SummaryThere is a drive to improve the sustainability of agricultural systems including the biodiversity component. Cultivar mixtures offer yield benefits from the same land area, but the mechanisms behind this overyielding have not been completely worked out. One potential mechanism is improved competition with weeds. We use an experimental approach of varying barley (Hordeum vulgare L.) genotypic and phenotypic diversity to test the hypothesis that increases in diversity have an impact on weed growth strategies and community assembly, or if responses are driven by barley performance. There was no effect of increasing barley mixture diversity on weed traits, either in terms of species means or the community‐weighted mean. However, Functional Richness of the weed community decreased with increasing barley mixture diversity mainly as a result of reduced specific leaf area Functional Richness. This pattern was driven by a reduction in species richness of the weed community rather than by reduced variation within species. Whilst barley phenotype had different impacts on weed traits, there were no specific mixtures of phenotypes or genotypes that had consistent effects on community assembly or weed species responses. The competitive exclusion of weed species could have agronomic and environmental benefits, through better targeting or less frequent use of herbicides. Growing crop mixtures is one of many strategies available to improve agricultural sustainability and resilience, and one that has clear benefits.

Abstract Cultivar mixtures of winter barley and spring barley, together with their component monocultures, were grown in field trials to assess the effect of cultivar combinations on both straw and grain yield. The overall grain yields for all trials were significantly higher for the cultivar mixtures than for the corresponding component monocultures. Also, significant decreases in rhynchosporium disease severity for cultivar mixtures were recorded for most non-fungicide treatments. The size of these responses was often significantly correlated with the component number of the mixtures. The amount of straw produced in mixtures was sometimes changed significantly, but not always in a positive direction and it was only correlated with increasing mixture component number in two environments. No correlation of straw yield potential of cultivars with performance in mixtures was found. Cultivar × cultivar mixture × environment interactions appeared to affect the relative yield of grain and straw differentially and therefore it was not possible to predict the effect of mixtures on the harvest index.

SUMMARYA range of wheat cultivars, including elite cultivars, older cultivars and some preferred by organic growers, were trialled under high and low nitrogen (N) conventional and organic conditions to determine whether cultivars that yield highly under organic conditions have the same relative yield under conventional conditions. A range of cultivar mixtures was also assessed to see whether these gave yield advantages or superiority in either farming system. The conventional trials were grown with and without full fungicide programmes, which largely controlled disease. Amongst the cultivars, Alchemy showed superior yield under organic conditions as did Pegassos, but under conventional conditions Pegassos was always one of the low-ranking cultivars. Under conventional conditions the more recent cultivars Alchemy, Glasgow and Istabraq yielded highly, while an older one, Consort, yielded highly under low fertilizer conditions, and both Ambrosia and Deben also yielded highly generally. Fungicide and high N favoured the disease-susceptible, high-yield cultivars such as Glasgow whereas Consort, an older susceptible cultivar, was favoured by fungicide and low N. Together this demonstrates that whilst the yield characteristics of some elite germplasm are also expressed under organic conditions, at least one cultivar that yielded poorly under conventional conditions showed adaptation towards the organic conditions of these trials. Other cultivars yielding poorly under conventional conditions also gave poor yield under organic conditions. The equal proportion mixtures of cultivars grown under conventional conditions showed no evidence of differences in yield from the mean of the component cultivars grown separately, but combinations of Glasgow, Alchemy and Istabraq gave consistently high yield.

Cereal farming is moving rapidly towards reduced tillage, with over 100 million ha of land currently tilled using minimum tillage implements. This study reports five years of data from a field experiment investigating the response of different barley cultivars and mixtures to soil tillage practice and nitrogen fertiliser levels. Five tillage treatments were established that imposed different amounts of :;oil disturbance: (T1) zero tillage, (T2) minimum tillage to 7 cm depth and ploughed treatments followed by power harrowing consisting of (T3) conventional plough to 20 cm depth, (T4) plough to 20 cm followed by compaction by wheeling the entire plot with a tractor fitted with 8.8 Mg total load and (T5) deep plough to 40 cm depth, all on the same site each year. Four winter barley cultivars (Sumo, Fanfare, Pastoral and Pipkin) were selected based on contrasting rooting characteristics, disease resistance and yield sensitivity. They were planted in plots as monocultures and as all 2-, 3- and 4-component mixtures thereof. Significant differences in soil physical properties and carbon content were observed over the five years of the study. Grain yield varied by 13% between tillage treatments, with conventional and deep plough conditions generally the highest yielding and zero tillage the lowest. Sumo gave the highest yield overall under deep plough conditions, whereas Pipkin was the best cultivar under conventional and zero tillage conditions. Rhynchosporium was the most common disease and the mixture gave decreased infection in all years and tillage conditions. Complex mixtures gave around 32% less disease than the simple mixtures. There was an overall differential cultivar response to soil tillage conditions that was buffered by cultivar mixtures. Mixtures offered benefits in both yield response and disease control under all soil tillage conditions. (C) 2011 Elsevier B.V. All rights reserved.

The effects of field-trial scale, including plot size, and its interaction with fertiliser level and barley germplasm on mixture efficacy in controlling powdery mildew were investigated. Two groups of cultivars, one from cultivars grown in the UK and the other from cultivars grown in Poland, along with all their respective three-component mixtures, were grown in three field trials, one with 13.5 m(2) plots, one with 0.4m(2) plots, and the third in intermediate size plots, which included a mixture of all the cultivars in both groups in equal size treatments of structured spatial arrangements. Another trial utilised similar structured spatial arrangements to trial 3 was carried out using a combination of cultivars with appropriate matching virulence levels (trial 4). Mixtures showed either a reduction in powdery mildew infection compared with the component monoculture mean, or no significant effect. There was a trend towards greater reductions at the low fertiliser level and smaller plot size. The most structured spatial arrangement was most effective for reducing infection in trials 3 and 4, but the complex-homogeneous arrangement in the fourth trial was also effective. Rhynchosporium was reduced by a similar and substantial amount by both spatial treatments in trial 3. The plot size and structuring effects may be explained by the interaction between host heterogeneity structure and pathogen dispersal scale effects, but resource exploitation for yield was best in the complex-homogeneous arrangement in trial 3 where all components were present to respond directly. These findings have implications for methods of trialling mixtures for large-scale field use and for testing component combining ability. (C) 2011 Elsevier B.V. All rights reserved.

An experimental field infested with Phytophthora fragariae var. fragariae (Pff) and used for strawberry red core fungicide and cultivar resistance trials until 1981 was surveyed for the presence of inoculum of the pathogen 11 and 12 years later. Alpine strawberries, highly susceptible to all races of Pff, were grown from true seed and planted as a bait crop on a 0·5 m‐spaced grid. Rapid and widespread red core infection was observed, which provided good evidence that oospores had survived in soil for this extended period. Site elevation and the distribution of red core infected plants showed a strong correlation, with a higher frequency of infected and dead plants in the lowest areas of the field. The race designation of 18 recovered isolates were determined and AFLP fingerprint patterns of some of these and their single‐spore derivatives were analysed. The isolates differed little in race type, and the majority were genetically identical at 433 AFLP loci. Races used to inoculate the site in the 1970s were recovered. The fingerprints of the single variant isolate matched that of an isolation made by Hickman in the 1950s, originally used to inoculate the site. Clearly Pff is a very stable and long‐lived pathogen able to retain its genetic integrity and lie dormant in soil for many years, ensuring its survival between epidemiologically favourable conditions which occur erratically.

Three winter barley cultivars were mixed in equal proportions but in different ways to give different patterns of spatial heterogeneity. The mixtures were sown in large field plots over three successive years and disease severity and yield were compared between the mixtures and the mean of the components grown as monocultures. Most mixtures significantly reduced disease in all years. The mixture composition, which appeared to generate a discrete pattern of small patches of the component cultivars, gave a yield advantage in 2 years, while the mixture which was pre-mixed most homogeneously gave no significant yield advantage in these trials. The cost of homogeneous mixing is therefore unlikely to be recovered in increased yields, compared with a simpler, imprecise mixing of the components in the seed hopper prior to sowing.

Barley grown under constant temperature and light regimes in controlled environment cabinets may not respond to pathogen challenge in the same way as under variable, 'stressed' environments. We demonstrate that variable temperature regimes increase levels of inducible resistance in barley to powdery mildew, and that different genotypes have differing levels of inducible resistance. This has implications for gene discovery and expression studies of plant resistance mechanisms, and for field evaluation of resistance.

A range of mixtures including one set made from cultivars grown in the UK and one from cultivars grown in Poland, were included, along with their component cultivars, in nine trials at Scottish Crop Research Institute, Dundee, UK or at the Experimental Plant Breeding Station of the IHAR, Baków near Kluczbork, Poland, over five seasons. The effects of inoculum pressure, plot size, fertiliser level and germplasm on mixture efficacy were investigated. In the four trials where inoculum pressure was controlled, mixtures reduced infection more at lower inoculum pressures, but this did not translate into yield benefit. Smaller plots increased mildew in monocultures but not mixtures. Fertiliser levels increased mildew levels but did not affect mixture efficacy. There were large differences between both Polish and UK germplasm, and between Polish and UK trial sites, but the performance of the mixtures compared with their respective monoculture components was similar within both germplasm groups and trial sites. Mixtures reduced lodging and affected plant height and heading date. The advantages of mixtures for improving yield, reducing fungicide applications and improving agronomic characteristics was demonstrated and there seems to be great potential for their further improvement and exploitation.

A local population of Rhynchosporium secalis was sampled and the race structure, pathogenicity, RAPD profile and rDNA ITS sequence examined. RAPDs were also used to compare this population with a larger sample of isolates including some isolated more than 30 years ago. Both the local population and the wide sample revealed much variation for all characteristics with no two isolates identical. The reliability of detached leaf pathogenicity tests was improved over previous studies by pre-washing spores and brushing the leaves prior to inoculation. Isolate aggressiveness was highly correlated with lesion number but not lesion size. There was no correlation between race and aggressiveness. RAPD profiles showed high levels of variability but more similarity was observed within a site than between sites. Clusters of similarity were primarily associated with samples from the same locality but samples taken 8 years apart from within the same field were not closely related. ITS2 sequences were all identical whereas ITS I sequences showed 0.8 % polymorphism, ITS polymorphisms fell mostly within one RAPD cluster. ITS sequence comparisons with the EMBL. database gave sonic indications of the taxonomic placement of R. secalis.

Two sets of barley cultivar mixtures, one from cultivars grown in the UK and one from cultivars grown in Poland, were included, along with their component cultivars, in trials at the Scottish Crop Research Institute over two seasons. In the second year, two levels of nitrogen fertilisation were compared. Laboratory scale malting revealed three mixtures with extracts equal to, or significantly higher than, those of all of their components. Increased nitrogen fertilisation gave higher diastatic power, but reduced hot water extract in mixtures and component cultivars. Polish mixtures and their component cultivars showed a higher Kolbach index but a slower rate of filtration, following malt extraction, than their UK counterparts. It was concluded that the malting performance of the mixtures was largely determined by the nature of the germplasm from which they were constructed and the conditions under which they were grown. Careful selection of components should, therefore, permit development of barley mixtures acceptable for malting.

The effect of inoculum pressure on disease tolerance was studied on a range of spring barley genotypes by manipulating the quantity of spores being deposited on field plots from surrounding plots. Genotypes included previously selected tolerant and non-tolerant types and the modifying effects of fertilizer levels were also studied. Under low fertilizer and 'normal' inoculum pressure and especially high fertilizer and high inoculum pressure conditions, where host and pathogen development were likely to be near equilibrium, designation of barley genotypes as tolerant or non-tolerant was more reliable than under the other conditions tested. Single trials were not reliable for designating tolerance.

A range of winter barley cultivars was grown. either as pure stands or as components of mixtures, in a trial in 1995-96. For malting quality characters, mean values of mixtures did not differ from mean values of the appropriate monocultures. except for decreases in homogeneity, as determined by a fluorescence test of cell wall modification. When disease pressure was modified by fungicide, hot water extract was not significantly altered, although this may have been due to the restriction of malting to grain retained by a 2.5 mm sieve. One mixture, comprised of three related winter malting cultivars, gave higher hot water extracts than its components, as pure stands, with no adverse effects on homogeneity. This was not simply attributable to lower grain nitrogen content.

This study, carried out under field conditions, assessed the extent to which temporary breakdown of mlo-resistance, following relief of water-stress, was determined by genetic background and mlo allele. Commercial barley cultivars expressing the mlo gene for resistance to powdery mildew (Erysiphe graminis (Blumeria graminis) f. sp. hordei) were tested as well as doubled haploid progeny from spring barley genotypes, a proportion of which were sown in the field in two successive years. Plants were protected from natural rain by a mobile rain shelter and either watered by trickle-irrigation or allowed to dry. Percentage mildew infection resulting from natural inoculum was recorded and the doubled haploid genotypes were classified as resistant, intermediate or susceptible on the basis of their control (watered) treatment scores. In each of the three designated classes, particular genotypes developed infection levels, following relief of water-stress, that were higher than those observed on the well-watered controls. This was found not to be related to the mlo allele as there was no significant difference between the increases observed on resistant plants carrying mlo9 and resistant plants carrying mlo11. Differences in the degree of breakdown of resistance were attributed to genetic background rather than to the specific mlo allele.