Purpose Common scab of potatoes (CS) is influenced by plant-microbe-soil interactions, which are locally specific. The study aims to identify any common trends in such interactions across a landscape differentiated by soil pH and CS severity. Methods The 33 sites and 17 cultivars were characterized using soil and periderm nutrient contents and microbial communities. Quantitative PCR and Illumina amplicon sequencing were used to assess abundance of bacteria, actinobacteria and pathogens, and community composition. Results Comparisons between bulk and tuberosphere soil compartments as well as potato cultivars divided to three categories of CS susceptibility revealed that nitrogen was elevated in tuberosphere soil and N, Mg and Fe were lowered in periderm of resistant cultivars. The susceptible cultivar Agria grown at 7 sites had higher Ca content in tuberosphere soil, while the resistant cultivar Adela grown at 10 sites had higher S, P and Mg contents in its tuberosphere soil and P and Fe in periderm. That suggests further interactions between plants and bacterial community involving nutrient uptake. Diversity of bacteria was positively correlated with CS severity suggesting interactions between the Streptomyces pathogen populations and the local soil community. Conclusions Overall, pathogen abundance assessed by quantifying the thaxtomin biosynthetic txtB genes were randomly dispersed among the sites without connections to CS severity or soil pH. Thus, the significant differences between bacterial communities of bulk and tuberosphere soils together with cultivar CS susceptibility showed that the susceptible cultivars select bacterial community relatively similar to the bulk soil, while the resistant cultivars promote more distinct communities.

Control of common scab disease can be reached by resistant cultivars or suppressive soils. Both mechanisms are likely to translate into particular potato microbiome profiles, but the relative importance of each is not known. Here, microbiomes of bulk and tuberosphere soil and of potato periderm were studied in one resistant and one susceptible cultivar grown in a conducive and a suppressive field. Disease severity was suppressed similarly by both means yet, the copy numbers of txtB gene (coding for a pathogenicity determinant) were similar in both soils but higher in periderms of the susceptible cultivar from conducive soil. Illumina sequencing of 16S rRNA genes for bacteria (completed by 16S rRNA microarray approach) and archaea, and of 18S rRNA genes for micro-eukarytes showed that in bacteria, the more important was the effect of cultivar and diversity decreased from resistant cultivar to bulk soil to susceptible cultivar. The major changes occurred in proportions of Actinobacteria, Chloroflexi, and Proteobacteria. In archaea and micro-eukaryotes, differences were primarily due to the suppressive and conducive soil. The effect of soil suppressiveness × cultivar resistance depended on the microbial community considered, but differed also with respect to soil and plant nutrient contents particularly in N, S and Fe.

Connections between the structure of bacterial communities in suppressive soils and potato resistance to common scab (CS) are not well understood. In this study, one resistant and one susceptible cultivar were grown in a conducive and suppressive field to assess cultivar resistance x soil suppressiveness interactions. The resistant cultivar had a higher Mg content in periderm compare to susceptible cultivar, while suppressive soil had lower pH (5.3 vs 5.9), N, C, P, Ca contents but higher Fe and S compared with the conducive soil. Bacteria and actinobacteria numbers were higher in the conducive soil. Copy numbers of txtB gene (coding for a pathogenicity determinant) were similar in both soils but were higher in the conducive soil (for periderm samples) and in the susceptible cultivar (for conducive soil samples). Taxonomic microarray analysis and Illumina sequencing of 16S rRNA genes amplicon showed that bacterial community differed between resistant vs susceptible cultivar and to a lesser extend between suppressive vs conducive soil. Bacteria participating in soil suppression belonged to Pseudomonadaceae, Bradyrhizobiaceae, Acetobacteraceae and Paenibacillaceae, while resistant cultivars selected a bacterial community resembling that of the suppressive soil, which was enriched in Nitrospirae and Acidobacteria. Thus, the analysis of soil suppressiveness x cultivar resistance interactions enabled to gain new insight to CS control in the field.

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.

Common scab (CS) severity was assessed in relationship to mineral contents of potato periderm in 18 cultivars varying in maturity period and CS susceptibility. Two field experiments, one repeated in 3 years, were conducted. The effect of site, cultivar and growing period on the disease severity was always significant but not that of year to year variability. Zn, Mn and Fe periderm contents were related to site, while Ca content to cultivar effects. In both experiments, the CS severity was positively correlated to calcium and negatively to phosphorus periderm content, while correlations of CS severity to other nutrients were dependent on site or year. Since soil pH, Ca and P soil contents were not different between sites, relatively small differences in other soil chemical characteristics combined with specific cultivar nutrient requirements seemed to determine the CS severity.