Somaclonal selection in potato for resistance to common scab provides concurrent resistance to powdery scab

Variant somaclones of potato cultivar Russet Burbank, selected for resistance to common scab using in vitro cell selection techniques, were tested for resistance to powdery scab, another important disease of potato caused by Spongospora subterranea. This pathogen also invades roots, producing root galls. Most variants consistently showed increased resistance to powdery scab, both in field and glasshouse challenge, when compared to the parental cultivar, several significantly so. On average, the best variant reduced powdery scab incidence by 51% and severity (tuber surface coverage) by 64%. In contrast, no improvement in the extent of root infection and root galling was seen. These results suggest host interactions during root and tuber infection are distinct. Correlation analyses of disease indices amongst the selected variants showed no association between Sp. subterranea root infection and gall scores, nor between root infection and tuber disease severity. However, a weak positive association was found between root gall score and tuber disease, and a strong correlation between tuber disease incidence and severity scores. Interestingly, positive correlations were also found between the extent of powdery and common scab resistance expressed and both incidence and severity of these diseases within the variants, suggesting a common defence mechanism. The role of thaxtomin A in selecting for concurrent resistance to both diseases is discussed.     

Relationship of resistance to common scab disease and tolerance to thaxtomin A toxicity within potato cultivars

Thaxtomin A has a central and implicit role in common scab disease expression in potato. Thaxtomin A tolerance has been suggested as a possible rapid means for screening potato germplasm for disease resistance, during breeding selections. We have tested a range of genetically diverse cultivars with varying resistances to common scab disease in both pot and field based studies and measured their mean necrosis response to thaxtomin A. We found no association between resistance to common scab disease and tolerance to thaxtomin A toxicity. For example, disease resistant cultivars ‘Russet Burbank’ and ‘Atlantic’ were sensitive and tolerant to thaxtomin A toxicity respectively. Similarly; disease susceptible cultivars ‘Bismark’ and ‘Tasman’ showed susceptibility and tolerance to thaxtomin A. This demonstrates that whilst thaxtomin A is critical to disease expression, reaction to this toxin is only one component influencing resistance to common scab disease and many other anatomical, physiological or biochemical factors are critical to defence against this disease.     

Enhanced resistance to common scab of potato through somatic cell selection in cv. Iwa with the phytotoxin thaxtomin A

A somatic cell selection approach, developed using thaxtomin A as a positive selection agent, recovered 113 potato cell colonies of cv. Iwa from which 39 regenerated plants within a single subculture period. In glasshouse pathogenicity trials, 13 of these regenerated variants exhibited significantly greater resistance to common scab disease than the unselected parent cultivar, both in mean tuber surface coverage with lesions and the proportion of disease free tubers. The best variant had on average an 85–86% lower disease score (or 91–92% less estimated tuber surface cover) than the unselected parent cultivar. The disease-resistant variants varied in their response to thaxtomin A in detached leaflet and tuber slice bioassays. This suggested that the cell selection procedure and thaxtomin A screening had limited success in selecting for toxin tolerance, and that thaxtomin A tolerance was not necessarily the driving factor for induction of disease resistance observed. Variation in relative necrosis scores between leaflet and tuber assays for individual variants possibly suggests differential expression of thaxtomin A tolerance in the different tissues. Yield estimates from the glasshouse grown potatoes showed the majority of disease-resistant variants had tuber yields equivalent to the unselected parent cultivar. This suggests that selection for disease resistance was not associated with negative tuber yield attributes and that these variants may have commercial merit and are worthy of further agronomic testing.     

Resistance of potato genotypes to common and netted scab-causing species of Streptomyces

Common and netted scabs are two disfiguring bacterial diseases of potato tubers, caused by various groups of Streptomyces species. Common scab, caused primarily by Streptomyces scabies and Streptomyces europaeiscabiei , is characterized by more or less deep pustules on the tuber surface, while symptoms of netted scab, caused mainly by Streptomyces reticuliscabiei , are superficial, corky alterations of the tuber periderm. Some isolates of S. europaeiscabiei are able to induce both common and netted scab symptoms, and therefore constitute a third pathogenicity group. Like most bacterial diseases, potato scabs would be best controlled by using resistant cultivars. Repeated experiments with soil artificially infested with isolates of three species representative of the three pathogenicity groups showed the level and stability of cultivar resistance, as well as the existence of a range of aggressiveness among different isolates. The distribution of scab severity indexes recorded on a collection of 16 potato cultivars and 27 breeding clones grown in soil infested with common scab-inducing isolates was continuous, suggesting isolate nonspecific quantitative resistance. Least susceptible cultivars were Nicola, BF15, Sirtéma, and Charlotte, while Urgenta, Désirée, Ondine and Bintje were very susceptible. The same genotypes proved either highly susceptible (e.g. cvs Bintje, Désirée or Carmine) or highly resistant (e.g. cvs Charlotte, Sirtéma, Monalisa, BF15 or Belle de Fontenay) to isolates forming netted scab symptoms, suggesting isolate-specific qualitative resistance. The ability was confirmed of some isolates of S. europaeiscabiei to induce one or the other type of symptoms depending on cultivar and soil temperature.     

Increased abundance of patatins,lipoxygenase and miraculins in a thaxtomin A-habituated potato Russet Burbank somaclone with enhanced resistance to common scab

Potato common scab caused by the actinobacterium Streptomyces scabiei is characterized by the formation of corky lesions on tubers that reduce their marketability. Management of common scab is very complex and often ineffective under various environmental conditions. Using potato varieties that are more resistant to common scab remains one of the most efficient strategies to control this disease. However, very little is known about the factors associated with resistance to common scab. Somaclone RB9 regenerated from thaxtomin A-habituated potato Russet Burbank calli produced tubers more resistant to common scab than the original variety. Comparison of the RB9 tuber proteome with that of Russet Burbank using label-free quantitative proteomic analysis revealed changes in the accumulation of defence-related proteins from the patatin and lipoxygenase (LOX) families, which are involved in the metabolism of lipids, and of two miraculins of the Kunitz-type protease inhibitors family. The implication of LOX during common scab infection was studied using synchronized minitubers developed from leaf-bud cuttings. S. scabiei infection stimulated the accumulation of LOX in both Russet Burbank and RB9 minitubers, but this accumulation was intensified in RB9 minitubers. Infection also increased LOX activity in Russet Burbank and RB9 minitubers. However, LOX activity measured in noninfected RB9 minitubers was similar to that of infected Russet Burbank minitubers, indicating endogenous activation of LOX activity in RB9 minitubers. We discuss how increased LOX abundance and activity in the somaclone RB9 may contribute to improving tuber defence against common scab.     

黑龙江省部分地区马铃薯疮痂病菌种类及致病性鉴定

为明确黑龙江省马铃薯疮痂病病原菌的种类及其特征,2012-2013年从黑龙江省克山县、绥化市、哈尔滨市、杜尔伯特蒙古族自治县采集具有疮痂病病斑的马铃薯块茎,从中分离纯化病原菌,根据16SrRNA基因的差异采用分子手段对所分离的菌株进行种类和致病性鉴定,并对txtAB阳性菌株采用萝卜幼苗法或马铃薯致病性试验测定其致病性。共分离出74株菌株,鉴定出致病性菌株26株,其中Streptomyces scabies或S.europaeiscabiei 21株,S.turgidiscabies 3株和S.acidiscabies 2株。所有的致病性菌株中共有4种致病岛基因型,即nec1+/tomA+、nec1-/tomA+、nec1+/tomA-和nec1-/tomA。     

Isolation and characterization of Streptomyces species from potato common scab lesions in Norway

The present survey was conducted to isolate and characterize Streptomyces species from common scab lesions of potato in Norway. Bacteria were isolated from scab lesions on tubers sampled in two consecutive years at different locations in Norway spanning ～1400 km from south to north. In total, 957 independent isolations from individual tubers were performed, with 223 putative pathogenic isolates obtained from 29 different potato cultivars and 130 different fields. Streptomyces europaeiscabiei was the most abundant species isolated from common scab lesions (69%), while 31% of the isolates obtained were S. turgidiscabies. Streptomyces scabies was not found. Pathogenicity of selected Streptomyces isolates was tested on potato. The ability of the bacterial isolates to infect potato was consistent with the presence of the txtAB operon. The results revealed no pattern in geographical distribution of S. europaeiscabiei and S. turgidiscabies; both could be found in the same field and even the same lesion. Four different pathogenicity island (PAI) genotypes were detected amongst the txtAB positive isolates: nec1+/tomA+, nec1–/tomA+, nec1+/tomA? and nec1?/tomA?. The current findings demonstrate that there is genetic variability within species and that the species are not spread solely by clonal expansion. This is thought to be the most comprehensive survey of Streptomyces species that cause common scab of potato in a European country.     

Effects of concanamycins produced by <Emphasis Type="Italic">Streptomyces scabies</Emphasis> on lesion type of common scab of potato

Streptomyces scabies, causative agent of common scab of potato, produces the phytotoxins concanamycin and thaxtomin. In a potato tuber slice assay to study the contribution of concanamycins to lesion development, concanamycin A had weak necrosis-inducing activities; >10× the amount of thaxtomin A was needed to produce equivalent lesion severity. Concanamycins were detected in tubers inoculated with S. scabies, which caused deep-pitted lesions but not in those inoculated with Streptomyces acidiscabies, which caused corky, raised lesions. In field-grown, diseased potatoes, concanamycin content tended to be higher in tubers with deep-pitted lesions than in those with corky, raised lesions.     

Coincidence of maximum severity of powdery mildew on grape leaves and the carbohydrate sink‐to‐source transition

Grapevine leaves infected with powdery mildew are a source of inoculum for fruit infection. Leaves emerging on a single primary shoot of Vitis vinifera cv. Cabernet Sauvignon were exposed to average glasshouse temperatures of 18°C (0·23 leaves emerging/day) or 25°C (0·54 leaves emerging/day). All leaves on 8–10 shoots with approximately 20 leaves each were inoculated with Erysiphe necator conidia to assess disease severity after 14 days in the 25°C glasshouse. Two photosynthetic ‘source’ leaves per shoot on the remaining 8–10 shoots were treated with ¹⁴CO₂ to identify, by autoradiography, the leaf position completing the carbohydrate sink‐to‐source transition. There was a clear association between the mean modal leaf position for maximum severity of powdery mildew (position 3·7 for 18°C; position 4·4 for 25°C) and the mean position of the leaf completing the sink‐to‐source transition (position 3·8 for 18°C; position 4·7 for 25°C). The mean modal leaf position for the maximum percentage of conidia germinating to form secondary hyphae was 4·2 for additional plants grown in the 25°C glasshouse. A higher rate of leaf emergence resulted in a greater proportion of diseased leaves per shoot. A Bayesian model, consisting of component models for disease severity and leaf ontogenic resistance, had parameters representing the rate and magnitude of pathogen colonization that differed for shoots developing in different preinoculation environments. The results support the hypothesis that the population of leaves in a vineyard capable of supporting substantial pathogen colonization will vary according to conditions for shoot development.     

Comparison of screening methods to evaluate the response of St. Augustine grass to Magnaporthe oryzae

The objective of this study was to develop a reliable and high throughput screening method to evaluate the response of St. Augustine grass (Stenotaphrum secundatum) genotypes to the grey leaf spot (GLS) caused by Magnaporthe oryzae infection. Whole plant, detached stolon and detached leaf assays under growth chamber conditions were compared to field conditions on eight commercial and nine advanced breeding lines of St. Augustine grass. Disease was assessed using two variables, lesion size (LS) and overall plant disease severity (SEV). LS and SEV were highly correlated for field and growth chamber screening methods using the whole plant assay (LS r² = 0·79; SEV r² = 0·83; P ≤ 0·001), the detached stolon assay (LS r² = 0·75; SEV r² = 0·72; P ≤ 0·001), and the detached leaf assay (LS r² = 0·46; SEV r² = 0·60; P ≤ 0·001). Genotypic variation for resistance in 17 St. Augustine grass genotypes was identified using all screening methods for LS (P < 0·05) and SEV (P < 0·05). The rank‐sum method was used to classify St. Augustine grass genotypes into highly resistant (HR), resistant (R), moderately resistant (MR), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) classes based on the rank‐sum values of LS and SEV. Two introduced African polyploids used as parents, and two F₁ interploid progeny obtained using an in vitro embryo rescue technique, were classified as highly resistant (HR), or resistant (R), across all screening methods.     

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, <10 and >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 < 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 < 0·01).     

Recent Progress in Breeding for Fireblight Resistance in Apples and Pears in North America1

Resistance in apple is evaluated by needle–inoculation of succulent shoot tips with 10⁶–10⁷ cells of a virulent isolate of Erwinia amylovora (Burr.) Winslow et al. (the incitant of fireblight) and measurement of the resulting cortical lesion when extension is complete. Data are now available on practically all commercial cultivars, some of which have a useful level of resistance. Some newer cultivars, particularly those with resistance to scab (Venluria inaequalis [Cooke] Wint.) derived from Malus floribunda, have good resistance to E. amylovora. A very high level of resistance is present in Asiatic Malus species, including M. x robusta, M. x sublo–bata, M. x atrosanguinea, and M. prunifolia, and in the North American species M. fusca. This type of resistance appears to be inherited polygenically, and because of its detectability in young seedlings can be used conveniently in breeding. Objectives of pear breeding programs are aimed at developing superior fruit quality combined with resistance to fireblight, psylla, and Fabraea leaf spot. Many high quality cultivars of Pyrus communis are extremely susceptible to E. amylovora and sensitivity appears to be controlled by a dominant gene Se. A high level of resistance is present in P. ussuriensis but varies considerably between clonal selections of other Pyrus species. Pear seedlings from controlled pollinations are artificially inoculated in the glasshouse with a similar bacterial suspension as used for apples, and only the most resistant ones are selected and planted in the field for future evaluation. In Beltsville, heritability studies of crosses between non–sensitive parents have indicated that selection for resistance within progenies results in a high degree of genetic gain. Interspecific hybridization has an advantage over P. communis crosses mainly when insect or fungus resistance is required.     

Phytotoxin produced by Streptomyces sp. causing potato russet scab in Japan

A phytotoxin isolated from the Stretomyces sp. causing potato russet scab in Japan (Kamenoko-byo in Japanese) was identified as a 16-membered macrolide, FD-891. It induced necrosis in potato tuber slices above 50 µg/disk, which is about 1/100 the activity of thaxtomin A. The phytotoxin was detected in potatoes infected by the pathogen and was produced by other pathogenic strains with different geographic origins.     

Isolation of antagonistic <Emphasis Type="Italic">Streptomyces</Emphasis> 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.     

Relationship between the extent of colonization by Verticillium dahliae and symptom expression in strawberry (Fragaria × ananassa) genotypes resistant to verticillium wilt

Eleven strawberry (Fragaria × ananassa) genotypes from the University of California breeding programme known to be resistant to verticillium wilt were inoculated with Verticillium dahliae. Individual plants were given a resistance score based on the severity of visual symptoms, and the extent of colonization was quantified as the percentage of petioles not colonized by the pathogen. Both resistance scores and the percentage of pathogen‐free petioles decreased significantly from May to June (P < 0·05) during each of two growing seasons, indicating a progression of both colonization and symptom expression. Even the most resistant genotypes had plants with some infected petioles, and manifested some symptoms of verticillium wilt. Significant (P < 0·05) genotypic variance was detected for the percentage of pathogen‐free petioles, but not for resistance score. The percentage of pathogen‐free petioles had a strongly positive genotypic correlation (r_g = 0·77, P < 0·01) with resistance score, indicating that about 60% of the genotypic variation for visual symptoms in this set of resistant genotypes was explained by the extent of colonization of individual plants by V. dahliae. Conversely, the genotypic correlation between the percentage of pathogen‐free petioles and the resistance score for plants sampled in May (r_g = 0·74, P < 0·01) was smaller than that for plants harvested in July (r_g = 0·93, P < 0·01). Together, these results suggest that the overall performance of strawberry genotypes in the presence of V. dahliae can be enhanced by both resistance and tolerance, but that tolerance may be less stable over the course of a season. Distinguishing between these two mechanisms may require evaluations that supplement visual assessments of resistance.     

Development of a rapid,accurate glasshouse bioassay for assessing fusarium wilt disease responses in cultivated Gossypium species

A rapid glasshouse‐based bioassay method to screen large numbers of cotton plants for responses to Fusarium oxysporum f. sp. vasinfectum (Fov) was developed. Different Fov inoculum concentrations and methods of inoculation were assessed using resistant and susceptible cotton cultivars. Cotton seeds were planted directly into Fov‐inoculated soil. Studies of seed germination, seedling establishment, seedling mortality and fusarium wilt symptoms (i.e. stunting, foliar symptoms and vascular browning) were performed to optimize the bioassay parameters. Growing seedlings in Fov‐inoculated soils at 5 × 10⁴ or 1 × 10⁵ CFU g^?1 soil, in individual seedling tubes with 12 h at 28–30°C and 12 h at 15–18°C, gave consistent results when assessing Fov disease responses 6 weeks after inoculation. When fusarium wilt resistance ranks (FWRRs) and vascular browning index (VBI) means of 18 Australian and other cotton cultivars from the Fov glasshouse bioassay were compared against their fusarium field performance ranks (F‐ranks), assessed on adult plants for cotton cultivar release, Pearson’s correlation was highly significant for both comparisons. The level of congruence between field and glasshouse data indicated that this protocol should be an effective tool for large‐scale screening for Fov‐resistance responses in diverse germplasm and breeding populations and for advancing genetic research to develop molecular markers for Fov resistance in cotton.     

Precise, simple screening for resistance in potato varieties to common scab using paper pots

Resistance to common scab pathogen Streptomyces turgidiscabies of seven potato varieties was compared in the field with a newly developed paper pot method. Seedlings raised in soil in paper pots containing inocula at 1 × 10³ to 10⁷cfu/g soil were transplanted into a scab-free field and grown for 3 months. The disease severity of the seven varieties in the field trials differed in iteration and from year to year, even though their resistance levels were approximately similar at the expected levels. With the paper pot method, the seven varieties had different resistance levels, which were almost completely consistent with the results of the field trials, at more than 1 × 10⁵cfu/g soil. Significant differences in disease severity between resistant and susceptible varieties were observed (P = 0.05) for 2 years, and the resistance level of the varieties was elucidated.     

Control of Avena fatua L. and Cirsium arvense (L.) Scop. with mixtures of 3,6-dichloropicolinic acid and four herbicides for control of A. fatua

Control of Avena fatua (L.) (wild oat) with diclofop methyl applied at 0·7 kg ha^?1 at the two-leaf stage and difenzoquat at 0·84 kg ha^?1 at the four-leaf stage in wheat (Triticum aestivum L.) under field conditions was good and not affected when either of these herbicides was mixed with 3,6-dichloropicolinic acid as the monoethanolamine salt at 0·14, 0·20 or 0·30 kg ha^?1. In the glasshouse, mixtures containing 3,6-dichloropicolinic acid at rates as high as 0·6 kg ha^?1 also did not affect control of A. fatua. When barban at 0·35 kg ha^?1, or flamprop methyl at 0·56 kg ha^?1 was mixed with similar rates of 3,6-dichloropicolinic acid and applied at the two-leaf and four-leaf stage of A. fatua respectively, a reduction in control of A. fatua (antagonism) occurred under both field and glasshouse conditions. The herbicides for control of A. fatua did not influence the fresh weight suppression of C. arvense shoots obtained in the glasshouse with 3,6-dichloropico-colinic acid at 0·3 kg ha^?1. Early tolerance of wheat (cv. Neepawa) was acceptable with all mixtures. Wheat yields with diclofop methyl or difenzoquat alone or in mixture with 3,6-dichloropicolinic acid were increased over the yields from the A. fatua-infested control.     

Allelopathic effect of Salvia syriaca L. (Syrian sage) in wheat

The allelopathic effect of Salvia syriaca L. (Syrian sage) was examined against wheat in glasshouse and laboratory experiments. The germination of wheat grains was delayed, and the development of wheat seedlings was decreased in laboratory experiments by both shoot and rhizome extract. The inhibitory effect of both extracts was most pronounced at 20°C compared with 10 or 15°C. Shoot extracts had more drastic effects than the rhizome extract on germination percentage, shoot and root lengths. In glasshouse experiments fresh and dried shoot of S. syriaca added to soil drastically decreased germination and development of wheat.