Common Scab Disease Research Articles

Comparison between Pathogenic <i>Streptomyces scabies</i> Isolates of Common Scab Disease

Streptomyces scabies (Thaxter) causes destructive and serious damages to many vegetable field crops, including potato. Fourteen pure isolates were obtained from naturally diseased potato tubers showing symptoms of common scab disease, collected from different localities of Sohag governorate, Egypt. All tested isolates were identified as S. scabies (Stc) according to morphological and biochemical tests. Isolate Stc 10 exhibited the highest activity of polyphenoloxidase enzyme, followed by isolate Stc 11, while isolate Stc 2 produced the lowest activity of this enzyme. Concerning the peroxidase activity, the isolates varied in their production; Stc 11 exhibited the highest activity enzyme, followed by isolate Stc 2, whereas isolate Stc 10 produced the lowest activity of enzyme. In regard with Tyrosine Amonnia Lyase (TAL) activity, isolate Stc 2 exhibited the highest activity, followed by isolate Stc 10, whereas isolate Stc 11 exhibited the lowest activity. Agarose gel electrophoresis of the PCR amplification products revealed a band representing the expected 279 bp DNA fragment in each DNA extracted from the highly pathogenic isolates Stc 10 and 11. The results demonstrated that PCR amplification of the nec1 gene could be used as a reliable marker for detecting pathogenic Streptomyces isolates on potato tubers.

Role of Certain Potato Tubers Constituents in Their Susceptibility to Bacterial Common Scab Caused by Streptomyces scabies

Streptomyces common scab is one of the most bacterial diseases that costs economic loses to potato plants. Streptomyces scabiei (Thaxter 1891) Lambert and Loria (syn. Streptomyces scabies) is the main causal agent of common scab in potato in the field. Eight potato cultivars namely, Diamont, Lady Rossete, Burna, Lady Bellford, Skenta, Flora, Brofent and Everest were tested for their susceptibility to common scab disease under greenhouse conditions. They varied to some extent in the degree of their susceptibility. Lady Rosette appeared to be the highest susceptible one followed by Burna, Everest, and Skenta. Lady Bellford, Diamont respectively, and Brofent showed intermediate disease severity. Flora cultivar exhibited the least degree of scab symptoms. The role of certain potato tubers constituents in the physiology of disease susceptibility was investigated (sugar contents, pectin substances and calcium contents), Lady Bellford cultivar contained the highest concentration of total sugar followed by Flora, Burna, and Everest but Skenta, Lady Rosette respectively and Diamont were approximately had equal in total sugar contents, while, Brofent cultivar contained the lowest content of total sugar. Skenta cultivar contained the highest concentration of pectin substances and calcium contents followed by Flora and Lady Bellford finally Burna cultivars respectively.

Breeding of Pest and Disease Resistant Potato Cultivars in Japan by Using Classical and Molecular Approaches

Solanum tuberosum L.) is among the most important upland crops cultivated for many end uses throughout Japan. Potatoes are consumed in different ways, such as table use, food processing, starch production, and others. At the same time, various cropping systems are adopted according to environmental conditions. Although potato-breeding programs in Japan are conducted by consider- ing these demands, pest and disease resistance is one of the most important traits required for all modern cultivars. There are many pests and diseases affecting potato production in Japan, where conferring resistance against potato cyst nematodes, late blight, common scab, bacterial wilt, and viral diseases has become a main target of breeding. Artificial inoculation tests and cultivation in infested fields were traditionally conducted, in order to evaluate the levels of resistance among breed- ing materials. However, several DNA markers for pest and disease resistance genes have recently been developed for the selection of resistant genotypes. We have taken both approaches toward the selection of pest and disease resistant genotypes at each breeding step. This review introduces our approaches to develop new pest and disease resistant potato cultivars by using classical and molecular approaches.

Molecular and functional characteristics of streptomycete communities in relation to soil factors and potato common scab

Naturally-occurring disease-suppressive soils provide control of plant pathogens via the activities of indigenous microbes. While all soils contain antagonistic microbes, there is little systematic understanding of the correlates of variation in indigenous antagonist populations and on their relationships with plant diseases. We characterized the population densities, inhibitory capacities, and phylogenetic composition of soil streptomycete communities in a potato field over time in relation to edaphic factors, antagonist inoculation, and potato common scab severities. Antagonistic Streptomyces populations were highly variable in time and space. Similarly, metagenomic analyses of streptomycete communities showed extensive spatial and temporal variation in composition. Soil characteristics (pH, potassium, organic matter, nitrate, and phosphorous) sometimes explained up to 50% of the spatial variation in Streptomyces population densities, proportion of inhibitory isolates, or pathogen suppressive capacity among locations in the field. Soil pH was positively correlated with common scab severity, and negatively correlated with the proportion of pathogen-inhibitory Streptomyces among locations in the field. This suggests that high pH may have direct beneficial effects on pathogen populations and disease development, and/or indirect effects on pathogen populations via reductions in pathogen-inhibitory Streptomyces populations. Mean pathogen suppressive capacity of antagonistic Streptomyces was negatively correlated with common scab disease. Streptomyces inoculants had no discernible effect on streptomycete community composition, antagonistic capacities, or potato common scab severities, likely reflecting poor colonization of inoculants. Further understanding of the relationships between indigenous antagonist populations and plant diseases will be important to harnessing the potential of indigenous communities to contribute to sustainable disease management.

Characterization of the Coronatine-Like Phytotoxins Produced by the Common Scab Pathogen Streptomyces scabies.

Streptomyces scabies is an important causative agent of common scab disease of potato tubers and other root crops. The primary virulence factor produced by this pathogen is a phytotoxic secondary metabolite called thaxtomin A, which is essential for disease development. In addition, the genome of S. scabies harbors a virulence-associated biosynthetic gene cluster called the coronafacic acid (CFA)-like gene cluster, which was previously predicted to produce metabolites that resemble the Pseudomonas syringae coronatine (COR) phytotoxin. COR consists of CFA linked to an ethylcyclopropyl amino acid called coronamic acid, which is derived from L-allo-isoleucine. Using a combination of genetic and chemical analyses, we show that the S. scabies CFA-like gene cluster is responsible for producing CFA-L-isoleucine as the major product as well as other minor COR-like metabolites. Production of the metabolites was shown to require the cfl gene, which is located within the CFA-like gene cluster and encodes an enzyme involved in ligating CFA to its amino acid partner. CFA-L-isoleucine purified from S. scabies cultures was shown to exhibit bioactivity similar to that of COR, though it was found to be less toxic than COR. This is the first report demonstrating the production of coronafacoyl phytotoxins by S. scabies, which is the most prevalent scab-causing pathogen in North America.

Streptomyces – from Basic Microbiology to Role as a Plant Pathogen

Streptomycetes are spore-forming Gram-positive bacteria found in soil in large numbers world-wide. More than 600 Streptomyces species have been described. The complex developmental morphology and molecular genetics of Streptomyces is connected to production of a wealth of secondary products, including more than 60% of antibiotics in industrial and pharmaceutical use. Streptomycetes have important roles in soil ecology as decomposers, though specific roles in microbial community structure and plant health are poorly understood. Some species are used as biocontrol agents while others have specific associations with potatoes as endophytes, pathogens or as part of plant rhizosphere communities. Very few species are plant pathogenic (ca. 1%), causing common scab disease on underground tubers in potatoes and root diseases in a broad range of host plants. Several unique aspects of Streptomyces as a plant pathogen are that (a) there is a main dominant pathogenicity determinant (thaxtomin); (b) only the developing underground stems, stolons and tubers are susceptible to potato common scab (CS); (c) Streptomyces does not incite a plant defense response; and (d) CS is not easily managed. The best available control is the use of resistant potato cultivars and there is wide variation in resistance (tolerance) among potato cultivars, though none is completely resistant. New molecular genetic tools, including the complete genome sequences of a number of plant pathogenic Streptomyces species and association mapping using the potato genome sequence, promise greater understanding of the genetics of CS tolerance and of regulation of thaxtomin production and contributory pathogenicity factors for better management of potato CS.

Determination of factors associated with natural soil suppressivity to potato common scab.

Common scab of potatoes is a disease, which is difficult to manage due to complex interactions of the pathogenic bacteria (Streptomyces spp.) with soil, microbial community and potato plants. In Bohemian-Moravian Highlands in the Czech Republic two sites (Vyklantice and Zdirec) were selected for a study of common scab disease suppressivity. At both sites, a field with low disease severity occurs next to one with high severity and the situation was regularly observed over four decades although all four fields undergo a crop rotation. In the four fields, quantities of bacteria, actinobacteria and the gene txtB from the biosynthetic gene cluster of thaxtomin, the main pathogenicity factor of common scab, were analyzed by real-time PCR. Microbial community structure was compared by terminal fragment length polymorphism analysis. Soil and potato periderm were characterized by contents of carbon, nitrogen, phosporus, sulphur, calcium, magnesium, and iron. Quality of organic matter was assessed by high performance liquid chromatography of soil extracts. The study demonstrated that the suppressive character of the fields is locally specific. At Zdirec, the suppressivity was associated with low txtB gene copies in bulk soil, while at Vyklantice site it was associated with low txtB gene copies in the tuberosphere. The differences were discussed with respect to the effect of abiotic conditions at Zdirec and interaction between potato plant and soil microbial community at Vyklantice. Soil pH, Ca soil content or cation concentrations, although different were not in the range to predict the disease severity. Low severity of common scab was associated with low content of soil C, N, C/N, Ca and Fe suggesting that oligotrophic conditions may be favorable to common scab suppression.

Thaxtomin A Production and Virulence Are Controlled by Several bld Gene Global Regulators in Streptomyces scabies

Streptomyces scabies is the main causative agent of common scab disease, which leads to significant annual losses to potato growers worldwide. The main virulence factor produced by S. scabies is a phytotoxic secondary metabolite called thaxtomin A, which functions as a cellulose synthesis inhibitor. Thaxtomin A production is controlled by the cluster-situated regulator TxtR, which activates expression of the thaxtomin biosynthetic genes in response to cello-oligosaccharides. Here, we demonstrate that at least five additional regulatory genes are required for wild-type levels of thaxtomin A production and plant pathogenicity in S. scabies. These regulatory genes belong to the bld gene family of global regulators that control secondary metabolism or morphological differentiation in Streptomyces spp. Quantitative reverse-transcriptase polymerase chain reaction showed that expression of the thaxtomin biosynthetic genes was significantly downregulated in all five bld mutants and, in four of these mutants, this downregulation was attributed to the reduction in expression of txtR. Furthermore, all of the mutants displayed reduced expression of other known or predicted virulence genes, suggesting that the bld genes may function as global regulators of virulence gene expression in S. scabies.

Chemical control of potato common scab disease under field conditions

The aim of this study was to evaluate certain fungicides against Streptomyces scabies (Thaxter), the main causal agent of common scab disease in potato and other crops, in vitro and in vivo. Fourteen isolates of S. scabies were isolated from naturally infected potato tubers showing common scab symptoms. All isolates were pathogenic to potato tubers and produced typical symptoms of common scab. Isolate (No. 11) caused highest disease index (DSI) followed by isolates 10, 8 and 5 (61.13%). Seven fungicides (Rizolex, Capitan, Moncut, Ridomil, Maxim, Topsin, and Oxyplus) were screened in vitro for their toxicity against the pathogen isolate (No. 11). Results showed that four of them (Rizolex, Capitan, Moncut, and Ridomil) exhibited inhibition zone ranging from 5.33 to 26.33 mm. Capitan, Ridomil, and Rizolex were able to reduce DSI under field condition but they varied in their effects. Capitan was the best fungicide which aids in the reduction of disease (33.8%) followed by Ridomil (31.5%) while Rizolex (21.2%) was the lowest one.

Survey On The Incidence And Severity Of Common Scab Of Potato In Bangladesh

A survey on the incidence and severity of common scab (Streptomyces sp.) of potato was made in the major potato growing districts of Bangladesh during the cropping season of 2007-2008. A total of 150 fields was surveyed for collection of potato common scab infected samples. It was observed that there was a lot of variation in disease severity and its incidence in different districts of Bangladesh. Highest per cent scab incidence (71.41 %) was recorded in Kahalo Upazila (Bogra) and lowest (13.79 %) in Pirgacha Upazila (Rangpur). Disease incidence also varied among the tested varieties namely, Cardinal, Diamant, Granola, Binella and Raja. Highest incidence was observed in Cardinal (54.08 %) followed by Binella (50.71 %) and it was lowest in Raja (3.07 %). Cardinal and Binella were found to be highly susceptible and Raja was resistant to common scab disease. Diamant, the commercial variety showed medium susceptible reaction to the disease. J. Asiat. Soc. Bangladesh, Sci. 39(1): 35-41, June 2013 DOI: http://dx.doi.org/10.3329/jasbs.v39i1.16031

Thaxtomin A-deficient endophytic Streptomyces sp. enhances plant disease resistance to pathogenic Streptomyces scabies

Each plant species in nature harbors endophytes, a community of microbes living within host plants without causing any disease symptom. However, the exploitation of endophyte-based phytoprotectants is hampered by the paucity of mechanistic understandings of endophyte-plant interaction. We here reported two endophytic Streptomyces isolates IFB-A02 and IFB-A03 recovered from a stress-tolerant dicotyledonous plant Artemisia annua L. After the determination of their non-pathogenicity at the genomic level and from the toxin (thaxtomin A, TXT) level, the endophytism of both isolates was supported by their successful colonization in planta. Of the two endophytes, IFB-A03 was further studied for the mechanism of endophyte-conferred phytoprotection owing to its plant growth promotion in model eudicot Arabidopsis thaliana. Using the endophyte-Arabidopsis co-cultivation system into which pathogenic Streptomyces scabies was introduced, we demonstrated that IFB-A03 pre-inoculation could activate the salicylic acid (SA)-mediated plant defense responses upon pathogen challenge. Moreover, IFB-A03 was shown to partially rescue the defense deficiency in eds5 (enhanced disease susceptibility 5) Arabidopsis mutants, putatively acting at the upstream of SA accumulation in the defense signaling pathway associated with the systemic acquired resistance (SAR). These data suggest that endophytic Streptomyces sp. IFB-A03 could be a promising candidate for biocontrol agents against S. scabies--a causative pathogen of common scab diseases prevailing in agronomic systems.

Relationship Between the Application of Foliar Chemicals to Reduce Common Scab Disease of Potato and Correlation with Thaxtomin A Toxicity.

Production of the phytotoxin thaxtomin A by pathogenic Streptomyces spp. is essential for induction of common scab disease in potato. The disease can be significantly reduced by a range of chemicals applied as foliar sprays before tuber initiation. We tested a range of chemicals that had previously demonstrated varying capacities to reduce common scab for both disease suppression and their ability to inhibit thaxtomin A toxicity in both 'Desiree' and 'Russet Burbank' potato. Our results for disease suppression generally supported previous studies. Our tuber slice assays with thaxtomin A showed a strong correlation between the ability of the chemical to suppress common scab symptom development and the ability of the chemical to inhibit thaxtomin A toxicity. A Bayesian measurement error linear regression model was derived for each cultivar and trial and demonstrated a clear positive relationship between disease and thaxtomin-A-induced necrosis. The relationships obtained were much stronger than would have been obtained without adjustment for measurement error. This demonstrates that disease mitigation using chemical foliar sprays is strongly correlated with the ability of the chemical to inhibit thaxtomin A toxicity, suggesting this mechanism as a key mode of action for understanding this novel disease control strategy.

Multiplex real-time PCR (TaqMan) assay for the simultaneous detection and discrimination of potato powdery and common scab diseases and pathogens

To develop a multiplex real-time PCR assay using TaqMan probes for the simultaneous detection and discrimination of potato powdery scab and common scab, two potato tuber diseases with similar symptoms, and the causal pathogens Spongospora subterranea and plant pathogenic Streptomyces spp. Real-time PCR primers and a probe for S. subterranea were designed based on the DNA sequence of the ribosomal RNA ITS2 region. Primers and a probe for pathogenic Streptomyces were designed based on the DNA sequence of the txtAB genes. The two sets of primer pairs and probes were used in a single real-time PCR assay. The multiplex real-time PCR assay was confirmed to be specific for S. subterranea and pathogenic Streptomyces. The assay detected DNA quantities of 100 fg for each of the two pathogens and linear responses and high correlation coefficients between the amount of DNA and C(t) values for each pathogen were achieved. The presence of two sets of primer pairs and probes and of plant extracts did not alter the sensitivity and efficiency of multiplex PCR amplification. Using the PCR assay, we could discriminate between powdery scab and common scab tubers with similar symptoms. Common scab and powdery scab were detected in some tubers with no visible symptoms. Mixed infections of common scab and powdery scab on single tubers were also revealed. This multiplex real-time PCR assay is a rapid, cost efficient, specific and sensitive tool for the simultaneous detection and discrimination of the two pathogens on infected potato tubers when visual symptoms are inconclusive or not present. Accurate and quick identification and discrimination of the cause of scab diseases on potatoes will provide critical information to potato growers and researchers for disease management. This is important because management strategies for common and powdery scab diseases are very different.

Comparative assessment of genetic and epigenetic variation among regenerants of potato (Solanum tuberosum) derived from long-term nodal tissue-culture and cell selection

Three long-term nodal tissued cultured Russet Burbank potato clones and nine thaxtomin A-treated regenerant lines, derived from the nodal lines, were assessed for genetic and epigenetic (in the form of DNA methylation) differences by AFLP and MSAP. The treated regenerant lines were originally selected for superior resistance to common scab disease and acceptable tuber yield in pot and field trials. The long-term, tissue culture clone lines exhibited genetic (8.75-15.63% polymorphisms) and epigenetic (12.56-26.13% polymorphisms) differences between them and may represent a stress response induced by normal plant growth disruption. The thaxtomin A-treated regenerant lines exhibited much higher significant (p < 0.05) genetic (2-29.38%) and epigenetic (45.22-51.76%) polymorphisms than the nodal cultured parent clones. Methylation-sensitive mutations accumulated within the regenerant lines are significantly correlated (p < 0.05) to disease resistance. However, linking phenotypic differences that could be of benefit to potato growers, to single gene sequence polymorphisms in a tetraploid plant such as the potato would be extremely difficult since it is assumed many desirable traits are under polygenic control.

INNOVATIVE TECHNOLOGIES: Tobacco Bio-oil Kills Agricultural Pests

Cigarette smoking continues to be the leading cause of preventable death and disease in the United States,1 but tobacco has potentially beneficial uses as well as deadly ones. Gardeners have long known that homemade mixtures of tobacco and water can kill insect pests. But these homemade brews kill desirable insects, too, and could poison animals that ingest them. Now researchers at the University of Western Ontario are finding new ways to turn tobacco into a more selective eco-friendly pest control agent.2 A team led by chemical engineer Cedric Briens heated finely ground tobacco leaves to 500°C in a vacuum, a process called pyrolysis, then collected the condensate. (Since publishing the paper, the team has found they can use the entire plant—leaves and stalks—which makes it easier and cheaper to harvest the tobacco.) The bio-oil was tested against the Colorado potato beetle (Leptinotarsa decemlineata), 11 fungi, and 4 bacteria, all of which are agricultural pests. The bio-oil blocked the growth of the bacteria Streptomyces scabies and Clavibacter michiganensis and the fungus Pythium ultimum. S. scabies causes a common potato scab disease that makes potatoes unmarketable, C. michiganensis kills young plants and deforms fruits, especially tomatoes, and P. ultimum kills seedlings of eggplant, peppers, lettuce, tomatoes, and cucumbers. The bio-oil also killed 100% of Colorado potato beetles, a resistant pest that can destroy potato crops. The other organisms were unaffected. Nicotine, a key toxin in tobacco, has known insecticidal properties on its own. But even after removing nicotine from the bio-oil, it still potently killed these few pests.2 The authors say the active components probably include a mixture of phenols with known pesticidal properties working synergistically. They analyzed the bio-oil using gas chromatography–mass spectrometry and note that some of the constituents defy detection. It’s possible new pesticidal molecules are being formed in the high heat conditions of pyrolysis. “We do know that no single molecule is effective, and we seem to have discovered a natural cocktail,” Briens says. The probable mixture of active chemicals suggests agricultural pests may not readily develop resistance to the bio-oil. Control of the Colorado potato beetle is especially challenging because the beetle is notorious for its ability to adapt rapidly to new pesticides that are applied.3 “Insecticides that work now will be obsolete in a few years, and we’ll need new insecticides,” Briens says. The ability of the bio-oil to target certain agricultural pests could be an asset for future commercialization, because it could spare desirable insects such as honeybees. Some pesticide manufacturers are watching the bio-oil work, but they want to know the active molecules before becoming involved. Then the active components of the bio-oil will require toxicity testing to assess their impact on the environment. Briens’ study “is a logical and efficient approach to identify a useful by-product of tobacco plants, creating a value-added pesticidal fraction,” says Joel Coats, a professor of entomology and toxicology at Iowa State University in Ames. “The possibility of discovering a novel pesticidal molecule makes the project very worthwhile.”

Habituation to thaxtomin A in hybrid poplar cell suspensions provides enhanced and durable resistance to inhibitors of cellulose synthesis.

BackgroundThaxtomin A (TA), a phytotoxin produced by the phytopathogen Streptomyces scabies, is essential for the development of potato common scab disease. TA inhibits cellulose synthesis but its actual mode of action is unknown. Addition of TA to hybrid poplar (Populus trichocarpa x Populus deltoides) cell suspensions can activate a cellular program leading to cell death. In contrast, it is possible to habituate hybrid poplar cell cultures to grow in the presence of TA levels that would normally induce cell death. The purpose of this study is to characterize TA-habituated cells and the mechanisms that may be involved in enhancing resistance to TA.ResultsHabituation to TA was performed by adding increasing levels of TA to cell cultures at the time of subculture over a period of 12 months. TA-habituated cells were then cultured in the absence of TA for more than three years. These cells displayed a reduced size and growth compared to control cells and had fragmented vacuoles filled with electron-dense material. Habituation to TA was associated with changes in the cell wall composition, with a reduction in cellulose and an increase in pectin levels. Remarkably, high level of resistance to TA was maintained in TA-habituated cells even after being cultured in the absence of TA. Moreover, these cells exhibited enhanced resistance to two other inhibitors of cellulose biosynthesis, dichlobenil and isoxaben. Analysis of gene expression in TA-habituated cells using an Affymetrix GeneChip Poplar Genome Array revealed that durable resistance to TA is associated with a major and complex reprogramming of gene expression implicating processes such as cell wall synthesis and modification, lignin and flavonoid synthesis, as well as DNA and chromatin modifications.ConclusionsWe have shown that habituation to TA induced durable resistance to the bacterial toxin in poplar cells. TA-habituation also enhanced resistance to two other structurally different inhibitors of cellulose synthesis that were found to target different proteins. Enhanced resistance was associated with major changes in the expression of numerous genes, including some genes that are involved in DNA and chromatin modifications, suggesting that epigenetic changes might be involved in this process.

The possible role of factor C in common scab disease development

The Gram-positive soil-borne streptomycetes exhibit a complex life cycle that is controlled by extracellular regulatory molecules. One interesting autoregulator is the protein factor C, originally isolated from the culture fluid of S. albidoflavus 45H. Southern hybridizations and database searches revealed that although homologues of factor C are not present in most Streptomyces strains, an exception is the plant pathogenic S. scabies , which causes common scab disease on potato. In S. scabies and related strains pathogenicity involves a large pathogenicity island that carries thaxtomin biosynthetic genes, the nec1 necrogenic factor and other putative virulence genes. Extracellular enzymes, including extracellular esterases, that attack the surface of the tubers and disintegrate the external protective layer are also known to be involved in pathogenicity. In S. albidoflavus 45H, factor C coordinates the expression of many secreted hydrolases. To find out whether esterase is also regulated by factor C, we made a factor C null mutant of strain 45H. The mutant showed a bald phenotype and was impaired in pathogenicity and esterase activity. This is a first indication that extracellular regulatory factors may play a role in the development of potato scab.

Yield and cooking qualities of somaclonal variants of cv. Russet Burbank selected for resistance to common scab disease of potato

We previously obtained somaclonal variants of the important French fry processing cultivar Russet Burbank with significantly enhanced resistance to common scab disease. In this study we have shown the commercial merit of a proportion of these variants through comparison of relative yield and tuber quality with the parent cultivar Russet Burbank. Whilst we showed a weak negative correlation between tuber yield (as assessed by weight of tubers per plant) and relative disease resistance within selected variants, we identified several with equivalent yields to the parent cultivar. Furthermore, two disease-resistant variants (TC-RB8 and NZ-24B) consistently yielded more tuber mass than the parent. The majority of our Russet Burbank variants showed equivalent tuber quality characteristics (occurrence of defects, tuber specific gravity and dry matter content, and flesh colour) and cooking qualities (fry colour and presence of dark end defects) to the parent cultivar. Independent testing by a commercial French fry processor confirmed these quality characteristics. We present data demonstrating that highly common scab disease-resistant somaclonal variants of Russet Burbank have commercially acceptable tuber yield and quality characteristics, comparable to the industry standard and parent Russet Burbank cultivar. We also demonstrate the value of in vitro cell selection techniques for potato cultivar improvement.

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.

Greenhouse and Field Nursery Evaluation for Potato Common Scab Tolerance in a Tetraploid Population

Potato common scab (Streptomyces scabies (Thaxt.) Waksman & Henrici) is a major disease of potato (Solanum tuberosum L.), due to the unmarketability of affected tubers. For identification of the most common scab-tolerant material, and for developing molecular markers for common scab tolerance, more information is needed on the genetic basis of common scab tolerance. Phenotyping common scab susceptibility is difficult because of the large variability in disease symptoms among tubers from a single plant, ranging from no common scab to severe pits. Two years of field data were collected for scab reaction on a segregating tetraploid population (MSL603, 160 individuals). Continuous variation in common scab susceptibility phenotype was observed among the progeny, with a normal distribution suggesting common scab disease phenotype is a genetically complex trait. Transgressive segregants were also observed, but they are skewed toward susceptibility. A greenhouse-based screening procedure was evaluated to discern tolerant from susceptible potato lines. A subset of ten individuals from this population were selected (five resistant, five susceptible). For the greenhouse study, soil was inoculated with a pathogenic S. scabies strain MSDPZ at a concentration of 3 X 108 CFU/ml. This greenhouse assay effectively discerned tolerant and susceptible individuals. There was a moderate correlation between the greenhouse study and the field trial. The greenhouse assay may provide information that would complement field data in identifying resistant clones.