The factors influencing the spatial distribution of fungal communities are commonly examined over large spatial scales but not at smaller scales. Given this, the extent to which soil properties and topographic features contribute to the diversity and distribution of fungal communities in an agricultural field needs to be further explored. We investigated the spatial distribution of soil fungal community composition from an similar to 1100 m long transect with 83 sampling points in a commercial potato field with a rolling landform. The relative abundance of Ascomycota, Basidiomycota, and Mortierellomycota showed medium to strong spatial dependence with an autocorrelation range varying from similar to 43 to 92 m, similar to the autocorrelation range of soil properties and topographic features. Most of the variability in fungal and saprotrophic community composition was explained by soil properties (15% and 11%, respectively) and spatial distance (16% and 15%, respectively) while topographic features contributed 8% and 5% of variability to total fungi and saprotrophic community composition, respectively. The fungal and saprotrophic community compositions were correlated with soil organic carbon, pH, and slope curvature, however, richness and Pielou's evenness of the fungal communities and fungal biomass were not correlated with soil properties or topographic features. The results suggest that the spatial variation in fungal and saprotrophic community composition in response to soil properties and topographic features in this agricultural landscape was due to differences in assemblages of fungal amplicon sequence variants (ASVs) but not in differences in the number of fungal ASVs or fungal biomass measured using phospholipids fatty acids.

Chemical fumigation is used to reduce soil-borne diseases in agricultural production systems; however, non -targeted soil microorganisms may also be affected. This study investigated the effects of chemical fumigation and substrate carbon (C) availability on the soil bacterial and fungal community diversity under controlled conditions over 128 days. This study consisted of a 3 x 3 factorial arrangement of three fumigant treatments (fumigation with chloropicrin [CP], metam sodium [MS], or no fumigation) and three soil amendment treat-ments (amendment with young barley, mature barley, or no amendment). MS fumigation transiently decreased bacterial species evenness when combined with young barley residues; however, it did not affect fungal diversity indices. CP fumigation, regardless of soil amendment or substrate C availability, decreased bacterial species evenness and richness that did not recover over time. However, CP fumigation only decreased fungal species evenness and richness when combined with young or mature barley residues. Although all treatments resulted in a bacterial and fungal community that was significantly dissimilar to the non-fumigated unamended soil, CP fumigated soils had the most dissimilar bacterial and fungal 0-diversity after 128 days. This study demonstrated that the addition of young or mature barley residues to chemically fumigated soil did not recover microbial diversity. Instead, the addition of plant residues to chemically fumigated soil had a greater impact on microbial diversity and community composition compared to chemical fumigation used alone, subsequently promoting a less diverse and selective community for both fumigation and organic C additions.

Chemical fumigation and biofumigation are used to reduce soil-borne diseases in agricultural production systems; however, non-targeted soil microorganisms may also be affected. This study compared the effects of chemical fumigation, either used alone or combined with an organic amendment, and biofumigation on soil bacterial community diversity and composition under controlled conditions over 160 days. Treatments included: fumigation with chloropicrin (CP), fumigation with metam sodium used alone (MS) or combined with barley plant residues (MSBR); biofumigation with mustard plant residues; addition of barley plant residues; and untreated control. Biofumigation had a greater impact on bacterial diversity at early time points, transiently decreasing species evenness and yielding the most dissimilar β-diversity after 3 days. MS fumigation did not affect bacterial diversity indices; however, MSBR transiently decreased species evenness after 8 days. CP-treated soil had decreased species evenness that did not recover over time and had the most dissimilar β-diversity at the end of the incubation compared to all other treatments. This study demonstrated that CP fumigation had the greatest and most persistent impact on bacterial diversity, whereas MS fumigation and biofumigation led to transient decreases in bacterial diversity.

Potato cultivars susceptible to common scab were previously reported to harbor five to six times more abundant pathogenic Streptomyces spp. in the rhizosphere soils compared with tolerant cultivars. It is still unclear if the diversity of soil bacterial communities is related to the abundance of pathogenic Streptomyces spp. This study evaluated the effects of potato cultivar on the diversity of bacterial communities in three spatial locations (soil located close to the plant [SCP], in the rhizosphere soil [RS], and in the geocaulosphere soil [GS]) in 2013 and 2014. Common scab tolerant (Goldrush and Hindenburg) and susceptible cultivars (Green Mountain and Agria) were planted in a field infested with pathogenic Streptomyces spp. causing common scab. The β-diversity of the bacterial community was significantly different between years and on dates within each year according to a permutational multivariate analysis of variance. The β-diversity also varied significantly among spatial locations (i.e., SCP, RS, and GS), probably due to changes in soil properties, but did not change significantly among potato cultivars. The architecture of the bacterial network in RS in 2014 was more complex compared with 2013 with a 2.5-fold increase in the number of bacteria included according to a co-occurrence analysis. These results indicated that the soil bacterial community diversity changed temporally and spatially. However, bacterial community diversity and richness were not affected by potato cultivar, suggesting that there were no relationships between bacterial community diversity or richness and the abundance of pathogenic Streptomyces spp.

Topography drives spatial variation of soil edaphic factors at the landscape scale however, it is unclear how it influences the spatial distribution of bacterial communities in distances relevant to agro-ecosystem management. This study examined the influence of soil physico-chemical properties and topographic features on bacterial communities and diversity in a commercial potato field with a rolling landform. Eighty-three soil samples were systematically collected across a transect 1100 m long. A significant negative correlation (r = -0.73) between soil pH (range 4.3-7.0) and slope gradient (range 1.8-11.9%) was observed. Regressions and/or a canonical correspondence analysis showed that pH, slope gradient and organic carbon were the major factors influencing bacterial a-diversity based on 16S rRNA gene sequences. Semivariogram analyses revealed that the bacterial adiversity, the relative abundance of most phyla, pH and slope gradient showed strong to medium spatial autocorrelations with a range between 20.8 and 217.8 m. These results evidenced that soil pH and slope gradient were the major factors explaining variation in the spatial structure of the bacterial community. Our results showed that the soil bacterial communities varied in a systematic and predictable pattern in an agricultural field in response to variation in soil physico-chemical properties and topographic features.

Potato cultivars vary in their tolerance to common scab; however, how they affect common scab-causing Streptomyces spp. populations over time is poorly understood. This study investigated the effects of potato cultivar on pathogenic Streptomyces spp. abundance, measured using quantitative PCR, in three spatial locations in a common scab-infested field: (i) soil close to the plant (SCP); (ii) rhizosphere soil (RS); and (iii) geocaulosphere soil (GS). Two tolerant (Gold Rush, Hindenburg) and two susceptible cultivars (Green Mountain, Agria) were tested. The abundance of pathogenic Streptomyces spp. significantly increased in late August compared with other dates in RS of susceptible cultivars in both years. Abundance of pathogenic Streptomyces spp., when averaged over locations and time, was significantly greater in susceptible cultivars compared with tolerant cultivars in 2014. Principal coordinates analysis showed that SCP and RS soil properties (pH, organic carbon, and nitrogen concentrations) explained 68 and 76% of total variation in Streptomyces spp. abundance among cultivars in 2013, respectively, suggesting that cultivars influenced common scab pathogen growth conditions. The results suggested that the genetic background of potato cultivars influenced the abundance of pathogenic Streptomyces spp., with five to six times more abundant Streptomyces spp. in RS of susceptible cultivars compared with tolerant cultivars, which would result in substantially more inoculum left in the field after harvest.

The ideal marker system for tetraploid potato would be dosage-sensitive and have the ability to distinguish heterozygous genotypes with multiple haplotypes within the genomic region targeted by the marker. The objective of this study was to evaluate the utility of high-resolution DNA melting (HRM) for genotyping and polymorphism detection in diploid and tetraploid potato. Amplicon scanning, unlabelled probe, and short amplicon assays were developed for four candidate genes affecting tuber skin and flesh colour, and starch, and a marker linked to nematode resistance. Genotyping a set of 95 potato clones revealed several examples of clones with three distinct haplotypes. Combined probe and amplicon analysis identified between 29 and 44 unique genotypes for the same assays. Assays developed for four of the five target genes are suitable for marker-assisted selection in potato breeding programs. This study illustrates the use of HRM in potato genetics. Further advances in the technology and associated data analysis should make HRM a useful tool for basic and applied studies of potato.