Common Scab Disease Research Articles

First Report of Streptomyces brasiliscabiei Causing Common Scab on Potato in China.

Severe typical deep-pitted lesions of Potato Common Scab (PCS) disease were observed in two locations in China, Dingxi, Gansu Province, and Shuozhou, Shaanxi Province, in 2021. Potato farms in Dingxi growing cultivar Huangxin 226 (26 hectares) exhibited a scab disease incidence of 10%, while cultivar Jinshu 15# (4 hectares) in Shuozhou showed a disease incidence of 30% (Fig. 1). During harvest, tubers displaying PCS symptoms were collected for pathogen isolation. To obtain pathogen isolates, surface-sterilized tuber tissue with scab lesions was ground in sterile water, serially diluted, and plated onto ISP5 agar medium plates (Handique et al. 2022). Five pure colonies of Streptomyces isolate were obtained, designated as ZRIMU1503, ZRIMU1502, ZRIMU1320, ZRIMU1321and ZRIMU791. Genomic DNA was extracted and sequenced using Illumina technology. Sequencing data of the 5 isolates were uploaded to NCBI GenBank and annotated (Accession numbers: JBBAYL010000000, JBBAYM010000000, JBBAYN010000000, JBBAYO010000000 and JBBAYP010000000, respectively) using the PGAP pipeline (Tatusova et al. 2016). Average Nucleotide Identity (ANI) values (97.52 %, 97.53 %,97.54 %,97.57 % and 97.52 %, respectively) indicated the identity of the five isolates to the type strain S. brasiliscabiei IBSBF 2867T. Additionally, pairwise comparisons of Digital DNA Hybridization (DDH) value (76.2%, 76.3%, 76.4%, 76.4% and 76.2% respectively) of all the Streptomyces type strains show the highest identity to S. brasiliscabiei IBSBF 2867T. Twelve housekeeping genes (acnA, atpD, dnaN, gap, gyrA, gyrB , infB, mdh, recA, rplB, rpoB, and trpB) were extracted from the genome sequence of the five isolates to construct a multi-locus sequence analysis (MLSA) tree. The evolutionary distance of the five isolates was constructed using MEGAX software (Kumar et al., 1994), along with other Streptomyces strains that are known to cause PCS. The resulting cladogram demonstrated the isolated strains clustered together with S. brasiliscabiei IBSBF 2867T (Fig.2). Koch's postulates were fulfilled by inoculating a perlite potting mix with spore suspensions of each isolate (104 CFU/ml), planting tubers (cv. Favorita), and reproducing PCS symptoms at harvest after three months. Negative control received water treatment. The plants were kept in greenhouse with 12 h of light per day and irrigated regularly. The experiment was repeated twice, once in April 2022 and again in April 2023. On harvest, all five isolates exhibited development of severe symptoms of PCS (Fig.1), while the negative controls had no lesions. The pathogen was reisolated from the lesions and confirmed to be identical to the original isolate by 16S rRNA gene sequences. To our knowledge, this is the first report of S. brasiliscabiei causing PCS in China. S. brasiliscabiei was identified as a new species to cause PCS in Brazil and was identified based on morphology, pathogenicity, and genetic features (Corrêa et al. 2021). Multiple pathogen-causing PCS has been recognized in China and a surge of disease incidence in potato fields has been reported (Handique et al. 2022; Wu et al. 2023). S. brasiliscabiei causes severe symptoms which makes potatoes unmarketable. The disease epidemiology of this pathogen needs to be investigated.

Occurrence of New Pathogenic Species of Streptomyces Causing Common Scab Disease on Potato Tubers in Egypt

Occurrence of New Pathogenic Species of Streptomyces Causing Common Scab Disease on Potato Tubers in Egypt

The Role of Soil Abundance of TxtAB in Potato Common Scab Disease Severity.

Common scab is an economically costly soilborne disease of potato endemic in many potato-growing regions. The disease is caused by species of Streptomyces bacteria that produce the phytotoxin thaxtomin A. The primary disease management tool available to growers is planting resistant cultivars, but no cultivar is fully resistant to common scab, and partially resistant cultivars are often not the preferred choice of growers because of agronomic or market considerations. Therefore, growers would benefit from knowledge of the presence and severity of common scab infestations in field soils to make informed planting decisions. We implemented a quantitative PCR diagnostic assay to enable field detection and quantification of all strains of Streptomyces that cause common scab in the United States through amplification of thaxtomin A biosynthetic genes. Greenhouse trials confirmed that pathogen abundance was highly correlated with disease severity for five distinct phytopathogenic Streptomyces species, although the degree of disease severity was dependent on the pathogen species. Correlations between the abundance of the thaxtomin biosynthetic genes from field soil with disease on tubers at field sites across four U.S. states and across 2 years were not as strong as correlations observed in greenhouse assays. We also developed an effective droplet digital PCR diagnostic assay that also has potential for field quantification of thaxtomin biosynthetic genes. Further improvement of the PCR assays and added modeling of other environmental factors that impact disease outcome, such as soil composition, can aid growers in making informed planting decisions.

Physiological Functions of the Cello-Oligosaccharides Binding CebE in the Pathogenic Streptomyces sp. AMCC400023.

Potato common scab, an economically important disease worldwide, is caused by pathogenic Streptomyces strains mainly through the effects of thaxtomin. The cello-oligosaccharides binding protein CebE is proposed as a gateway to the pathogenic development of Streptomyces scabiei. In this study, two functional CebE encoding genes, GEO5601 and GEO7671, were identified in pathogenic Streptomyces sp. AMCC400023. With a higher binding affinity towards signal molecules, the deletion of GEO5601 severely impaired thaxtomin-producing capacity and reduced the strain's pathogenicity. Transcriptional analysis confirmed that CebE5601 is also responsible for the import and provision of carbon sources for cell growth. With lower binding affinity, the pathogenicity island (PAI)-localized CebE7671 may assume a new function of mediating the biological process of sporulation, given the significantly impaired formation of ΔGEO7671 spores. The mechanisms of action of CebE proteins unraveled in Streptomyces sp. AMCC400023 will help pave the way for more effective prevention of the potato common scab disease.

Nitrogen sources enhance siderophore-mediated competition for iron between potato common scab and late blight causative agents.

How do pathogens affecting the same host interact with each other? We evaluated here the types of microbe-microbe interactions taking place between Streptomyces scabiei and Phytophthora infestans, the causative agents of common scab and late blight diseases in potato crops, respectively. Under most laboratory culture conditions tested, S. scabiei impaired or completely inhibited the growth of P. infestans by producing either soluble and/or volatile compounds. Increasing peptone levels correlated with increased inhibition of P. infestans. Comparative metabolomics showed that production of S. scabiei siderophores (desferrioxamines, pyochelin, scabichelin, and turgichelin) increased with the quantity of peptone, thereby suggesting that they participate in the inhibition of the oomycete growth. Mass spectrometry imaging further uncovered that the zones of secreted siderophores and of P. infestans growth inhibition coincided. Moreover, either the repression of siderophore production or the neutralization of their iron-chelating activity led to a resumption of P. infestans growth. Replacement of peptone by natural nitrogen sources such as ammonium nitrate, sodium nitrate, ammonium sulfate, and urea also triggered siderophore production in S. scabiei. Interestingly, nitrogen source-induced siderophore production also inhibited the growth of Alternaria solani, the causative agent of the potato early blight. Overall, our work further emphasizes the importance of competition for iron between microorganisms that colonize the same niche. As common scab never alters the vegetative propagation of tubers, we propose that S. scabiei, under certain conditions, could play a protective role for its hosts against much more destructive pathogens through exploitative iron competition and volatile compound production.

Morphological, Biochemical and Molecular Identification Bacteria Streptomyces spp. Causing Common Scab Disease on Potato in Iraq

This study was carried out at the University of Anbar - College of Agriculture - Department of Plant Protection. The study aimed to isolate and diagnose the bacterium that causes scabies on potatoes, Streptomyces spp., depending on the phenotypic characteristics, biochemical interactions, and molecular methods. The results of isolation from different regions and on PDA medium showed that it was obtained On 45 pure isolates of the pathogenic bacteria Streptomyces spp, and the results of growth in different media showed that the best medium for growth was PDA and Nutrient agar. The results of phenotypic diagnosis and biochemical tests revealed that 8 isolates belong to the type Streptomyces scabies and two isolates belong to the type Streptomyces acidiscabies. DNA was extracted from 10 isolates of pathogenic bacteria, and the results of molecular diagnosis using the 16S rRNA gene showed that light bands of 1250 pairs were obtained. The nitrogenous base sequences of these isolates were determined and deposited in GenBank under accession codes (OQ8191, OQ819106, OQ815918, OQ789975, OQ789966, OQ815949, OQ815950).

Efficiency of some Biological and Chemical Agents in Controlling Common Scab Disease in Potatoes Caused by Streptomyces scabies

This study was carried out at the University of Anbar - College of Agriculture - Department of Plant Protection. This study aimed to evaluate the efficiency of using biological and chemical control agents such as Azotobacter chroococcum, Glomus mosseae and ammonium sulfate (NH4)2SO4 in controlling common scab disease on potatoes caused by Streptomyces spp. A study of its effect on reducing the severity of disease, evidence of scabies, and some studied parameters such as plant speed, plant diameter, plant diameter, and stem diameter Azotobacter chroococcum was superior to the rest of the treatments in reducing the percentage of infection severity and the percentage of evidence of scab, which amounted to (16.90 and 24.000), respectively, and an increase in the growth parameters of the crop, the speed of emergence, the height of the plant, the number of aerial stems, the diameter of the stem, the yield of one plant, and the total yield in tons, which amounted to (2.667%,41.111 6.000, 4.900, 1227.67, 7.00).

First Report of Streptomyces niveiscabiei Causing Potato Common Scab in Shanxi and Gansu, China.

Multiple species of Streptomyces cause common scab disease in potato (Solanum tuberosum) (Kämpfer et al. 1991). Potato tubers (cv. Jinshu1 #5 and Longshu #6) with severe pitted common scab symptoms were observed from two farms in Chaozhou in Shanxi Province and in Tianzhu in Gansu Province during the national disease survey of bacterial diseases of potatoes in 2021. The disease incidence was around 30% on the 6.7 ha of the Chaozhou farm and 10% on the 0.7 ha on the Tianzhu farm. Three tubers with scab symptoms were surface disinfested with 3% sodium hypochlorite for 1 min. The symptomatic tissue was then ground in sterile water. Serially diluted ground samples were cultured on Streptomyces ISP Medium 5 agar plates (Shirling and Gottlieb 1966) and incubated at 280C for 5 days. Eight pure Streptomyces colonies were obtained and sequenced for identification using the universal 16S rRNA gene primers 27F (5'-AGAGTTTGATCMTGGCTCAG-3') and 1492R (5'-TACGGYTACCTTGTTACGACTT-3') (Monciardini et al. 2002) by colony PCR. Blast results of the sequences against the NCBI GenBank for the eight isolates, ZRIMU1508, ZRIMU1510, ZRIMU1511, ZRIMU1512, ZRIMU1514, ZRIMU1515, ZRIMU1516 and ZRIMU1530 (Accession numbers: OP941573 - OP941580), showed more than 99% sequence identity to S. niveiscabiei NRRLB-24457T type strain. Additionally, 12 housekeeping gene sequences, acnA (OP997624 - OP997625), atpD (OP997622 - OP997623), dnaN (OP997620 - OP997621), gap (OP997618 - OP997619), gyrA (OP997614 - OP997615), gyrB (OP997612 - OP997613), infB (OP997610- OP997611), mdh (OP997608 - OP997609), recA (OP997602 - OP997603), rplB (OP997600 - OP997601), rpoB (OP997598- OP997599), and trpB (OP997594 - OP997595), were extracted from the genome sequences of two strains, ZRIMU1510 and ZRIMU1530, and uploaded to GenBank. Genes for pathogenicity, txtA (OP997593 - OP997594), tomA (OP997596 - OP997597) and Nec1(OP997606 - OP997607), were also identified from the genome sequence and uploaded to GenBank. The housekeeping genes and the pathogenicity genes showed over 98% identity with S. niveiscabiei. Phylogenetic trees were constructed using concatenated housekeeping gene sequences (Kumar et al. 1994) and the cladogram showed that the isolates ZRIMU1510 and ZRIMU1530 grouped with the type strain NRRLB-24457T. Pathogenicity tests were done by drench application of 100 ml spore suspensions (104 CFU/ml) of ZRIMU1530, ZRIMU1510, or phosphate buffer into pots with potato plants (cv. Favorita) grown in potting mix. Five tubers were planted and inoculated with each pathogen or phosphate buffer as the negative control. The plants were then placed in a greenhouse with 12 h of light per day, irrigated regularly, and harvested after 3 months. The newly formed tubers were checked for disease symptoms. Tubers from pots inoculated with ZRIMU1530 and ZRIMU1510 exhibited typical symptoms of common scab with raised corky lesions with deep pits, but the negative controls remained asymptomatic. The pathogens were reisolated from the lesions and confirmed to be identical to the original isolates by 16S rRNA gene sequences, thus completing Koch's postulates. The pot experiment was conducted twice: first in May 2022 and second in February 2023. To our knowledge, this is the first report of S. niveiscabiei causing common scab of potato in Shanxi and Gansu, China. S. niveiscabiei was first reported in Korea (Park et al. 2003) and this report will draw attention to the study and management of scab pathogens in China.

Diverse mobile genetic elements shaped the evolution of Streptomyces virulence

Mobile genetic elements can innovate bacteria with new traits. In plant pathogenic Streptomyces, frequent and recent acquisition of integrative and conjugative or mobilizable genetic elements is predicted to lead to the emergence of new lineages that gained the capacity to synthesize Thaxtomin, a phytotoxin neccesary for induction of common scab disease on tuber and root crops. Here, we identified components of the Streptomyces -potato pathosystem implicated in virulence and investigated them as a nested and interacting system to reevaluate evolutionary models. We sequenced and analysed genomes of 166 strains isolated from over six decades of sampling primarily from field-grown potatoes. Virulence genes were associated to multiple subtypes of genetic elements differing in mechanisms of transmission and evolutionary histories. Evidence is consistent with few ancient acquisition events followed by recurrent loss or swaps of elements carrying Thaxtomin A-associated genes. Subtypes of another genetic element implicated in virulence are more distributed across Streptomyces . However, neither the subtype classification of genetic elements containing virulence genes nor taxonomic identity was predictive of pathogenicity on potato. Last, findings suggested that phytopathogenic strains are generally endemic to potato fields and some lineages were established by historical spread and further dispersed by few recent transmission events. Results from a hierarchical and system-wide characterization refine our understanding by revealing multiple mechanisms that gene and bacterial dispersion have had on shaping the evolution of a Gram-positive pathogen in agricultural settings.

Common scab disease: structural basis of elicitor recognition in pathogenic Streptomyces species.

In Streptomyces scabiei, the main causative agent of common scab disease of root and tuber crops, the interaction between the substrate-binding protein (SBP) CebE (CebEscab) and cellotriose released by the plant host (KD in the nanomolar range) is the first event for the onset of its pathogenic lifestyle. Here, we report the structure of CebEscab in complex with cellotriose at a resolution of 1.55 Å, adopting a general fold of the B subcluster of SBPs. The interaction between CebEscab and cellotriose involves multiple direct or water-mediated hydrogen bonds and hydrophobic interactions, with the glucose monomer at the non-reducing end occupying the most conserved part of the substrate-binding cleft. As main interactions between the two domains of CebE involve cellotriose itself, the closed conformational state of CebE is performed via an induced-fit ligand binding mechanism where cellotriose binding triggers the domain movement. Analysis of regulon predictions revealed that the signaling pathway from CebE-mediated cellotriose transport to the transcriptional activation of thaxtomin phytotoxin biosynthesis is conserved in Streptomyces spp. causing common scab, except for Streptomyces ipomoeae, which specifically colonizes sweet potatoes and responds to other and yet unknown virulence elicitors. Interestingly, strains belonging to the pathogenic species turgidiscabies and caniscabiei have a cellotriose-binding protein orthologous to the CebE protein of the saprophytic species Streptomyces reticuli with lower affinity for its substrate (KD in the micromolar range), suggesting higher cellotriose concentrations for perception of their host. Our work also provides the structural basis for the uptake of cellobiose and cellotriose by non-pathogenic cellulose-decomposing Streptomyces species.IMPORTANCECommon scab is a disease caused by a few Streptomyces species that affects important root and tuber crops including potato, beet, radish, and parsnip, resulting in major economic losses worldwide. In this work, we unveiled the molecular basis of host recognition by these pathogens by solving the structure of the sugar-binding protein CebE of Streptomyces scabiei in complex with cellotriose, the main elicitor of the pathogenic lifestyle of these bacteria. We further revealed that the signaling pathway from CebE-mediated transport of cellotriose is conserved in all pathogenic species except Streptomyces ipomoeae, which causes soft rot disease in sweet potatoes. Our work also provides the structural basis of the uptake of cellobiose and cellotriose in saprophytic Streptomyces species, the first step activating the expression of the enzymatic system degrading the most abundant polysaccharide on earth, cellulose.

Artificial network inference analysis reveals the impact of biostimulant on bacterial communities in fumigated soil for potato production against common scab

Introduction and methodsAs part of a study on control methods of common scab disease impact on potato yield and quality, high-throughput sequencing was used to measure the effects of soil fumigant chloropicrin alone or in combination with a Bacillus species-based biostimulant on soil bacterial diversity in terms of richness and composition, as well as on soil bacterial network interactions.Results and discussionThe results showed that common scab caused significant net yield losses of more than 46.25% in potatoes of control plots (T1), while the use of the fumigant alone (T3) and the use of the fumigant with the biostimulant (T4) reduced net yield losses to less than 2.5%. These treatments also promoted gross yield increases of 23.5 cwt. acre-1 (7.06%) and 28 cwt. acre-1 (8.41%) respectively. The study found that using the soil fumigant chloropicrin significantly and persistently altered the composition of the soil bacterial community over the growing season. The modifications of the soil bacterial community induced by the inoculation of the Bacillus species-based biostimulant are distinct by the end of the growing season depending on whether the soil has been fumigated (T4) or not (T2). Interestingly, artificial network inference analysis showed that the T2 treatment had the highest number of edges and linkages, contrary to the T3 treatment that had the lowest number of edges and linkages. The fumigation alone treatment leads to a reduction in interactions, while the application of the biostimulant, in both non-fumigated and fumigated soil, results in increased interactions and a higher number of connections within a phylum or between different taxa. Furthermore, the treatment combining the fumigant and the biostimulant exhibits a moderate increase in various network properties, providing evidence for the positive effect of biostimulant inoculation on bacterial communities in fumigated soils. Our results provide a more detailed understanding of the bacterial community structure and diversity in the soil of the different treatments. Moreover, deciphering network interactions in soil bacterial communities is fundamentally important for research in soil microbial ecology of potato cropping systems.

Microencapsulation of Thymus vulgaris and Tagetes minuta essential oils: Volatile release behavior, antibacterial activity and effect on potato yield

Thymus vulgaris and Tagetes minuta essential oils (EOs) are recognized due to their antibacterial activity against Streptomyces scabiei , the main causal agent of potato common scab disease. However, EOs have phytotoxic characteristics and are susceptible to degradation by environmental factors, which make their storage and application difficult. Microencapsulation technology represents an alternative for EOs handle, allowing their protection and gradual spread of their compounds. In the present work, microcapsules of maltodextrin and hydroxypropyl methylcellulose containing T. minuta and T. vulgaris EOs were evaluated, analyzing their features, their stability at storage conditions and the control release of their volatile compounds when they are applied on humid substrate. Additionally, the in vitro antibacterial activity of microencapsulated EOs against S. scabiei, and their effects on potato crop yield were evaluated. The obtained microcapsules presented good yield (48.28–51.01 %), low moisture (3.87–3.98 %), spherical shape, and variable size. The release rate of volatiles was high and constant over the 29 days of storage, for almost every compound detected from microcapsules containing T. vulgaris EO. Additionally, the microcapsules containing T. minuta EO showed a variable but high release rate of its main compound dihydrotagetone. The application of microcapsules on humid substrate resulted in a greater release of volatiles for 14 days. The application of T. vulgaris and T. minuta EOs on potato plants (var. Spunta) showed no effect on plant growth and photosynthetic activity, but reduced tuber yield (9.07 and 9.40 g tubers fresh weight/ plant, respectively). The microencapsulation of these EOs reduced this effect (10.14 and 10.29 g tubers fresh weight/plant, respectively), and maintained the bacteriostatic activity on S. scabiei , making them a promising tool for potato common scab control.

Role of Alternative Elicitor Transporters in the Onset of Plant Host Colonization by Streptomyces scabiei 87-22.

Plant colonization by Streptomyces scabiei, the main cause of common scab disease on root and tuber crops, is triggered by cello-oligosaccharides, cellotriose being the most efficient elicitor. The import of cello-oligosaccharides via the ATP-binding cassette (ABC) transporter CebEFG-MsiK induces the production of thaxtomin phytotoxins, the central virulence determinants of this species, as well as many other metabolites that compose the 'virulome' of S. scabiei. Homology searches revealed paralogues of the CebEFG proteins, encoded by the cebEFG2 cluster, while another ABC-type transporter, PitEFG, is encoded on the pathogenicity island (PAI). We investigated the gene expression of these candidate alternative elicitor importers in S. scabiei 87-22 upon cello-oligosaccharide supply by transcriptomic analysis, which revealed that cebEFG2 expression is highly activated by both cellobiose and cellotriose, while pitEFG expression was barely induced. Accordingly, deletion of pitE had no impact on virulence and thaxtomin production under the conditions tested, while the deletion of cebEFG2 reduced virulence and thaxtomin production, though not as strong as the mutants of the main cello-oligosaccharide transporter cebEFG1. Our results thus suggest that both ceb clusters participate, at different levels, in importing the virulence elicitors, while PitEFG plays no role in this process under the conditions tested. Interestingly, under more complex culture conditions, the addition of cellobiose restored thaxtomin production when both ceb clusters were disabled, suggesting the existence of an additional mechanism that is involved in sensing or importing the elicitor of the onset of the pathogenic lifestyle of S. scabiei.

Alteration in Biochemical Responses in Leaves of Potato due to Common Scab Disease

The current study was designed at the research area of Plant Pathology Research Institute, Ayub Agricultural Research Institute, Faisalabad, Pakistan during 2017– 2020. Fresh leaves of inoculated and un-inoculated potato varieties highly susceptible (FD 76-36), moderately susceptible (FD 73-110), and resistant (Esmee)/ test lines were collected during 2017-18 and 2018-19, at 35, 65 and 95 days after sowing for biochemical analysis. Superoxide dismutase activity, Catalase activity, Per-oxidase activity, Protein and Total phenolics contents were quantified. FD 76-36 exhibited minimum SOD activity (67.733, 45.637, 24.910) %, followed by FD 73-110 (70.303,57.893, 42.513) % and Esmee (84.567, 65.167, 45.873) % after 35, 65 and 95 days of inoculation respectively as compared to control. FD 76-36 expressed minimum CAT activity (10.990, 7.473, 3.413) %, followed by Esmee (13.537, 9.630, 8.147) % and FD 73-110 (19.277, 8.147, 6.170) % after 35, 65 and 95 days of inoculation respectively as compared to control. FD 76-36 exhibited minimum POD activity (0.2133, 0.2767, 0.3600) %, followed by FD 73-110 (0.2900, 0.5567, 0.6300) % and Esmee (0.8800, 1.2733, 1.5433) % after 35, 65 and 95 days of inoculation respectively as compared to control. FD 76-36 expressed minimum protein contents (3.097, 2.873, 1.260) %, followed by FD 73-110 (7.907, 5.423, 4.267) % and Esmee (12.163, 8.633, 5.127) % after 35, 65 and 95 days of inoculation respectively as compared to control. FD 76-36 expressed minimum TPC (136.31, 115.58, 70.77) %, followed by FD 73-110 (165.85, 136.75, 86.89) % and Esmee (188.42, 158.38, 109.00) % after 35, 65 and 95 days of inoculation respectively as compared to control. The experiment was conducted with a randomized complete block design (RCBD) and three replications. The experimental data were analyzed using Fisher’s analysis of variance technique and treatment means were compared by the least significance difference (LSD) test at a 5% probability level.

Isolation and characterization of Streptomyces spp. from potato and peanut in Israel

AbstractPotato common scab (CS) and peanut pod wart diseases cause substantial economic losses every year in Israel. In this comprehensive study on the Streptomyces spp. population, isolates were collected during 2004–2016 from potato tubers with symptoms grown in Israel, seed tubers imported from Europe and from peanut pods with symptoms grown in Israel. A total of 142 isolates were characterized by PCR using three primer sets (txtA, tomA and nec1 genes) and by three pathogenicity tests. Seven species were identified among the isolates, S. bottropensis, S. europaeiscabiei, S. griseus, S. scabiei, S. sampsonii, S. turgidiscabies, and S. venezuelae. S. europaeiscabiei was the most dominant among isolates from imported seed tubers, while S. scabiei and S. turgidiscabies were isolated from tubers grown in Israel. Isolates originating from peanuts were significantly more virulent than those from imported seed tubers, as determined by a pathogenicity assay on radish plants. The presence of pathogenic Streptomyces was detected by quantitative PCR (qPCR) in 289 soil samples collected from 31 commercial potato fields with CS history. Eighty percent to 100% of samples collected from fields with a high disease incidence on tubers (50%) gave positive results in qPCR, whereas in samples collected from fields with low CS incidence (2%–30%) the qPCR results were very variable. Potato seed lots imported from Europe during 2008–2021 were found contaminated with CS symptoms at different levels on visual examination. The results of this study indicate that Streptomyces species, in particular S. europaeiscabiei, were introduced to Israel through seed lots imported from Europe.

The Role of Thaxtomin A and Necrosis Protein in Virulence of Streptomyces scabies that Cause Potato Common Scab Disease

The Role of Thaxtomin A and Necrosis Protein in Virulence of Streptomyces scabies that Cause Potato Common Scab Disease

Hybridizations Between formae speciales of Venturia inaequalis Pave the Way for a New Biocontrol Strategy to Manage Fungal Plant Pathogens.

Hybridization and adaptation to new hosts are important mechanisms of fungal disease emergence. Evaluating the risk of emergence of hybrids with enhanced virulence is then key to develop sustainable crop disease management. We evaluated this risk in Venturia inaequalis, the fungus responsible for the common and serious scab disease on Rosaceae hosts, including apple, pyracantha, and loquat. Field isolates from these three hosts and progenies obtained from five crosses between formae speciales isolates collected from pyracantha (f. sp. pyracantha) and apple (f. sp. pomi) were tested for their pathogenicity on the three hosts. We confirmed a strict host specificity between isolates from apple and pyracantha and showed that most isolates were able to cause disease on loquat. None of the 251 progeny obtained from five crosses between V. inaequalis f. sp. pyracantha and V. inaequalis f. sp. pomi could infect apple. If confirmed on more crosses, the inability of the hybrids to infect apple could lead to a novel biocontrol strategy based on a sexual hijacking of V. inaequalis f. sp. pomi by a massive introduction of V. inaequalis f. sp. pyracantha in apple orchards. This strategy, analogous to the sterile insect approach, could lead to the collapse of the population size of V. inaequalis and dramatically reduce the use of chemicals in orchards.

Biocontrol efficacy of antagonistic and endophytic Streptomyces sp. against common scab disease

Biocontrol efficacy of antagonistic and endophytic Streptomyces sp. against common scab disease

Plant Natural Products as Antimicrobials for Control of Streptomyces scabies: A Causative Agent of the Common Scab Disease.

The common scab disease caused by Streptomyces scabies, a soil-dwelling Gram-positive bacterium, is an economically important disease of potatoes and other tuber crops. The lack of effective treatments against this disease accounts for large economic losses globally. Plant extracts were screened to find several that effectively inhibited Streptomyces scabies growth in culture. Seven tinctures showed the greatest inhibition of S. scabies growth by reducing pathogen growth in culture by 75% or more. These extracts were myrrh, garlic, cayenne, barberry, frankincense, wild indigo root, and lavender. Myrrh extract from Commiphora myrrha, a resin made from tree sap, showed strong antibacterial activity by reducing the growth of S. scabies to 13% of the control. Additionally, a flavonoid library was screened to identify several compounds that were effective to control the pathogen growth. The flavonoids that showed the greatest inhibition of Streptomyces scabies growth were sophoraflavanone G, jaceosidin, baicalein, and quercetin. Minimum inhibitory concentrations for the effective flavonoids were calculated to be 6.8 ± 0.4 μM, 100.0 ± 2.1 μM, 202.9 ± 5.3 μM, and 285.2 ± 6.8 μM, respectively. The mean lethal doses for these flavonoids against Streptomyces scabies were 2.0 ± 0.1 μM, 22.6 ± 0.5 μM, 52.9 ± 1.3 μM, and 37.8 ± 1.0 μM, respectively. A live/dead assay showed complete cell death in the presence of sophoraflavanone G indicative of a bactericidal mechanism for flavonoid action on Streptomyces scabies. Scanning electron and transmission electron microscopy imaging showed damaged cell membrane morphologies when Streptomyces scabies was exposed to these flavonoids. Mycelia appeared as flat and deflated structures with contents seen as spewing from branching hyphae with numerous holes and tears in the membrane structure indicative of cell death. Sophoraflavanone G showed the greatest potency and potential as a natural antibiotic from the library of tested flavonoids. These results suggest that these plant compounds act on the pathogen through a bactericidal mechanism involving cell membrane destabilization and disruption leading to cell death.

Streptomyces rhizophilus Causes Potato Common Scab Disease.

Common scab (CS) caused by Streptomyces spp. is a significant soilborne potato disease that results in tremendous economic losses globally. Identification of CS-associated species of the genus Streptomyces can enhance understanding of the genetic variation of these bacterial species and is necessary for the control of this epidemic disease. The present study isolated Streptomyces strain 6-2-1(1) from scabby potatoes in Keshan County, Heilongjiang Province, China. PCR analysis confirmed that the strain harbored the characteristic Streptomyces pathogenicity island (PAI) genes (txtA, txtAB, nec1, and tomA). Pathogenicity assays proved that the strain caused typical scab lesions on potato tuber surfaces and necrosis on radish seedlings and potato slices. Subsequently, the strain was systemically characterized at morphological, physiological, biochemical, and phylogenetic levels. Phylogenetic analysis based on 16S rRNA gene sequences revealed that strain 6-2-1(1) shared 99.86% sequence similarity with Streptomyces rhizophilus JR-41T, isolated initially from bamboo in rhizospheric soil in Korea. PCR amplification followed by Sanger sequencing of the 16S rRNA gene of 164 scabby potato samples collected in Heilongjiang Province from 2019 to 2020 demonstrated that approximately 2% of the tested samples were infected with S. rhizophilus. Taken together, these results demonstrate that S. rhizophilus is capable of causing potato CS disease and may pose a potential challenge to potato production in Heilongjiang Province of China.