encountered within urban landscapes.

Reação de cultivares de batata a Streptomyces scabies, agente causal da sarna comum profunda

Este trabalho visou caracterizar quanto a critérios morfológicos e fisiológicos isolados de Streptomyces causadores de sarna comum profunda em batata; avaliar o comportamento de cultivares em relação à doença e a variação na agressividade entre os isolados da bactéria. Os isolados de Streptomyces apresentaram coloração cinza em meio extrato de levedura e malte e cadeias de esporos espiraladas, produzidas sobre um micélio aéreo. Ocorreu produção de melanina em meio de tirosina-ágar e a utilização de oito fontes de carbono recomendadas, propriedades estas que correspondem às descritas para S. scabies. Batatas-semente sadias de seis cultivares foram plantadas em substrato infestado com seis isolados de S. scabies separadamente e as plantas cultivadas em vasos e ambiente aberto. A severidade da doença foi estimada com auxílio de uma escala diagramática e avaliou-se o rendimento de tubérculos (g/planta). As cvs. Mondial e Jaete Bintje foram as mais resistentes à sarna comum com severidade média nos dois experimentos de 10,5% e 14,0%, respectivamente, seguidas por Asterix (17,4%), Ágata (21,8%), Monalisa (23,0%) e Cupido (23,3%). A agressividade dos isolados variou quantitativamente, com severidade maior para o isolado M4 (34,7%) e menor para o isolado M1 (6,2%). Redução da produção de tubérculos foi verificada na cv. Jaete Bintje, no primeiro experimento, e quando as cultivares foram infectadas pelos isolados A1, M2 e M3, no primeiro experimento, e pelo isolado M4, no segundo experimento.

Towards improvement of marker assisted selection of apple scab resistant cultivars: Venturia inaequalis virulence surveys and standardization of molecular marker alleles associated with resistance genes

Molecular breeding for pathogen resistance faces two major problems that delay its widespread adoption, resistance breakdown and difficulties in unambiguously identifying the alleles of the markers associated with specific resistance genes. Since the breakdown of the Rvi6 (Vf) gene in the Northern part of Europe breeders have intensified the search for new resistance sources to be introduced into their breeding programs. Alternative major genes to Rvi6 are available (e.g. Rvi2, Rvi4, Rvi5, Rvi10; Rvi11, Rvi12, Rvi13, and Rvi15, respectively Vh2, Vh4, Vm, Va, Vbj, Vb, Vd, Vr2 according to the old apple scab resistance gene nomenclature) but, with few exceptions (i.e., Rvi4, Rvi5 and, Rvi13), they have so far not been incorporated in commercial varieties. Pyramiding, i.e., combining several of these major resistance genes (R-genes) in individual plants, is one of the most promising strategies currently available to develop apple cultivars with durable apple scab resistance. But, which genes are the best suited to produce such new cultivars? Although the most interesting genes are surely those whose resistance so far has not been broken by the pathogen, genes with resistance that has been overcome coupled with only limited spread of the virulence may also be used in the pyramiding process. However, obtaining information on whether an R-gene is overcome and if so, the extent of the spread of the virulence is difficult and time consuming. Furthermore, often such reports are not up-to-date and the correctness of the data is difficult to verify. To solve these problems, the initiative “Monitoring of Venturia inaequalis virulences” has been proposed. The monitoring is based on a network of orchards of selected differential hosts. Incidence and severity of scab on these genotypes will be collected yearly; and after validation, the data will be published through the homepage of the project (www.vinquest.ch). Here, we present an outline of this initiative. A second major obstacle for broad adoption of marker assisted selection is the lack of tools to align marker analyzes performed in different laboratories to unambiguously identify the alleles linked to specific resistances. The identification of the alleles of the markers in coupling with the resistance genes is often very difficult, if the same genotype used to develop the markers is not simultaneously analyzed. In this paper we present an approach to standardize the size of the alleles in coupling with the resistance genes, using easily accessible cultivars. The proposed procedure has been applied to selected markers for the apple scab resistance genes Rvi2, Rvi4, Rvi5, Rvi6, Rvi11, Rvi12, Rvi13, Rvi14 and Rvi15 (respectively Vh2, Vh4, Vm, Vf, Vbj, Vb, Vd, Rvi14 and Vr2 according to the old nomenclature).

Evaluation of film forming polymers to control apple scab ( Venturia inaequalis (Cooke) G. Wint.) under laboratory and field conditions

A detached leaf bioassay was used to determine the influence of several film forming polymers and a conventional triazole fungicide on apple scab ( Venturia inaequalis (Cooke) G. Wint.) development under laboratory in vitro conditions, supported by two field trials using established apple cv. Golden Delicious to further assess the efficacy of foliar applied film forming polymers as scab protectant compounds. All film forming polymers used in this investigation (Bond, Designer, Nu-Film P, Spray Gard, Moisturin, Companion PCT12) inhibited germination of conidia, subsequent formation of appressoria and reduced leaf scab severity using a detached leaf bioassay. Regardless of treatment, there were no obvious trends in the percentage of conidia with one to four appressoria 5 days after inoculation. The synthetic fungicide penconazole resulted in the greatest levels of germination inhibition, appressorium development and least leaf scab severity. Under field conditions, scab severity on leaves and fruit of apple cv. Golden Delicious treated with a film forming polymer (Bond, Spray Gard, Moisturin) was less than on untreated controls. However, greatest protection in both field trials was provided by the synthetic fungicide penconazole. Higher chlorophyll fluorescence Fv/Fm emissions in polymer and penconazole treated trees indicated less damage to the leaf photosynthetic system as a result of fungal invasion. In addition, higher SPAD values as measures of leaf chlorophyll content were recorded in polymer and penconazole treated trees. Application of a film forming polymer or penconazole resulted in a higher apple yield per tree at harvest in both the 2005 and 2006 field trials compared to untreated controls. Results suggest application of an appropriate film forming polymer may provide a useful addition to existing methods of apple scab management.