Genotypic diversity and migration of clonal lineages of Botrytis cinerea from chickpea fields of Bangladesh inferred by microsatellite markers

An analysis of allelic diversity at nine microsatellite loci provided an insight into the population structure of Botrytis cinerea from four fields (sampled in 2003 and 2004) that represented important regional locations for chickpea production in Bangladesh. Although three populations were limited by sample size after clone‐correction, a total of 51 alleles were amplified among 146 B. cinerea isolates from Bangladesh, which revealed a high amount of within‐population and overall genetic diversity (H_S = 0·48 and H_T = 0·54, respectively). The percentage of maximal genotypic diversity (G) ranged between populations (G = 23–40), with a total of 69 haplotypes detected (G = 25). Bayesian cluster analysis depicted two major clusters distributed among the four Bangladesh populations, indicating population admixture from two origins that have spread throughout these regions. Genotype flow between regions was detected and indicated the spread of clonal lineages, consistent with relatively low differentiation among the four populations (mean G_ST = 0·1, P < 0·05). These results highlighted the potential threat of host resistance breakdown as a result of considerable genetic diversity, genotype flow and the evolutionary potential of B. cinerea.     

SSR-based genetic linkage analysis of resistance to crown rust (Puccinia coronata f. sp. lolii) in perennial ryegrass (Lolium perenne)

Crown rust (caused by Puccinia coronata f. sp. lolii) is a serious foliar disease of the pasture and turfgrass perennial ryegrass (Lolium perenne). Previous genetic studies have detected both qualitative and quantitative resistance mechanisms, and interpretation of the genetic system is complicated by variation within the sexually reproducing pathogen. Resistant and susceptible parental genotypes of ryegrass were identified using a composite urediniospore population collected from three geographically distinct locations. A two-way pseudo-testcross mapping population was obtained as the F₁ progeny of the pair-cross between ryegrass parental genotypes Vedette₆ and Victorian₉. Both parents showed intermediate resistance against a pathogen population collected in a single geographical zone (Hamilton, Victoria), but in the F₁ population, significant variation for a range of resistance-associated characters was detected. Statistical analysis of phenotypic data suggested a major gene effect, hence bulked segregant analysis with map-assigned simple sequence repeat (SSR) markers was used to scan the genome. A marker showing strong association with resistance was assigned to linkage group (LG) 2 of perennial ryegrass. Analysis of 11 LG2 SSR markers defined an interval between loci xlpssrh03f03 and xlpssrk02e02 as containing the gene or genes (LpPc1) conferring crown rust resistance. Resistance gene determinants were inherited from both parents, with up to 80% of the total phenotypic variation explained by markers segregating from Vedette₆ and up to 26% of the variation explained by markers segregating from Victorian₉. The two contributions together resulted in an additive increase in effect, with fully resistant individuals requiring determinants from both parents. A conserved syntenic relationship was observed with linkage group B of Avena strigosa, which is the location of a cluster of resistance genes to the oat form of crown rust. The implications of this study for marker-assisted selection of disease resistance in perennial ryegrass are discussed.     

Recovery of fungicide-resistant <Emphasis Type="Italic">Botrytis</Emphasis> isolates from strawberry nursery plants

In a four-year survey of strawberry nursery plants, 340 Botrytis isolates were collected and examined for fungicide resistance. High percentages of isolates with resistance to members of all fungicide classes registered on strawberries in Germany were found, i.e. trifloxystrobin (90.3%), boscalid (53.8%), cyprodinil (41.5%), fludioxonil (28.2%) and fenhexamid (15.6%). Of these isolates, 10.3% possessed multiple resistance to all compounds. At 5.0%, resistance to the recently registered fungicide fluopyram was low. Dutch nursery material harboured significantly higher proportions of isolates with resistance to any or all fungicides than plants produced in Germany. These data point to nursery plants as a possible route of introducing fungicide-resistant Botrytis strains into commercial strawberry fields.     

Spatial genetic structure and dispersal of the cacao pathogen Moniliophthora perniciosa in the Brazilian Amazon

Moniliophthora perniciosa is the causal agent of witches’ broom in Theobroma cacao (cacao). Three biotypes of M. perniciosa are recognized, differing in host specificity, with two causing symptoms on cacao or Solanaceae species (C‐ and S‐biotypes), and the third found growing endophytically on lianas (L‐biotype). The objectives of this study were to clarify the genetic relationship between the three biotypes, and to identify those regions in the Brazilian Amazon with the greatest genetic diversity for the C‐biotype. Phylogenetic reconstruction based on the rRNA ITS regions showed that the C‐ and S‐biotypes formed a well‐supported clade separated from the L‐biotype. Analysis of 131 isolates genotyped at 11 microsatellite loci found that S‐ and especially L‐biotypes showed a higher genetic diversity. A significant spatial genetic structure was detected for the C‐biotype populations in Amazonia for up to 137 km, suggesting ‘isolation by distance’ mode of dispersal. However, in regions containing extensive cacao plantings, C‐biotype populations were essentially ‘clonal’, as evidenced by high frequency of repeated multilocus genotypes. Among the Amazonian C‐biotype populations, Acre and West Amazon displayed the largest genotypic diversity and might be part of the centre of diversity of the fungus. The pathogen dispersal may have followed the direction of river flow downstream from Acre, Rondônia and West Amazon eastward to the rest of the Amazon valley, where cacao is not endemic. The Bahia population exhibited the lowest genotypic diversity, but high allele richness, suggesting multiple invasions, with origin assigned to Rondônia and West Amazon, possibly through isolates from the Lower Amazon population.     

Soil carbon increases with long-term cattle stocking in northern temperate grasslands

Grassland management aimed at enhancing carbon (C) in soil is an important tool in mitigation of rising atmospheric CO₂, yet little is known of how grassland soil C changes with livestock stocking rate (SR). We relate soil organic and inorganic C mass (t ha⁻¹ to 60 cm depth) with cattle stocking over periods of 7–27 year for 32 paddocks distributed across nine community pastures in the mixed-grass prairie of Saskatchewan, Canada. Initial analysis comparing Akaike information criterion models showed that cattle SR explained a greater proportion of variance in soil C, particularly soil organic C, than rainfall. Soil organic C mass increased with cattle SR (R² = .293; p = .001), even when the latter was normalized to account for differences in vegetation composition and growing conditions among pastures. Normalized SR varied from 0.49 to 2.30 times recommended levels, over which SOC increased from 24.7 to 57.4 t ha⁻¹. Increases in soil organic C under greater stocking coincided with increased abundance of introduced vegetation, particularly the rhizomatous grass Poa pratensis. Inorganic soil C accounted for 34.6% of total soil C, being particularly large below 30 cm soil depth, but did not vary with stocking rate. These findings indicate that both organic and inorganic C are important pools of C in northern temperate grassland soils, with soil organic C positively associated with long-term cattle SR. Further studies are recommended to understand more fully the mechanisms regulating grazing impacts on soil C mass in northern temperate grasslands.     

河套盐碱地不同利用方式土壤盐碱化特征差异分析

为研究河套平原盐碱地在不同利用方式对土壤盐碱化特征分布的影响,通过野外调查取样和室内分析相结合的方法,对5种土地利用方式下(林地、农用地、牧草地、改良地、盐荒地)不同深度土壤剖面形态特征、可溶盐含量、盐分离子组成、总碱度、pH、碱化度等指标的变化规律进行了系统研究。结果表明:不同利用方式土壤剖面土体结构、颜色、根系分布、石灰性反应及碱化层分布均有明显差异;土壤剖面的可溶性盐、阴阳离子分布因土地利用方式不同而存在差异,牧草地和林地土壤剖面可溶性盐呈橄榄型、盐荒地和改良地呈表聚型、农用地呈底聚型;土壤pH、碱化度和总碱度在剖面中的分布特征基本一致,呈S型;经不同利用方式土壤的盐化程度和碱化程度均有所下降,与盐荒地相比,农用地盐碱化指标下降的最为显著(P0.05),其中0—20 cm土壤全盐量降至0.88 g/kg,pH降至7.83,总碱度、ESP降至0.17 cmol/kg和12.54%,表明苏打盐碱地经农用后更有利于土壤脱盐化过程和脱碱化过程;土壤全盐量、Ca~(2+)、Mg~(2+)、SO_4~(2-)、总碱度、pH可作为控制区域盐碱化的主要因子。研究结果为阐明河套平原盐碱地的盐碱化过程及盐碱化土壤利用模式提供技术参考。     

Hybrid breeding for biomass yield in winter triticale: II. Combining ability and hybrid prediction

Accurate hybrid prediction and knowledge about the relative contribution of general (GCA) and specific combining ability (SCA) are of utmost importance for efficient hybrid breeding. We therefore evaluated 91 triticale single-cross hybrids in field trials at seven environments for plant height, heading time, fresh biomass, dry matter content and dry biomass. Fresh and dry biomass showed the highest proportion (23%) of variance due to SCA. Prediction accuracies based on GCA were slightly higher than based on mid-parent values. Utilizing parental kinship information yielded the highest prediction accuracies when both parental lines have been tested in other hybrid combinations, but still moderate-to-low prediction accuracies for two untested parents. Thus, hybrid prediction for biomass traits in triticale is currently promising based on mid-parent values as emphasized by our simulation study, but can be expected to shift to GCA-based prediction with an increasing importance of GCA due to selection in hybrid breeding. Moreover, the performance of potential hybrids between newly developed lines can be predicted with moderate accuracy using genomic relationship information.     

Application of Triticum monococcum for the improvement of triticale resistance to leaf rust (Puccinia triticina)

The aim of this work was to evaluate the leaf rust resistance introduced into introgressive triticale lines with Triticum monococcum genes, and to study the expression of these genes at the hexaploid level. The introgressive lines were developed by incorporating diploid wheat (T. monococcum s.s.) genes into hexaploid triticale LT 522/6 using the synthetic allotetraploid T. monococcum/Secale cereale (A^mA^mRR) as a bridging form. A group of 44 those lines, parental stocks and check cultivars were inoculated at the seedling stage (in a greenhouse) and at the adult‐plant stage (in the field) with four pathotypes of Puccinia triticina. At the seedling stage the assessment of infection type showed that four lines had resistance to all pathotypes as high as in the T. monococcum donor. Adult plant examinations showed some introgressive lines with complete resistance and also lines with partial resistance, expressed in area under the disease progress curve (AUDPC) calculations as slow rusting. Some lines comprise low AUDPC with complete resistance at seedling stage.