Potato Scab Research Articles

Potato Root Exudation and Release of Spongospora subterranea Resting Spore Germination Stimulants are Affected by Plant and Environmental Conditions

AbstractVariation in plant and environmental conditions were studied to determine the effect thereof on the exudation of low‐molecular‐weight organic compounds by potato roots. The results of the phytochemical analyses showed that among the conditions investigated, root vigour, potato cultivar, nutrients in incubation solution and temperature influenced the number and the type of primary metabolites released. Moreover, these conditions influenced our detection of compounds known to stimulate germination of resting spores of the pathogen Spongospora subterranea, causal agent of powdery scab and root diseases of potato. We conclude that changes in plant and environmental conditions can affect the release of specific compounds that stimulate germination of S. subterranea resting spores. The impact of the factors affecting potato root exudation on subsequent disease development is discussed.

The effect of peat and iron supplements on the severity of potato common scab and bacterial community in tuberosphere soil.

The control of common scab (CS) of potatoes includes resistant cultivars, specific fertilization, increase of soil moisture and chemical treatments. Yet, these management practices do not have common or reproducible results at differing sites. In order to determine the effects of soil organic matter, iron and pH on CS development, peat and DTPA-chelated iron were supplemented to pots filled with soil conducive for CS. All results were compared with the same data obtained for a suppressive soil, which has naturally low severity of CS and occurs nearby. Bacteria, Actinobacteria and the txtB genes from the biosynthetic cluster of thaxtomin, which is responsible for the disease development, were quantified by qPCR in tuberosphere soil and potato periderm. Illumina amplicon sequencing of bacterial 16S rRNA genes was performed for tuberosphere soils. Both peat and iron supplements controlled potato scab, and the combination of the two supplements reduced CS most effectively. The bacterial community was modified by all treatments but the highest number of operational taxonomic units (OTUs) changed towards the suppressive soil after the combined peat and iron treatment. It seemed that iron supplement supported plant defense while both iron and peat additions changed the bacterial community in favor of CS suppression.

Enzymatic hydrolysis by transition-metal-dependent nucleophilic aromatic substitution.

Nitroaromatic compounds are typically toxic and resistant to degradation. Bradyrhizobium species strain JS329 metabolizes 5-nitroanthranilic acid (5NAA), which is a molecule secreted by Streptomyces scabies, the plant pathogen responsible for potato scab. The first biodegradation enzyme is 5NAA-aminohydrolase (5NAA-A), a metalloprotease family member that converts 5NAA to 5-nitrosalicylic acid. We characterized 5NAA-A biochemically and obtained snapshots of its mechanism. 5NAA-A, an octamer that can use several divalent transition metals for catalysis in vitro, employs a nucleophilic aromatic substitution mechanism. Unexpectedly, the metal in 5NAA-A is labile but is readily loaded in the presence of substrate. 5NAA-A is specific for 5NAA and cannot hydrolyze other tested derivatives, which are likewise poor inhibitors. The 5NAA-A structure and mechanism expand our understanding of the chemical ecology of an agriculturally important plant and pathogen, and will inform bioremediation and biocatalytic approaches to mitigate the environmental and ecological impact of nitroanilines and other challenging substrates.

Sequential analysis of soil factors related to common scab of potatoes.

The severity of common scab (CS) of potatoes has been correlated with multiple environmental factors. This study aimed at separating the effect of factors related to local conditions from those correlated to the disease development at all studied sites using a mathematical adjustment of the variables' means for site and field. The experiment was conducted at two sites differing in soil conditions, where a field with low disease severity occurs next to one with high severity. Three cultivars susceptible to CS were grown in four replicates on each field. Bacteria, actinobacteria and the txtB gene, involved in the biosynthesis of the main CS pathogenicity factor, thaxtomin, were quantified by real-time PCR. Bulk soil, tuberosphere soil and potato periderm were characterized by carbon, nitrogen, phosphorus, sulfur, calcium, magnesium and iron contents. The adjustment of the data for field effects eliminated the confounding local conditions and showed that at all fields the CS severity was negatively correlated with soil S content while the number of txtB gene copies was positively correlated with soil C and N contents. Thus, those factors might have a more general relationship to the pathogen occurrence and disease severity, which needs to be verified in other environmental conditions.

Biocontrol of Potato Common Scab is Associated with High Pseudomonas fluorescens LBUM223 Populations and Phenazine-1-Carboxylic Acid Biosynthetic Transcript Accumulation in the Potato Geocaulosphere.

Pseudomonads are often used as biocontrol agents because they display a broad range of mechanisms to control diseases. Common scab of potato, caused by Streptomyces scabies, was previously reported to be controlled by Pseudomonas fluorescens LBUM223 through phenazine-1-carboxylic acid (PCA) production. In this study, we aimed at characterizing the population dynamics of LBUM223 and the expression of phzC, a key gene involved in the biosynthesis of PCA, in the rhizosphere and geocaulosphere of potato plants grown under controlled and field conditions. Results obtained from controlled experiments showed that soil populations of LBUM223 significantly declined over a 15-week period. However, at week 15, the presence of S. scabies in the geocaulosphere was associated with significantly higher populations of LBUM223 than when the pathogen was absent. It also led to the detection of significantly higher phzC gene transcript numbers. Under field conditions, soil populations of LBUM223 followed a similar decline in time when a single inoculation was applied in spring but remained stable when reinoculated biweekly, which also led to greater phzC gene transcripts accumulation. Taken together, our findings suggest that LBUM223 must colonize the potato geocaulosphere at high levels (10(7) bacteria/g of soil) in order to achieve biocontrol of common scab through increased PCA production.

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.

Production of the Streptomyces scabies coronafacoyl phytotoxins involves a novel biosynthetic pathway with an F420 -dependent oxidoreductase and a short-chain dehydrogenase/reductase.

Coronafacoyl phytotoxins are secondary metabolites that are produced by various phytopathogenic bacteria, including several pathovars of the Gram-negative bacterium Pseudomonas syringae as well as the Gram-positive potato scab pathogen Streptomyces scabies. The phytotoxins are composed of the polyketide coronafacic acid (CFA) linked via an amide bond to amino acids or amino acid derivatives, and their biosynthesis involves the cfa and cfa-like gene clusters that are found in P. syringae and S. scabies, respectively. The S. scabies cfa-like gene cluster was previously reported to contain several genes that are absent from the P. syringae cfa gene cluster, including one (oxr) encoding a putative F420 -dependent oxidoreductase, and another (sdr) encoding a predicted short-chain dehydrogenase/reductase. Using gene deletion analysis, we demonstrated that both oxr and sdr are required for normal production of the S. scabies coronafacoyl phytotoxins, and structural analysis of metabolites that accumulated in the Δsdr mutant cultures revealed that Sdr is directly involved in the biosynthesis of the CFA moiety. Our results suggest that S. scabies and P. syringae use distinct biosynthetic pathways for producing coronafacoyl phytotoxins, which are important mediators of host-pathogen interactions in various plant pathosystems.

Key events in pathogenesis of spongospora diseases in potato: a review

Spongospora subterranea (Wallr.) Lagerh. is the causative agent of powdery scab and root diseases of potato. Diseases induced by S. subterranea causes substantial economic losses to the global potato industry. The process of disease development in the S. subterranea-potato pathosystem have long been studied, but critical events prior to infection and disease development remain poorly understood because the subject have received relatively little attention. Nonetheless, new knowledge of host-pathogen-environment interactions have been gained in recent years. This paper provides the current knowledge of the key events which leads to the development of potato diseases caused by S. subterranea and highlights future research to address identified knowledge gaps. This will further our understanding of the interactions between S. subterranea and its potato host and contribute to improved disease control measures.

Isolation and Characterization of Plant-PathogenicStreptomycesSpecies Associated with Common Scab-Infected Potato Tubers in Newfoundland

Potato common scab (CS) is an economically important crop disease that is caused by several members of the genus Streptomyces. In this study, we characterized the plant-pathogenic Streptomyces spp. associated with CS-infected potato tubers harvested in Newfoundland, Canada. A total of 17 pathogenic Streptomyces isolates were recovered from potato scab lesions, of which eight were determined to be most similar to the known CS pathogen S. europaeiscabiei. All eight S. europaeiscabiei isolates were found to produce the thaxtomin A phytotoxin and to harbor the nec1 virulence gene, and most also carry the putative virulence gene tomA. The remaining isolates appear to be novel pathogenic species that do not produce thaxtomin A, and only two of these isolates were determined to harbor the nec1 or tomA genes. Of the non-thaxtomin-producing isolates, strain 11-1-2 was shown to exhibit a severe pathogenic phenotype against different plant hosts and to produce a novel, secreted phytotoxic substance. This is the first report documenting the plant-pathogenic Streptomyces spp. associated with CS disease in Newfoundland. Furthermore, our findings provide further evidence that phytotoxins other than thaxtomin A may also contribute to the development of CS by Streptomyces spp.

New kid on the block – the clubroot pathogen genome moves the plasmodiophorids into the genomic era

Plasmodiophora brassicae causes clubroot on cruciferous plants and causes worldwide huge economical losses on important Brassica crops. P. brassicae infection produces large root galls, the clubroots, which can also affect the upper plant parts by reduced water and nutrient uptake and redirection of assimilates from leaves to roots. P. brassicae is an obligate biotrophic protist in the plasmodiophorids within the eukaryote supergroup of Rhizaria and is unrelated to other known plant pathogens. Plasmodiophorids can be parasites of plants and oomycetes. The recently released genome of P. brassicae is only the third in the poorly studied Rhizaria and the first plant pathogenic genome of this eukaryotic group. The P. brassicae genome was estimated to be 25.5 Mb in size and predicted to contain 9730 gene models. A transcriptome of P. brassicae and Spongospora subterranea, the potato scab pathogen was also presented. Consequently, for the first time large scale data for a eukaryotic plant pathogen group outside the fungi and oomycetes are now available. This review highlights selected characteristics of the P. brassicae genome including molecular events shown or predicted to take place in each phase of its life-cycle, such as manipulation of: 1) host primary metabolism, 2) plant hormone homeostasis, and 3) plant defense. Further, future directions and challenges in the P. brassicae and plasmodiophorid genomic research are discussed.

Effect of Soil Moisture Management on the Development of Potato mop-top virus-Induced Tuber Necrosis.

Potato mop-top virus (PMTV), transmitted by Spongospora subterranea f. sp. subterranea, the causal agent of powdery scab in potato, has become one of the most important tuber necrosis viruses in the United States. The virus has been confirmed in six major potato-producing states in the United States since its identification in 2003. Currently, no control methods are available for PMTV or its vector. A growth-chamber experiment was conducted to investigate the potential of using moisture regime adjustments to manage tuber necrosis caused by PMTV. Two commercial potato cultivars with varying levels of sensitivity to PMTV, 'Dakota Crisp' and 'Ivory Crisp', were grown in soil obtained from a PMTV-infested field. Over the course of the plant growth cycle, plants of each cultivar were subjected to moisture regimes of wet throughout (WT), wet early/dry late (WEDL), dry early/wet late (DEWL), and dry throughout (DT). Soil moisture levels of 90 and 60% field capacity were considered wet and dry, respectively, while early and late refer to first and last 50 days after planting, respectively. Results of visual assessment conducted 3 months after storage showed significant differences in root gall formation, powdery scab on tubers, and PMTV tuber necrosis among moisture regimes. Powdery scab incidence was significantly higher in the WT and DEWL regimes than WEDL and DT regimes. PMTV tuber necrosis incidence did not differ between the WT and DEWL or between DEWL and WEDL moisture regimes. However, PMTV tuber necrosis incidence was significantly higher in WT than under WEDL and DT regimes. The incidence of PMTV tuber necrosis was significantly lower in the DT regime than in the other three moisture regimes. Severity of PMTV tuber necrosis was significantly higher in the WT than the other regimes, which did not differ statistically among themselves. A significant interaction was found between cultivar and moisture regime on root gall formation, with the highest number of galls found on Ivory Crisp grown in the WT moisture regime. A significant correlation was found between powdery scab incidence on tubers and PMTV-induced tuber necrosis incidence. The results of this study provide useful basic information upon which further work can be built for formulating irrigation management schemes that have the potential of reducing the likelihood of powdery scab infection and PMTV-induced tuber necrosis development in potato.

Management of common scab of potato in the field using biopesticides, fungicides, soil additives, or soil fumigants

ABSTRACTCommon scab, caused by Streptomyces scabiei is an economically important potato disease worldwide. The potato industry in New Brunswick, Canada experience $1.2 million loss every year due to this disease. Superficial, raised, or deep-pitted brownish lesions on infected tubers reduce the quality and marketability of both fresh-market and processing potatoes, and hence, common scab is considered a priority disease for which adequate control measures are lacking. The objective of this research was to compare various potential treatments in suppressing the disease. Two field experiments were conducted at McCain's Research Farm, Florenceville-Bristol, New Brunswick, Canada, in 2008 and 2009 to assess the efficacy of Bacillus subtilis, Enterobacter cloacae, Chloropicrin, Pic-Plus, manganese sulphate and mustard meal in comparison with the chemical controls fludioxonil and mancozeb against common scab of potato. The disease incidence was significantly reduced by 36% due to the addition of mustard meal to the soil; 35.4% due to fludioxonil seed treatment; 30.0% due to soil fumigation with Pic-Plus; or by 27.2% due to soil fumigation with Chloropicrin. Potato tubers with scab severity ≥5% which are considered unmarketable in Canada were significantly reduced by 56.1% due to seed treatments with B. subtilis; by 57.8% due to fludioxonil; or by 63.1% due to the soil addition of mustard meal. The same treatments significantly increased marketable yield by 32.5%, 24.6%, and 24.6%, respectively. Soil fumigation with Chloropicrin or Pic-Plus increased marketable yield by 9.5% or 7.1%, respectively. These findings indicate that, in addition to the fludioxonil seed treatment, the seed treatment with the biopesticide containing B. subtilis and the soil addition of mustard meal treatments are potential alternatives for managing common scab of potatoes.

Identification of genetic and environmental factors stimulating excision from Streptomyces scabiei chromosome of the toxicogenic region responsible for pathogenicity.

The genes conferring pathogenicity in Streptomyces turgidiscabies, a pathogen causing common scab of potato, are grouped together on a pathogenicity island (PAI), which has been found to be mobile and appears to transfer and disseminate like an integrative and conjugative element (ICE). However, in Streptomyces scabiei, another common scab-inducing species, the pathogenicity genes are clustered in two regions: the toxicogenic region (TR) and the colonization region. The S. scabiei 87.22 genome was analysed to investigate the potential mobility of the TR. Attachment sites (att), short homologous sequences that delineate ICEs, were identified at both extremities of the TR. An internal att site was also found, suggesting that the TR has a composite structure (TR1 and TR2). Thaxtomin biosynthetic genes, essential for pathogenicity, were found in TR1, whereas candidate genes with known functions in recombination, replication and conjugal transfer were found in TR2. Excision of the TR1 or TR2 subregions alone, or of the entire TR region, was observed, although the excision frequency of TR was low. However, the excision frequency was considerably increased in the presence of either mitomycin C or Streptomyces coelicolor cells. A composite TR structure was not observed in all S. scabiei and Streptomyces acidiscabies strains tested. Of the ten strains analysed, seven lacked TR2 and no TR excision event could be detected in these strains, thus suggesting the implication of TR2 in the mobilization of S. scabiei TR.

Pseudomonas fluorescens LBUM223 Increases Potato Yield and Reduces Common Scab Symptoms in the Field.

Common scab of potato, caused by pathogenic Streptomyces spp., is an important disease not efficiently controlled by current methods. We previously demonstrated that Pseudomonas fluorescens LBUM223 reduces common scab development under controlled conditions through phenazine-1-carboxylic (PCA) production, leading to reduced thaxtomin A production by the pathogen, a key pathogenicity and virulence factor. Here, we aimed at determining if LBUM223 is able to increase potato yield and control common scab under field conditions, while characterizing the biocontrol mechanisms involved. We investigated if a reduction in pathogen soil populations, activation of induced systemic resistance in potato, and/or changes in txtA gene expression, involved in thaxtomin A biosynthesis in pathogenic Streptomyces spp. were involved in common scab control by LBUM223. Common scab symptoms were significantly reduced and total tuber weight increased by 46% using biweekly applications of LBUM223. LBUM223 did not reduce pathogen soil populations, nor was potato systemic defense-related gene expression significantly altered between treatments. However, a significant down-regulation of txtA expression occurred in the geocaulosphere. This is the first demonstration that a Pseudomonas strain can directly alter the transcriptional activity of a key pathogenesis gene in a plant pathogen under field conditions, contributing to disease control.

Biological control of potato scab and antibiosis by antagonistic Streptomyces sp. WoRs-501

Streptomyces sp. isolate WoRs-501 suppressed potato scab caused by S. turgidiscabies and S. scabiei in a field pot trial. An inoculum of 2 × 107 colony-forming units (CFU)/g dry soil was sufficient to establish a population with a stable suppressive effect on the disease. In the field, mixing the WoRs-501 inoculum (approximately 109 CFU/g dry mass) at a ratio of 1:9 (v/v) with scab-infested field soil at 30 × 30 × 15 cm per plant decreased the disease severity score by 26–54 % in comparison with the untreated control. To understand establishment of WoRs-501 in soil and the mechanism of the suppressive effect of the isolate on potato scab, we analyzed the population dynamics of WoRs-501 and scab pathogen in soil using real-time quantitative competitive quenching probe PCR. In a pot trial using soil that was initially treated with 2 × 107 WoRs-501 CFU/g dry soil, the copy number of the WoRs-501 16S–23S rDNA ITS sequence was maintained approximately 107 copies/g dry soil during potato cultivation. Treatment with 2 × 107 WoRs-501 CFU/g dry soil reduced the copy number of the scab pathogen nec1 gene in infested soil below the detection threshold. Though the copy number of this gene increased gradually during potato cultivation, the nec1 copy number remained at a significantly lower value than in the untreated control. An extract of the WoRs-501 inoculum inhibited mycelial growth of S. turgidiscabies in vitro. Therefore, antibiotics from WoRs-501 decrease scab pathogen populations in soil early in potato cultivation and strongly suppress scab disease.

Effect of Powdery Scab Root Galls on Yield of Potato.

Powdery scab of potato, caused by Spongospora subterranea f. sp. subterranea, has increased in incidence since 1981 to become a major concern for potato production in the Columbia Basin of south-central Washington and north-central Oregon. Disease symptoms consist of pustules on the tuber surface and wart-like galls on roots, stolons, and tubers. In the current study, the effect of powdery scab root galls on potato yield was quantified by comparing yields of cultivars susceptible and resistant to root galling grown on soil naturally infested and noninfested with the powdery scab pathogen. Potato tuber yields for the three yield components of yield per plant, number of tubers, and weight of tubers were not negatively affected by powdery scab galls on roots of Umatilla Russet and Shepody, as indicated by yield ratios ≥1.0 in 3 of 4 years, and also by regression analyses in all 3 years. Regression lines with a nonsignificant slope or slopes that significantly increased with gall index also indicated that root galls did not negatively affect yield. Mean tuber weight increased as gall index increased for Umatilla Russet in 2 of 3 years. Consequently, control tactics directed solely at reducing powdery scab galls on roots are not justified economically under conditions encountered in the Columbia Basin of Washington and Oregon states. Effort has been made to develop resistant cultivars and search for soil applied chemicals to reduce root galls.

Complete Genome Sequence of Biocontrol Strain Pseudomonas fluorescens LBUM223.

Pseudomonas fluorescens LBUM223 is a plant growth-promoting rhizobacterium (PGPR) with biocontrol activity against various plant pathogens. It produces the antimicrobial metabolite phenazine-1-carboxylic acid, which is involved in the biocontrol of Streptomyces scabies, the causal agent of common scab of potato. Here, we report the complete genome sequence of P. fluorescens LBUM223.

Strategies for Selecting Stable Common Scab Resistant Clones in a Potato Breeding Program

Common scab (CS) of potato, caused by Streptomyces scabies, is an important disease in the US. CS problems can be avoided using resistant varieties. However, evaluating breeding clones can be complicated by high location and season-based soil and environmental variation. In this study we evaluated the efficacy of screening for CS resistance within the Wisconsin breeding program across multiple environments from 2006 to 2013. In each trial, 60–160 clones were evaluated. We compared the ability to select for CS resistance in a set of 18 dedicated CS screening trials (DST) versus 18 similar parallel standard breeding trials (SBT). Heritability for CS rating across DST was 0.83 vs. 0.53 in SBT. Data analysis from DST was able to separate CS susceptible cultivars (Atlantic, Snowden) from resistant cultivars (Pike, Snowden). However, same data analysis from SBT was not able to separate susceptible from resistant cultivars. Using DST datasets, we estimated the genotypic stability of scab performance across years and locations. We also calculated the probability distributions for the better or worse performance of a given clone vs. a standard variety. Using results from six or more DST we identified five round white clones that outperformed or matched CS tolerant Pike, eleven russet clones that outperformed or matched CS tolerant Russet Burbank and five red or yellow skin clones that outperformed or matched CS resistance level of Dark Red Norland. The approaches utilized here offer useful additional information for breeding programs which aim to improve selection efficiency for scab resistance.

Characterization of Suspected Cyclic di‐GMP Metabolizing Genes in Streptomyces scabies

Streptomyces scabies is a bacterial plant pathogen responsible for a condition known as common scab in potatoes and other tuber crops. Hyphae and spore development have been shown to be regulated b...

Introduction to 2013 Symposium on Bacterial Diseases of Potatoes

Numerous bacterial diseases limit potato productivity. Healthy potato seed tubers are vital for sustaining the continued supply of potatoes between seasons. The pathogens that cause soft rots and ring rots of tubers, and wilting and stem and foliar necrosis may be introduced as secondary-infecting pathogens after the plant has been compromised or be disseminated within seed tubers. Recurrent and persistent bacterial soil-borne diseases have long plagued potato production systems, such as common scab. Recently, a more insidious bacterial disease that is vectored by the potato psyllid, Zebra chip appeared in North America and New Zealand. Although conventional agrochemical pest control has historically been generally ineffective for control of bacterial diseases of potatoes, integrated methods of bacterial disease management is driven by increasing concern over economic and environmental sustainability of potato production. The 2013 Potato Association of America Symposium entitled “Bacterial Disease Issues, Basic to Applied Aspects“ was organized to summarize persistent and emerging bacterial disease issues in potato production. Four speakers presented their experience with the underlying theme common in many academic circles, the process of applying basic research and applied research to benefit the food production system. On a sad note, Dr. Leslie Wanner who presented information on common scab of potato passed away on Christmas day, 2014. All who knew her will miss her. We have all benefited from her excellent and extensive research and spirit of collegiality.