Responses of potato cultivars to the common scab pathogens, Streptomyces scabies and S. turgidiscabies

A glasshouse experiment was conducted to study the responses to Streptomyces scabies and S. turgidiscabies in potato cultivars Bellona, Matilda and Sabina (Solanum tuberosum). Potatoes were grown in a peat‐sand mixture inoculated with one of the two strains of either S. scabies or S. turgidiscabies. Logit models were used to analyse the data on disease incidence and severity, whereas the data on emergence and yield were tested by analysis of variance. S. turgidiscabies, a recently described potato pathogen in Finland, possessed a high ability to cause superficial, raised and pitted lesions on all three cultivars tested. Symptoms induced by S. turgidiscabies were similar to those of S. scabies, regardless of the cultivar, which suggests that the two causal organisms of common scab cannot be distinguished based on symptoms. Infection by S. turgidiscabies and S. scabies delayed emergence, had the tendency to decrease the yield, and increased the proportion of small tubers in the yield, regardless of the potato cultivar. Differences in the levels of resistance to common scab were evident between potato cultivars, since cvs. Matilda and Bellona showed higher disease incidence and more severe scab symptoms than cv. Sabina.     

Interactions and biocontrol of pathogenic Streptomyces strains co-occurring in potato scab lesions

Aims:  To test interactions between pathogenic strains of Streptomyces turgidiscabies , S. scabies and S. aureofaciens . To study biological control of S. turgidiscabies and S. scabies using the nonpathogenic Streptomyces strain (346) isolated from a scab lesion and a commercially available biocontrol agent ( S. griseoviridis strain K61; 'Mycostop').
Methods and Results:  Pathogenic strains of S. turgidiscabies and S. aureofaciens inhibited growth of S. scabies in vitro , whereas strain 346 and S. griseoviridis inhibited the pathogenic strains and were subsequently tested for control of scab in the greenhouse and field. Strains 346 and K61 suppressed development of common scab disease caused by S. turgidiscabies in the greenhouse. Strain 346 reduced incidence of S. turgidiscabies in scab lesions on potato tubers in the field.
Conclusions:  Streptomyces turgidiscabies shows antagonism against S. scabies that occurs in the same scab lesions and shares the ecological niche in the field. Biocontrol of S. turgidiscabies is possible with nonpathogenic Streptomyces strains but interactions may be complicated.
Significance and Impact of the Study:  Streptomyces turgidiscabies may have potential to displace S. scabies under the Scandinavian potato growing conditions. Biological control of the severe potato scab pathogen, S. turgidiscabies , is demonstrated for the first time. The results can be applied to enhance control of common scab.     

TxtH is a key component of the thaxtomin biosynthetic machinery in the potato common scab pathogen Streptomyces scabies

Streptomyces scabies causes potato common scab disease, which reduces the quality and market value of affected tubers. The predominant pathogenicity determinant produced by S. scabies is the thaxtomin A phytotoxin, which is essential for common scab disease development. Production of thaxtomin A involves the nonribosomal peptide synthetases (NRPSs) TxtA and TxtB, both of which contain an adenylation (A-) domain for selecting and activating the appropriate amino acid during thaxtomin biosynthesis. The genome of S. scabies 87.22 contains three small MbtH-like protein (MLP)-coding genes, one of which (txtH) is present in the thaxtomin biosynthesis gene cluster. MLP family members are typically required for the proper folding of NRPS A-domains and/or stimulating their activities. This study investigated the importance of TxtH during thaxtomin biosynthesis in S. scabies. Biochemical studies showed that TxtH is required for promoting the soluble expression of both the TxtA and TxtB A-domains in Escherichia coli, and amino acid residues essential for this activity were identified. Deletion of txtH in S. scabies significantly reduced thaxtomin A production, and deletion of one of the two additional MLP homologues in S. scabies completely abolished production. Engineered expression of all three S. scabies MLPs could restore thaxtomin A production in a triple MLP-deficient strain, while engineered expression of MLPs from other Streptomyces spp. could not. Furthermore, the constructed MLP mutants were reduced in virulence compared to wild-type S. scabies. The results of our study confirm that TxtH plays a key role in thaxtomin A biosynthesis and plant pathogenicity in S. scabies.     

Designing a crop rotation strategy to manage Streptomyces scabies causing potato scab in north India

Common scab of potato caused by various species in the genus Streptomyces has assumed serious proportions in certain potato-growing regions of North India. Although it does not have significant effect on tuber yield, it downgrades quality and reduces marketability. Being soil-borne, this disease is often difficult to manage. Therefore, the present studies were conducted to evaluate different potato-based crop rotations prevalent in north India as a part of a strategy to manage Streptomyces scabies population in scab sick fields. Our results showed that S. scabies population in soil can be effectively reduced either by keeping the field fallow after potato or by including mungbean or sunhemp as green manure, in a single year cropping sequence over a period of four years. Three crop rotations, viz. fallow–rice–potato, mungbean–rice–potato and sunhemp–rice–potato, showed maximum decrease in the population of S. scabies in soil resulting in reduced scab severity. The mungbean- and sunhemp-based crop rotations also enhanced rhizospheric soil microflora especially fluorescent pseudomonads and Trichoderma spp.     

A New Polyacetylene from Solidago altissima L.

A new polyacetylene was isolated from the roots of Solidago altissima L. by column and thin-layer chromatography. The structure of this polyacetylene was identified as methyl 10-(2-methyl-2-butenoyloxy)-cis-2,cis-8-decadiene-4,6-diynoate from the results of its spectroscopic and chemical analyses.

This polyacetylene, as well as dehydromatricaria ester, inhibited the growth of the seedlings of barnyard millet (Panicum crus-galli L. var. frunentaceum Trin.).     

Genome shuffling enhances biocontrol abilities of Streptomyces strains against two potato pathogens

Aims: To employ the genome shuffling technique for improving the phenotype of a biocontrol control agent of the genus Streptomyces. Methods and Results: Two rounds of genome shuffling (GS) were carried out with Streptomyces melanosporofaciens EF‐76, a geldanamycin producer. Six fusants that showed optimized in vitro antagonistic activity against Streptomyces scabies or Phytophthora infestans, two important pathogens of potato crops, were selected. All selected fusants retained the capacity to produce geldanamycin, but none overproduced this antibiotic. The higher antagonism ability appeared to result from a diversification of secreted metabolites. Seven or eight metabolites were detected in the HPLC profiles of parental strains, whereas 12–15 were detected in fusant strains. Biocontrol assays revealed that four of six fusants protected tubers more efficiently than parental strains. Conclusions: GS emerged as an elegant and rapid tool to optimize the antagonistic ability of Streptomyces strains. Optimization of the in vitro antagonistic activity against plant pathogens appears to be an effective approach to select for improved biocontrol agents. The enhanced phenotype did not depend on an overproduction of a specific antibiotic but rather on the secretion of a wider variety of secondary metabolites. Significance and Impact of the Study: Improved capacities of a biocontrol agent compensate for the lack of efficient chemical control of potato scab.     

Bacillus altitudinis strain AMCC 101304: a novel potential biocontrol agent for potato common scab

ABSTRACT

Potato common scab, caused by Streptomyces spp., is one of the leading causes of heavy commercial losses in the potato industry and is thus one of the most serious plant diseases worldwide. This study identified and assessed potential biocontrol agents against potato common scab. In total, 110 isolates were obtained through antagonistic tests; among which, Bacillus sp. strain AMCC 101304 was found to be most effective at inhibiting the potato common scab pathogen, Streptomyces scabies. Bacillus sp. strain AMCC 101304 was finally identified as Bacillus altitudinis by morphological observation, physiological and biochemical experimentation, as well as 16S rRNA sequencing and phylogenetic analysis. Pot experiments were conducted twice (in spring and autumn) to verify the biocontrol effect of B. altitudinis AMCC 101304 against potato common scab. In spring, the control efficiency reached 76.34%. In autumn, the disease incidence was reduced from 100% to 34.19% (one treatment with strain AMCC 101304) and 38.42% (two treatments with strain AMCC 101304), and the control efficiency reached 82.50% (one application) and 78.43% (two applications). The present study demonstrated the potential of an isolate, identified as B. altitudinis AMCC 101304, as an effective biocontrol agent for future use in the field.     

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.     

Effect of Pathogen Isolate, Potato Cultivar, and Antagonist Strain on Potato Scab Severity and Biological Control

Antibiotic-producing Streptomyces spp. have shown potential in biocontrol of potato scab caused by Streptomyces scabies. However, results have been inconsistent among field trials. Pathogen isolate, antagonist strain and potato cultivar were investigated as potential sources of variation in the efficacy of potato scab biocontrol. Biocontrol success varied significantly among pathogen isolates and was not correlated with in vitro sensitivity to antibiotic inhibition. Antagonists also varied in their effectiveness as biocontrol agents, and the relative effectiveness of different antagonists varied among growing seasons. Finally, biocontrol varied among potato cultivars in the field. The diverse origins of significant variation in potato scab biocontrol suggest that consistent control in the field is likely to be difficult to achieve.     

Genome Mining and Molecular Networking Guided Isolation of Antimycin Analogs with Antifeedant Activities from the Deep-Sea-Derived Streptomyces sp. NA13

Polyphagous insects could affect agricultural production, which leads to serious economic losses. Due to the negative effects of synthesized insecticides, finding eco-friendly and new biopesticides is emergent. To develop natural origin insecticides, an integrative approach combining antifeedant activity screening, genome mining, and molecular networking has been applied to discover antifeedant secondary metabolites from Streptomyces sp. NA13, which leads to the isolation of a novel antimycin Q ( 1 ) and six known antimycin analogs (antimycins A1a, A2a, A3a, A4a, A7a, and N-formylantimycic acid methyl ester, 2 – 7 ). Their structures were identified by high-resolution mass spectrometry (HR-MS) and nuclear magnetic resonance (NMR) spectroscopic. The absolute configuration of 1 was elucidated by the comparison of coupling constant, electronic circular dichroism (ECD) analysis, and NMR calculations. 1 – 6 exhibited different levels of antifeedant activities against Helicoverpa armigera, especially 1– 4 . At the same time, the antifeedant activity of antimycin was reported firstly.     

The Relative Importance of Seed-borne Inoculum to Common Scab Disease of Potato and the Efficacy of Seed Tuber and Soil Treatments for Disease Control

The importance of both seed and soil-borne inoculum in the epidemiology of common scab disease under Australian conditions was clearly demonstrated. In field trials the severity of disease in harvested potatoes was directly related to the severity of disease on the planted seed tubers. Chemical seed dressing treatments were assessed for common scab disease control under field conditions in four trials over 5 years. Where seed treatments were applied to both diseased and visibly clean seed pieces significantly more disease was found in tubers harvested from diseased seed than the corresponding clean seed treatment. In all but one trial, the treatments applied to diseased seed significantly reduced the incidence of common scab. Fluazinam, flusulfamide (at elevated rates), fenpiclonil, pentachloronitrobezene and mancozeb seed treatments were particularly effective as seed dressing treatments. Applications to visibly clean seed failed to significantly diminish disease levels below that found on untreated seed. Preliminary investigations of some chemical soil treatments gave disappointing levels of control.     

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.     

Detection of the nec1 virulence gene and its correlation with pathogenicity in Streptomyces species on potato tubers and in soil using conventional and real-time PCR

AIMS: To evaluate the virulence gene nec1 as a reliable marker for the detection of pathogenic Streptomyces species on potato tubers and in soil samples using conventional and real-time quantitative PCR assays. Methods AND RESULTS: Two pairs of conventional primers (outer and nested) and one set of primers/probe for use in real-time PCR were designed to detect the necrogenic protein encoding nec1 gene of Streptomyces scabiei strain ATCC 49173(T). The conventional PCR primers were also incorporated into a multiplex PCR assay to simultaneously detect the nec1 gene in conjunction with the potato pathogens Helminthosporium solani and Colletotrichum coccodes. The specificity of each PCR assay was confirmed by testing 32 pathogenic and nonpathogenic reference strains of Streptomyces representing 12 different species and 74 uncharacterized streptomycete strains isolated from diseased tubers. A clear correlation between pathogenicity and the detection of nec1 by PCR was demonstrated. The sensitivity and specificity of both the conventional and real-time PCR assays allowed the detection of nec1 on potato tubers in the absence of visible symptoms of common scab, and in seeded soil down to a level equivalent to three S. scabiei spores per gram soil. CONCLUSIONS: Reliable and quantitative PCR techniques were developed in this study for the specific detection of the virulence gene nec1 of pathogenic Streptomyces species on potato tubers and in soil samples, and the data demonstrated a clear correlation between pathogenicity in Streptomyces species and the presence of the nec1 gene. SIGNIFICANCE AND IMPACT OF THE STUDY: Together with the DNA extraction protocols, these diagnostic methods will allow a rapid and accurate assessment of tuber and soil contamination by pathogenic Streptomyces species.     

Taxonomic study on nonpathogenic streptomycetes isolated from common scab lesions on potato tubers

Numerical analysis was carried out to compare sixteen nonpathogenic actinomycetes isolated from common scab lesions on potato tubers with Streptomyces scabiei type strain as well as with other streptomycete groups. These isolates were divided into two classes according to their level of similarity with S. scabiei. Isolates resembling S. scabiei were associated with S. griseoruber or with S. violaceusniger while isolates exhibiting less than 61% of similarity with S. scabiei were phenotypically related to S. albidoflavus or to S. atroolivaceus. Sequence of the 16S rRNA gene of each isolate was obtained and compared against the GenBank nucleotide database. No significant match could be established between the sequences of two potato isolates and the ones available in the GenBank database. The other isolates were closely related with S. setonii (S. griseus), S. mirabilis, S. fimbriatus, S. violaceoruber, S. melanosporofaciens and S. thermocarboxydus.