Paula Persson

A study on microbial diversity in different cultivars of Brassica napus in relation to its wilt pathogen, Verticillium longisporum.

Oilseed rape (Brassica napus) is one of the major oilseed crops in the world but is vulnerable to attack by many pathogens and insect pests. In addition to the host plant genotype, micro-organisms present in the rhizosphere and within plant tissues affect the susceptibility to plant pathogens. While rapid progress has been achieved concerning the concept of plant resistance genes, information on the role of the microbial community in plant protection is less apparent. We have studied the endophytic bacterial populations present in different tissues of oilseed rape and also analysed several cultivars (Express, Libraska, Maluka and Westar), which differ in their susceptibility to the wilt pathogen Verticillium longisporum. The population diversity was studied using agar plating assay, fatty acid methyl ester analysis and functional characterisation of isolated strains. Our work shows that already in the seeds there exists diversity in populations as well as in the total microbial load between two of the four tested cultivars. About 50% of the strains isolated from cultivars Express and Libraska showed moderate to strong direct inhibition of V. longisporum. The diversity of the endophytic flora isolated from oilseed rape and its implications in crop protection are discussed.

The composition of bacterial populations in soil fractions differing in their degree of adherence to barley roots

Soil fractions differing in their degree of adherence to barley roots were sampled, and their microbial populations were cultivated on a nutrient-poor bacterial medium at low temperature. The biomass of each population was harvested and analysed to determine its FAME pattern. The greater the degree of adherence of the soil sample to roots, the higher were the relative proportions of the fatty acids 15 : 0, 15 : 0 Iso, 15 : 0 Iso 3OH, 15 : 1 Iso, 17 : 0 Iso 3OH and 17 : 1 Iso. This trend was both pronounced and consistent. The opposite relationship was found between relative proportions of the fatty acids 10 : 0 3OH, 12 : 0, 12 : 0 2OH, 12 : 0 3OH, 16 : 0 and 18 : 1 and degree of root adherence. Differences in the FAME patterns can best be interpreted as differences in the relative proportions of the various types of groups constituting the bacterial populations. Specifically, as the degree of soil adherence to roots increased, there was a shift in favour of bacteria belonging to the Cytophaga‐Flavobacterium group and a corresponding decrease in bacteria with FAME patterns similar to that of Pseudomonas. The distribution of the fatty acid 15 : 0 Anteiso indicated that the relative frequencies of certain Gram-positive bacterial strains with a high content of this acid were diminished in habitats close to roots. # 1999 Elsevier Science B.V. All rights reserved.

Fungicide effects on fungal community composition in the wheat phyllosphere.

The fungicides used to control diseases in cereal production can have adverse effects on non-target fungi, with possible consequences for plant health and productivity. This study examined fungicide effects on fungal communities on winter wheat leaves in two areas of Sweden. High-throughput 454 sequencing of the fungal ITS2 region yielded 235 operational taxonomic units (OTUs) at the species level from the 18 fields studied. It was found that commonly used fungicides had moderate but significant effect on fungal community composition in the wheat phyllosphere. The relative abundance of several saprotrophs was altered by fungicide use, while the effect on common wheat pathogens was mixed. The fungal community on wheat leaves consisted mainly of basidiomycete yeasts, saprotrophic ascomycetes and plant pathogens. A core set of six fungal OTUs representing saprotrophic species was identified. These were present across all fields, although overall the difference in OTU richness was large between the two areas studied.

Evidence for specificity of cultivable bacteria associated with arbuscular mycorrhizal fungal spores.

Bacteria associated with arbuscular mycorrhizal (AM) fungal spores may play functional roles in interactions between AM fungi, plant hosts and defence against plant pathogens. To study AM fungal spore-associated bacteria (AMB) with regard to diversity, source effects (AM fungal species, plant host) and antagonistic properties, we isolated AMB from surface-decontaminated spores of Glomus intraradices and Glomus mosseae extracted from field rhizospheres of Festuca ovina and Leucanthemum vulgare. Analysis of 385 AMB was carried out by fatty acid methyl ester (FAME) profile analysis, and some also identified using 16S rRNA gene sequence analysis. The AMB were tested for capacity to inhibit growth in vitro of Rhizoctonia solani and production of fluorescent siderophores. Half of the AMB isolates could be identified to species (similarity index 0.6) within 16 genera and 36 species. AMB were most abundant in the genera Arthrobacter and Pseudomonas and in a cluster of unidentified isolates related to Stenotrophomonas. The AMB composition was affected by AM fungal species and to some extent by plant species. The occurrence of antagonistic isolates depended on AM fungal species, but not plant host, and originated from G. intraradices spores. AM fungal spores appear to host certain sets of AMB, of which some can contribute to resistance by AM fungi against plant pathogens.

Occurrence of Glomeromycota spores and some arbuscular mycorrhiza fungal species in arable fields in Sweden

Fungi within the phylum Glomeromycota were investigated in arable fields throughout Sweden. Sweden is located between 55° and 69° North. The fungi within this phylum form arbuscular mycorrhizal symbiosis with plant roots. Sampling of soil was carried out to a depth of 30 cm in the rhizosphere. Arbuscular mycorrhizal fungi were found at all 45 sampling sites, at densities between 3 and 44 spores per g dry weight of soil. No significant differences in spore densities were found between different agro-climatic zones or between semi-natural grassland and ploughed fields. Our study revealed that the upper half (0–15 cm) of the soil profiles had significantly more spores than the lower half (15–30 cm). Spores from eight sampling sites were identified from the indigenous soils. Almost 90% were shown to belong to the genus Glomus. The other genera found were Gigaspora and Scutellospora.

Barley rhizobacterial population characterised by fatty acid profiling

The aim of the study was to explore the possibilities for using fatty acid analysis to characterise rhizobacterial populations. Fatty acid profiles were determined for 1188 bacterial isolates originating from barley roots growing in three different soils, two of which were clays and one a silt loam. A multivariate statistical treatment of the fatty acid data revealed three distinct groups of rhizobacteria. For 720 isolates (Group A), almost the whole fatty acid content of the cell consisted of straight-chain fatty acids with an even number of carbon atoms in the chains. In another group (B) of 435 isolates, most of the fatty acids had branched chains with an odd number of carbon atoms. A small group (C) with 33 isolates was mainly characterised by high contents of the fatty acid 15 : 0 Anteiso. The dominating bacterial genera in these three groups were Pseudomonas, Cytophaga and Gram positives, respectively. When roots had grown in clay soils bacteria from Group A were most frequent, Group B dominated in the loam. # 1999 Elsevier Science B.V. All rights reserved.

Cover Crop Residues—Effects on Germination and Early Growth of Annual Weeds

Abstract There is an increasing interest in the use of cover crops in agriculture, in Sweden mainly for the use as catch crops to reduce nitrogen leakage. Some of these crops are known for their allelopathic abilities, which may play a role in the control of weeds and contribute to reduced herbicide use. This study aimed to explore the possible suppressive effect of the cover crop species white mustard, fodder radish, rye, and annual ryegrass on the early growth of the weed species silky windgrass, shepherds-purse, and scentless false mayweed. In a greenhouse experiment using fresh cover crop residues, white mustard was the only crop that showed an effect. It reduced both seedling establishment, by 51 to 73%, and biomass, by 59 to 86%, of shepherds-purse and scentless false mayweed. In contrast, in a growth chamber experiment using frozen material, mean germination time of silky windgrass was extended by 20 to 66% by all cover crops. Also, three out of four cover crops reduced root growth in scentless false mayweed by 40 to 46%, and two out of four cover crops reduced root growth in shepherds-purse by 13 to 61%. However, considering seedling survival, white mustard was the most prominent cover crop, reducing survival by 21 to 57% in shepherds-purse and scentless false mayweed. In this paper we provide evidence that different weed species show different response to different cover crops under climatic conditions prevailing in Scandinavia. Such results emphasize the importance of understanding weed–cover crop interactions as necessary for developing cropping systems that can utilize cover crops to suppress local weed flora. Nomenclature: Annual ryegrass, Lolium multiflorum var. westerwoldicum Wittm. ‘Botrus’; fodder radish, Raphanus sativus L. ‘Adios’; silky windgrass, Apera spica-venti (L.) Beauv.; rye, Secale cereale L. ‘Amilo’; scentless false mayweed, Tripleurospermum perforatum (Mérat) M. Laínz; shepherds-purse, Capsella bursa-pastoris (L.) Medik.; white mustard, Sinapis alba L. ‘Architect’.

Agricultural factors affecting Fusarium communities in wheat kernels

Fusarium head blight (FHB) is a devastating disease of cereals caused by Fusarium fungi. The disease is of great economic importance especially owing to reduced grain quality due to contamination by a range of mycotoxins produced by Fusarium. Disease control and prediction is difficult because of the many Fusarium species associated with FHB. Different species may respond differently to control methods and can have both competitive and synergistic interactions. Therefore, it is important to understand how agricultural practices affect Fusarium at the community level. Lower levels of Fusarium mycotoxin contamination of organically produced cereals compared with conventionally produced have been reported, but the causes of these differences are not well understood. The aim of our study was to investigate the effect of agricultural factors on Fusarium abundance and community composition in different cropping systems. Winter wheat kernels were collected from 18 organically and conventionally cultivated fields in Sweden, paired based on their geographical distance and the wheat cultivar grown. We characterised the Fusarium community in harvested wheat kernels using 454 sequencing of translation elongation factor 1-α amplicons. In addition, we quantified Fusarium spp. using real-time PCR to reveal differences in biomass between fields. We identified 12 Fusarium operational taxonomic units (OTUs) with a median of 4.5 OTUs per field. Fusarium graminearum was the most abundant species, while F. avenaceum had the highest occurrence. The abundance of Fusarium spp. ranged two orders of magnitude between fields. Two pairs of Fusarium species co-occurred between fields: F. poae with F. tricinctum and F. culmorum with F. sporotrichoides. We could not detect any difference in Fusarium communities between the organic and conventional systems. However, agricultural intensity, measured as the number of pesticide applications and the amount of nitrogen fertiliser applied, had an impact on Fusarium communities, specifically increasing the abundance of F. tricinctum. There were geographical differences in the Fusarium community composition where F. graminearum was more abundant in the western part of Sweden. The application of amplicon sequencing provided a comprehensive view of the Fusarium community in cereals. This gives us better opportunities to understand the ecology of Fusarium spp., which is important in order to limit FHB and mycotoxin contamination in cereals.

Genus-Specific Primers for Study of Fusarium Communities in Field Samples.

ABSTRACT Fusarium is a large and diverse genus of fungi of great agricultural and economic importance, containing many plant pathogens and mycotoxin producers. To date, high-throughput sequencing of Fusarium communities has been limited by the lack of genus-specific primers targeting regions with high discriminatory power at the species level. In the present study, we evaluated two Fusarium-specific primer pairs targeting translation elongation factor 1 (TEF1). We also present the new primer pair Fa+7/Ra+6. Mock Fusarium communities reflecting phylogenetic diversity were used to evaluate the accuracy of the primers in reflecting the relative abundance of the species. TEF1 amplicons were subjected to 454 high-throughput sequencing to characterize Fusarium communities. Field samples from soil and wheat kernels were included to test the method on more-complex material. For kernel samples, a single PCR was sufficient, while for soil samples, nested PCR was necessary. The newly developed primer pairs Fa+7/Ra+6 and Fa/Ra accurately reflected Fusarium species composition in mock DNA communities. In field samples, 47 Fusarium operational taxonomic units were identified, with the highest Fusarium diversity in soil. The Fusarium community in soil was dominated by members of the Fusarium incarnatum-Fusarium equiseti species complex, contradicting findings in previous studies. The method was successfully applied to analyze Fusarium communities in soil and plant material and can facilitate further studies of Fusarium ecology.

Blackleg and Stem Rot of Potatoes in Sweden

Abstract Samples of potato plants showing symptoms of basal soft rot (blackleg) and aerial soft rot (stem rot) were collected from potato fields in Sweden during 1982–1984. Erwinia carotovora subsp. atroseptica (Eca) was the predominant species isolated from blackleg stems while both Eca and E. carotovora subsp. carotovora (Ecc) was isolated from aerial stem rot symptoms. Water samples were also collected from a stream and a reservoir used for irrigating potato fields. Potato pathogenic Ecc was found in the water. The cool temperature of Swedish potato soils is a major factor influencing that Eca is the dominant cause of blackleg. It is of importance though to notice that both Eca and Ecc can cause stem rot in warmer and more humid periods.